lcd panel contrast ratio free sample

As a first step, try using the calibration settings we recommend (provided we have reviewed your monitor). This will get good, basic contrast - meaning no additional contrast-enhancing settings - and with no loss of detail in dark portions of the image. You can find this information in the "Post Calibration" section of the review.

Contrast:Adjusting this will let you affect how much contrast the monitor has. We list a recommended setting with all of our reviews, but it"s almost always fine to just set this to the maximum. On rare occasions, gamma might be affected, leading to a loss of detail in highlights.

Local Dimming: The local dimming feature dims the backlight behind darker portions of the screen. It can deepen contrast, and it"s worth using when implemented well. It can introduce issues like light blooming off of light objects within dark areas, and when done especially poorly, can dim the entire image. We discuss local dimming in more detail here.

Backlight settings have a very minor impact on contrast, and so you should set it to whatever looks best in your viewing space. With LED Monitors, both white and black will become about equally brighter or dimmer when the backlight is adjusted, preserving the ratio of light to dark. With OLED monitors, adjusting the OLED light only increases the peak brightness; blacks are still perfectly black.

One frequently asked question is which is more important, a panel"s native contrast or contrast with local dimming? It"s a good question. The answer is a bit complicated, but basically, it depends. Unlike TVs, most monitors don"t have a local dimming feature. The few that do, generally speaking, don"t work very well. They usually have very small zone counts, and the algorithms can"t keep up with fast-paced motion, so the leading edge of a bright object in a dark scene ends up looking darker than the rest, and there"s a trail of light behind it.

Because of these issues with local dimming, it"s almost always more important to look at the native capabilities of a monitor instead of the contrast ratio with local dimming. Because most monitors have poor local dimming features, there"s usually not that much of a difference between the native contrast of the panel and the contrast with local dimming when tested with a checkerboard pattern. In fact, of the 23 monitors with local dimming that we"ve tested on our latest test bench, only 4 of them can improve contrast by 10% or more with our test pattern through local dimming.

There are different ways to measure contrast. We measure contrast with a checkerboard pattern and take the average black level from four squares, but some other review sites measure it differently, which can lead to a difference in posted numbers. Some of the other methods we"ve seen websites use include:

Full On/Off: Some websites measure the contrast using a full white screen, and a full black screen. This is generally considered a less accurate way to measure contrast, and it isn"t very realistic. Contrast measurements with local dimming tend to appear much better with this measurement technique, as it"s easy for any monitor with local dimming to turn the entire screen off at once.

Small Samples: Similar to the full-screen method, but instead of large slides, contrast is measured using small slides that only cover part of the screen. This method isn"t ideal either, as imperfect uniformity can significantly skew the results.

ANSI Checkerboard: The most generally accepted way to measure contrast; a checkerboard pattern very similar to ours is used, but with an asymmetric test pattern. The ANSI method measures the output in all 16 squares and averages the values for the white and black squares. It usually produces very similar results to our own.

Because of differences in measurement techniques, equipment used, and even differences between units, it"s extremely common for different websites to report different contrast measurements.

Monitors use different display technologies, each with advantages and disadvantages. Knowing which type of panel is used in your monitor can already give you a good indication of what to expect in terms of contrast ratio:

OLED: Very few OLED monitors exist, but they essentially have perfect contrast, as each pixel is self-emissive, the black level of black pixels is essentially zero.

Even within the same panel types, it"s normal for the contrast to vary a bit between units, even of the same model, due to manufacturing tolerances. Manufacturers used to provide the typical contrast ratio for each monitor, but recently, some brands, including LG, have started listing the minimum contrast ratio you could get. For IPS and TN panels, this difference usually isn"t very significant, and most people shouldn"t worry about it, but for VA panels, the variance between individual units and measurement techniques can be significant. For example, the LG 32GN600-B is advertised to have a typical contrast ratio of 3000:1, but according to LG, it could be as low as 1800:1 for some units. We measured a contrast ratio of 3248:1, almost double the minimum contrast for that model.

A monitor’s contrast ratio indicates the depth of blacks – a higher contrast ratio means deeper blacks – and, by extension, better picture quality. It’s a very important part of picture quality, so if you want something that looks good (particularly in a dark room), be sure to get a monitor that has good contrast.

There are a few things that can be done to improve contrast, but there are limits. As a good first step, look to our recommended picture settings (listed with every review), as those can help you get a good baseline. From there, you can enable or disable a few different settings that might help deepen blacks. Just remember that some of those settings will have other consequences on picture quality.

As you type, the contrast ratio indicated will update. Hover over the circle to get more detailed information. When semi-transparent colors are involved as backgrounds, the contrast ratio will have an error margin, to account for the different colors they may be over.

Evaluating your design for color contrast is a critical aspect of accessibility testing and organizations may benefit from appropriate user experience training and expertise to ensure proper contrast. While our color contrast checker is one of the best tools that allows you to determine contrast levels on your own, we would love to help you with your next project. Our accessibility and user experience experts have the skills required to ensure your next project meets your accessibility goals.

We can help you determine the appropriate success criteria, develop a roadmap to achieve your goals, and provide expert support along the way. Contact us today to discuss why you are looking for our color contrast checker or see if we can help with other aspects of your accessibility projects.

If you have any questions concerning our color contrast tool, would like to report bugs or suggest improvements, or contribute to the code, see the Colour Contrast Analyser (CCA) repository.

The current version of the Colour Contrast Analyser (CCA) has been rebuilt from the ground up using Electron. For the previous, non-Electron versions (“CCA Classic”), see the CCA-Win and CCA-OSX repositories.

If you’re in the market for a new TV, projector, camera, or any other type of display, you should pay attention to the contrast ratio. But what does this measurement mean, and how do you know whether your display has good contrast?

While most displays have a contrast setting that the viewer can manually adjust, the ratio refers to the panel’s limitations—in other words, the largest possible difference between its lightest (white) and darkest (black) areas.

Contrast ratio is the measurement of the difference between a display"s maximum and minimum brightness; put another way, it"s the ratio between the brightest white and the darkest black. For example, a contrast ratio of 1,000:1 means that the brightest white image is 1,000 times brighter than the darkest black.

Generally, a higher contrast ratio is better since a display with a 100,000:1 ratio can produce darker black levels and more saturated colors than one with a 1,000:1 rating, thus achieving a more natural-looking image. That said, a bigger number isn"t always better, as you need to take external lighting conditions into account the lighting conditions and the type of display into account.

As previously mentioned, a higher contrast ratio has its benefits but isn’t the only thing you should consider. For example, a projector with a lower contrast ratio could provide an optimal viewing experience if you’ll be using it in a room with a lot of ambient light.

Contrast ratios can also vary significantly across different display types. While a transmissive digital projector may only have a contrast ratio of 200:1, many newer TVs are over 4,000:1. But even these figures don’t tell the whole story, as contrast ratios are dependent on the underlying technology and how they are measured.

When looking at a display’s contrast ratio, it’s important to understand the various ways in which they are measured. The actual ratio you see can be broken down into two different types: Static Contrast and Dynamic Contrast.

Static Contrast, otherwise known as “native” or “onscreen,” is a ratio comparing the brightest and darkest shade a display system is capable of producing at the same time. Since this ratio reflects the results from when the panel was made, industry experts typically consider this a more accurate representation of a display’s capabilities.

Dynamic Contrast offers a more theoretical range of a display’s contrast ratio, as it’s heavily dependent upon the screen’s underlying technology. Here, the range between the lightest areas of an all-white/light scene and the darkest areas of a black/dark scene is measured.

The problem with dynamic contrast measurements is that they are typically dishonest, as you’re unlikely to experience such a wide contrast range in the same scene. On top of this, manufacturers can manipulate contrast to make a scene lighter or darker using a display’s backlighting and firmware.

Unfortunately, there is no standardized measurement of contrast ratio. Particularly in the TV market, manufacturers can essentially inflate their ratings due to a combination of measurement and unstated variables. That said, most contrast ratios are measured using one of two methods:

Displays that measure with this method tend to register lower contrast ratios as ANSI contrast provides a more realistic measurement of the screen’s capability. However, since the test can include a room’s lighting conditions in its measurement, it needs to be performed in an ideal environment for the most accurate reading.

This method measures an all-white screen with an all-black screen and reflects equal proportions of light from the display to the room and back. It"s the preferred method for many manufacturers, as it cancels out exterior lighting conditions and results in an ideal (and thus higher) contrast ratio. Unfortunately, dynamic contrast specs are often misleading since they can be inflated and don"t indicate much about how an average image"s contrast will look.

The eye test is the best tool at your disposal — if a display’s black levels look washed out and gray, its contrast ratio probably isn’t high enough. However, there are other ways to ensure you’re not being misled:

Look for vendors that publish ANSI contrast specs, as this is a more accurate reflection of the display’s true contrast range. Unfortunately, many companies don’t disclose these figures, as ANSI readings tend to be much lower than Full On/Off, and it’s simply a better marketing strategy for these companies to focus on the latter.

Pay attention to backlighting technology.If you’re looking for a TV with a high contrast ratio, an OLED display will offer a better viewing experience than an LCD panel, as the OLED’s pixels don’t rely on a backlight and can display deeper blacks without a “blooming” effect.

Stick to the same manufacturer when making comparisons.Since every company arrives at its contrast ratios through different means, comparing displays produced by the same manufacturer is an excellent way to get consistent figures.

As it pertains to monitors, the contrast ratio is the ratio between the brightest white’s highest lumination level and the deepest black color the monitor is capable of producing. If a monitor has a high contrast ratio, it means it offers deeper shades of black, indicating a higher level of picture quality overall.

Contrast ratio is crucial for projector image quality. The higher the contrast ratio, the more detail viewers can see on the image projected. A higher contrast ratio also means more color subtlety is available, and more shading is visible.

Modern computer LCD monitors typically have a contrast ratio of between 1000:1 and 3000:1. A good gaming monitor may range toward the higher end of the spectrum, but use your eyes when considering a monitor you"re comfortable with and note that ambient light will affect what you"re seeing.

Contrast ratio is the most important aspect of a TV"s performance. More than any other single metric, a set"s contrast ratio will be the most noticeable difference between two TVs.

In its simplest form, contrast ratio is the difference between the brightest image a TV can create and the darkest. In another way: white/black=contrast ratio. If a TV can output 45 foot-lamberts with a white screen and 0.010 ft-L with a black screen, it"s said to have a contrast ratio of 4,500:1.

There is no standard as to how to measure contrast ratio. In other words, a TV manufacturer could measure the maximum light output of 1 pixel driven at some normally unobtainable maximum, then measure that same pixel with no signal going to it at all. This hardly represents what you"d see at home, but without a standard, such trivialities don"t matter to TV manufacturers.

Worse, contrast ratio numbers have gotten so extreme, there is literally no way to measure some of them. What happens more often than not is the marketing department will come up with the number it needs to sell the product. The engineers will shuffle their feet, and stare at the wall, and magically the TV has that contrast ratio.

Because you"re reading this article on a device that has its own contrast ratio, I can"t give you real examples of what good and bad contrast ratios look like, so I"ll have to fake it. If you can, make sure your computer monitor is set decently; you can use

There are two more aspects of contrast ratio. Most often these are referred to as "native" and "dynamic." Native contrast ratio is what the display technology itself can do. With an LCD, this is what the liquid crystal panel itself is capable of. With DLP, it"s what the DMD chip/chips can do.

Imagine putting the image above on your TV"s screen. Native contrast ratio is how dark the darkest parts of the image are, compared with the brightest parts of the same image. I like to call this "intra-scene contrast ratio" though I"m certainly open to something better if anyone has an idea.

When an adjustable backlight, or a projector"s iris, is used in conjunction with circuitry to monitor the video signal, it is able to adjust the overall light output in real time depending on what"s onscreen. This dynamic contrast ratio looks like this:

A bright image is bright, a dark image is dark. Done well, this does increase the apparent contrast ratio of a display, but not nearly as much as the numbers would suggest. A TV with 5,000,000:1 contrast ratio would be unbelievable to look it. Too bad one doesn"t exist. A TV with a high dynamic contrast ratio may look better than a TV that has no such circuitry, but it won"t look as good as a display with a high native contrast ratio.

Yes, the LED"s of an LED LCD can turn off, creating a true black, but it will never do this when there is any amount of video on the screen. Picture the end credits of a movie. A display with a high native contrast will show this as a dark black background, and punchy white text. A display with a high dynamic contrast ratio may have a similarly dark background, but the text won"t be bright.

As you can see, a display with a high native contrast is the way to go, if that"s what you"re going for. The night sky is black, but the streetlights pop out. The day sky is bright, but the dark jacket is dark. This is more like CRT, more like film, more like life.

The technology with the highest native contrast ratio is... LCOS. At the moment, JVC front projectors using their version of the technology (D-ILA) have the highest native contrast ratios I"ve measured. Sony"s version (SXRD) comes in a rather distant second. Third is plasma, though some DLP projectors are close.

LCD has come a long way in the past decade, but still lags behind the other technologies. Thankfully, the better LCD manufactures know this and have come up with a few ways to mimic the high native contrast ratio of the other technologies.

The best way to get a high intra-scene contrast ratio with LCDs is with local dimming. This is when the backlight of the LCD is an array of LEDs, all of which can dim depending on what"s on screen. It"s not done on a per-pixel level, but LED zones are generally small enough that the overall effect is quite good. It"s far better than what the LCD panel can do itself. The downside is an artifact known as "halos" where the LEDs are lit behind small bright areas of the screen, but these areas are visible because the other parts of the screen are dark. This is very noticeable on specific types of content (like movie credits or star fields) but generally local dimming works really well. I was going to Photoshop some halos onto a screenshot of the one movie where I actually had a screen credit, but it came across more douchey

Most LED LCDs these days are "edge lit," as in their LEDs are along the sides (or the top and bottom, or both). Several companies have developed methods to dim areas of the screen with LED edge lighting, though the effect isn"t as good as full array LEDs. Again, every bit helps though, and many edge lit LED LCDs look amazing.

You may be asking yourself: How can you, as a consumer, find out what display has the best contrast ratio? Good question. You can"t tell in a store, as the store lighting will throw off any comparison (biasing towards LCDs or TVs with antireflective and/or antiglare screens that have better ambient light rejection). As mentioned, all manufacturers manufacture their numbers with little basis on reality, so spec sheets are out.

So that leaves reviews. Sadly, few review sites measure contrast ratio, and those that do don"t have consistency between them. There is no set standard for reviewers on how to measure contrast ratio either, so numbers are going to be extremely different. I may measure 20,000:1, while Joe Numbnutz over at TVAwesomeReviews.com measures 1,000:1 with his Datacolor Spyder (a decent product, but not a valid measurement tool for contrast ratio).

ANSI contrast ratio is a good addition. This is where eight-each white and black boxes in a checkerboard pattern are measured and averaged. This gives a good idea of what a display is doing, and is far more relevant to compare to actual video. Even this, though, is problematic, as the brightness of the white boxes can affect the measurement of the black boxes. Done right, it is also exceedingly time consuming. When I started measuring ANSI contrast ratio when I was at Home Theater, it nearly doubled the total amount of time spent measuring a television. Spending that much time on one measurement that most people will overlook is not an effective use of time.

Like nearly all TV buying guides say: It"s all in what you want to do with the TV. If you"re a movie buff and you watch TV in a dark room or at night, the added contrast of plasma will be very cinematic.

Somewhere in between is an LED LCD with some kind of local or zone dimming, offering better intra-scene contrast ratio than a "normal" LCD, but still offering that technology"s extreme light output.

Such factors are listed below.Display technology (Twisted Nematic panels, In-plane switching panels, Vertical alignment panels, and super vertical alignment)

Nothing beats the vertical alignment panels and super vertical alignment panels in contrast ratios regarding the display technology and probably the panel types.

The vertical alignment panels have liquid crystals aligned naturally to the glass, ensuring a more comprehensive contrast ratio range. These panels also have minimum light leakage; meaning prevents the backlights from reaching the deepest blacks. Its whites have better clarity and are uniform.

The display brightness determines the contest ratio integrity in the real action. It counters the surrounding lights" effect on the display. Calibrate the backlight luminance settings to match the display contrast calibration.

To enjoy the best contrast ratio experience, consider a monitor with high brightness levels of up to 5,000 nits. The high luminance counters the effect of ambient lighting.

Glare and reflection of light to the screen affect contrast ratio. When ambient light hits the screen surface, it reflects into your eyes, disrupting your vision. A monitor with an anti-glare coating addresses the glare and reflection issues effectively. Also, ensure your room has minimum ambient and natural light striking the screen directly.

Ambient light from the surroundings directed towards the monitor affects the contrast ratio. This light strikes the screen, scattering in all directions. The reflected light then strikes the eyes of the monitor used. This condition results in unclear image display due to lowered contrast quality.

The contrast ratio (CR) is a property of a display system, defined as the ratio of the luminance of the brightest shade (white) to that of the darkest shade (black) that the system is capable of producing. A high contrast ratio is a desired aspect of any display. It has similarities with dynamic range.

There is no official, standardized way to measure contrast ratio for a system or its parts, nor is there a standard for defining "Contrast Ratio" that is accepted by any standards organization so ratings provided by different manufacturers of display devices are not necessarily comparable to each other due to differences in method of measurement, operation, and unstated variables.projection screen or emitted by a cathode ray tube, and the only light seen in the room would come from the display device. With such a room, the contrast ratio of the image would be the same as the contrast ratio of the device. Real rooms reflect some of the light back to the displayed image, lowering the contrast ratio seen in the image.

Static contrast ratio is the luminosity ratio comparing the brightest and darkest shade the system is capable of producing simultaneously at any instant of time, while dynamic contrast ratio is the luminosity ratio comparing the brightest and darkest shade the system is capable of producing over time (while the picture is moving). Moving from a system that displays a static motionless image to a system that displays a dynamic, changing picture slightly complicates the definition of the contrast ratio, due to the need to take into account the extra temporal dimension to the measuring process.

Many display devices favor the use of the full on/full off method of measurement, as it cancels out the effect of the room and results in an ideal ratio. Equal proportions of light reflect from the display to the room and back in both "black" and "white" measurements, as long as the room stays the same. This will inflate the light levels of both measurements proportionally, leaving the black to white luminance ratio unaffected.

Some manufacturers have gone as far as using different device parameters for the three tests, even further inflating the calculated contrast ratio. With DLP projectors, one method to do this is to enable the clear sector of the color filter wheel for the "on" part and disable it for the "off" part

Another measure is the ANSI contrast, in which the measurement is done with a checker board patterned test image where the black and white luminosity values are measured simultaneously.

It is useful to note that the full on/full off method effectively measures the dynamic contrast ratio of a display, while the ANSI contrast measures the static contrast ratio.

An LCD technology is dynamic contrast (DC), also called advanced contrast ratio (ACR) and various other designations. When there is a need to display a dark image, a display that supports dynamic contrast underpowers the backlight lamp (or decreases the aperture of the projector"s lens using an iris), but proportionately amplifies the transmission through the LCD panel; this gives the benefit of realizing the potential static contrast ratio of the LCD panel in dark scenes when the image is watched in a dark room. The drawback is that if a dark scene contains small areas of superbright light, the resulting image will be over exposed.

It is also common to market only the dynamic contrast ratio capability of a display (when it is better than its static contrast ratio only on paper), which should not be directly compared to the static contrast ratio. A plasma display with a 4,000,000:1 static contrast ratio will show superior contrast to an LCD (with LED or CCFL backlight) with 30,000,000:1 dynamic and 20,000:1 static contrast ratio when the input signal contains a full range of brightnesses from 0 to 100% simultaneously. They will, however, be on par when input signal ranges only from 0 to 20% brightness.

This animated gif shows a rudimentary representation of how various backlight dimming technologies work on TV. Dimming technology can drastically affect the contrast ratio of the display.

In marketing literature, contrast ratios for emissive (as opposed to reflective) displays are always measured under the optimum condition of a room in total darkness. In typical viewing situations, the contrast ratio is significantly lower due to the reflection of light from the surface of the display, making it harder to distinguish between different devices with very high contrast ratios.luminance of the display, as well as the amount of light reflecting off the display.

Contrast in visual perception is a felt difference in appearance of two or more parts of a field seen simultaneously or successively (hence: brightness contrast, lightness contrast, color contrast, simultaneous contrast, successive contrast, etc.).

Contrast in physics is a quantity intended to correlate with the perceived brightness contrast, usually defined by one of a number of formulae (see below) which involve e.g. the luminances of the stimuli considered, for example: ΔL/L near the luminance threshold (known as Weber contrastH/LL for much higher luminances.

The contrast of electronic visual displays depends on the electrical driving (analog or digital input signal), on the ambient illumination and on the direction of observation (i.e. viewing direction).

The "luminance contrast" is the ratio between the higher luminance, LH, and the lower luminance, LL, that define the feature to be detected. This ratio, often called contrast ratio, CR, (actually being a luminance ratio), is often used for high luminances and for specification of the contrast of electronic visual display devices. The luminance contrast (ratio), CR, is a dimensionless number, often indicated by adding ":1" to the value of the quotient (e.g. CR = 900:1).

Two parts of a visual field can be of equal luminance, but their color (chromaticity) is different. Such a color contrast can be described by a distance in a suitable chromaticity system (e.g. CIE 1976 UCS, CIELAB, CIELUV).

During measurement of the luminance values used for evaluation of the contrast, the active area of the display screen is often completely set to one of the optical states for which the contrast is to be determined, e.g. completely white (R=G=B=100%) and completely black (R=G=B=0%) and the luminance is measured one after the other (time sequential).

Any two test patterns that are not completely identical can be used to evaluate a contrast between them. When one test pattern comprises the completely bright state (full-white, R=G=B=100%) and the other one the completely dark state (full-black, R=G=B=0%) the resulting contrast is called full-swing contrast. This contrast is the highest (maximum) contrast the display can achieve. If no test pattern is specified in a data sheet together with a contrast statement, it will most probably refer to the full-swing contrast.

When luminance and/or chromaticity are measured before the optical response has settled to a stable steady state, some kind of transient contrast has been measured instead of the static contrast.

When the image content is changing rapidly, e.g. during the display of video or movie content, the optical state of the display may not reach the intended stable steady state because of slow response and thus the apparent contrast is reduced if compared to the static contrast.

This illustration shows the repetitive impulse response between several states of gray when each state is applied for one frame only. It is obvious that the stationary luminance levels (indicated by dashed horizontal lines) are not reached within a single frame.

LCD-screens comprise a backlight unit which is permanently emitting light and an LCD-panel in front of it which modulates transmission of light with respect to intensity and chromaticity. In order to increase the contrast of such LCD-screens the backlight can be (globally) dimmed when the image to be displayed is dark (i.e. not comprising high intensity image data) while the image data is numerically corrected and adapted to the reduced backlight intensity. In such a way the dark regions in dark images can be improved and the contrast between subsequent frames can be substantially increased.digital signal processing required for implementation of the dynamic contrast control technique in a way that is pleasing to the human visual system (e.g. no flicker effects must be induced).

The contrast within individual frames (simultaneous contrast) can be increased when the backlight can be locally dimmed. This can be achieved with backlight units that are realized with arrays of LEDs.

In order to measure the highest contrast possible, the dark state of the display under test must not be corrupted by light from the surroundings, since even small increments ΔL in the denominator of the ratio (LH + ΔL) / (LL + ΔL) effect a considerable reduction of that quotient. This is the reason why most contrast ratios used for advertising purposes are measured under dark-room conditions (illuminance EDR ≤ 1 lx).

All emissive electronic displays (e.g. CRTs, PDPs) theoretically do not emit light in the black state (R=G=B=0%) and thus, under darkroom conditions with no ambient light reflected from the display surface into the light measuring device, the luminance of the black state is zero and thus the contrast becomes infinity.

When these display-screens are used outside a completely dark room, e.g. in the living room (illuminance approx. 100 lx) or in an office situation (illuminance 300 lx minimum), ambient light is reflected from the display surface, adding to the luminance of the dark state and thus reducing the contrast considerably.

A quite novel TV-screen realized with OLED technology is specified with a dark-room contrast ratio CR = 1.000.000 (one million). In a realistic application situation with 100 lx illuminance the contrast ratio goes down to ~350, with 300 lx it is reduced to ~120.

Since always the dark areas of a display are corrupted by reflected light, reasonable "ambient contrast" values can only be maintained when the display is provided with efficient measures to reduce reflections by anti reflection and/or anti-glare coatings.

When a test pattern is displayed that contains areas with different luminance and/or chromaticity (e.g. a checkerboard pattern), and an observer sees the different areas simultaneously, the apparent contrast is called concurrent contrast (the term

When a contrast is established between two optical states that are perceived or measured one after the other, this contrast is called successive contrast. The contrast between two full-screen patterns (full-screen contrast) always is a successive contrast.

Depending on the nature of the display under test (direct-view or projection) the contrast is evaluated as a quotient of luminance values (direct-view) or as a quotient of illuminance values (projection displays) if the properties of the projection screen is separated from that of the projector. In the latter case, a checkerboard pattern with full-white and full-black rectangles is projected and the illuminance is measured at the center of the rectangles.luminous flux from illuminance measurements, it does not define however a quantity named "ANSI lumen".

Luminance, contrast and chromaticity of LCD-screens is usually varying with the direction of observation (i.e. viewing direction). The variation of electro-optical characteristics with viewing direction can be measured sequentially by mechanical scanning of the viewing cone (gonioscopic approach) or by simultaneous measurements based on conoscopy.

Chen, Hanfeng; Sung, Junho; Ha, Taehyeun; Park, Yungjun (2007). "Locally pixel-compensated backlight dimming on LED-backlit LCD TV". Journal of the Society for Information Display. 15 (12): 981. doi:10.1889/1.2825108. S2CID 62621574.

Kelley, Edward F.; Lindfors, Max; Penczek, John (2006). "Display daylight ambient contrast measurement methods and daylight readability". Journal of the Society for Information Display. 14 (11): 1019. doi:10.1889/1.2393026. S2CID 61094696.

The screen size is measured diagonally, from ‘corner to corner.’ Common monitor sizes, in inches, are 15″, 17″, 19″, and 21″. In the case of CRT monitors, the diagonal measurement is taken from the outside edges of the display casing and, therefore, the active display area is actually a few inches lesser. In LCD monitors, the diagonal measurement is taken from the inside of the beveled edge. As a result, a 17″ LCD display is comparable to a 19″ CRT display[1] [Figure 1].

Screen sizes. In CRT monitors (A), the screen size is measured diagonally from the outside edges of the display casing. In LCD monitors (B), the screen size is measured diagonally from the inside of the beveled edges. Thus a 17-inch LCD display is comparable to a 19-inch CRT display. The CRT display includes 2 inches of casing, as well

This is the ratio of the width of a screen image to its height. The aspect ratio for most CRT monitors is 4:3. However, the trend today is in favor of flat screens and wide-screen LCD monitors with aspect ratios of 16:9 and, occasionally, even 16:10 or 15:9, are common today.[2]

Until recently, images in radiology were viewed in the landscape mode, a tradition set by the computer industry. This mode is clearly not optimal for use in diagnostic imaging.[3] LCD monitors, being lightweight and thin, can be easily turned around and placed in a portrait mode, which corresponds with the image format of radiology images. A 17 × 14 image is optimally viewed in the ‘portrait’ rather than ‘landscape’ mode. The ‘portrait’ mode is particularly useful for chest and skeletal radiography and for mammography [Figure 2].[4]

Landscape (A) versus portrait (B) mode. By virtue of being lightweight and thin, LCD monitors can be placed in a portrait mode that corresponds with the 17×14 format of images obtained in chest and skeletal radiography. Mammography images can be optimally viewed in the ‘portrait’ (B) rather than the ‘landscape’ mode (A), as shown here

Pixel or ‘picture element’ is the basic unit used for generating a video or computer picture and is measured in millimeters. In the CRT display screen, a pixel is the distance between phosphor dots, whereas in the LCD display screen it is the distance between cells of the same color. A dot pitch is essentially a dot with a given color and brightness value. A smaller dot pitch implies a sharper image, as there are more dots in an area of a given size. In computer monitors, the pixel numbers are dependent on the aspect ratio of the screen (its horizontal size compared to its vertical size) and the display size. Standard monitors with a 4:3 aspect ratio have a width of 1024 pixels and a height of 768 pixels. Widescreen display monitors with a 16:9 aspect ratio have a width of 1024 pixels and a height of 576 pixels.[1]

In a CRT monitor, the horizontal resolution is limited by bandwidth and spot size, while the vertical resolution is limited by line spacing and spot size.[5] In comparison, the resolution in an LCD monitor is determined by pixel pitch (the product of vertical and horizontal pixels)[5] and ranges commonly from 1 megapixel (MP) to 5 MP. With ongoing refinements in technology, high-resolution monitors with resolutions as high as 9 MP and higher have started becoming available.

In CRT monitors, the resolution can be changed to match the frequency emitted by the video signal. This advantageous feature is referred to as ‘multisync.’[6] LCD monitors, in contrast, have a fixed resolution, which is termed the native resolution.

With the availability of a variety of monitors in radiology, there is a need to ensure that different displays show the same image consistently and that too over a long period of time. An optimal grayscale response for monitors can be achieved by calibration of monitors according to the guidelines given in part 14 of the DICOM standard: the grayscale display function (GSDF).[7] The DICOM GSDF ‘specifies a standard relationship for the shades of gray.’[8] It recommends the use of calibration systems for measuring the monitor display"s white level, black level, and the shades of gray in between, with allowances for the ambient light. The use of lookup tables (LUTs) converts a particular display to a DICOM-standard display, thereby ensuring consistency.

In monitors, the term brightness refers to the ‘emitted luminous intensity on screen.’[10] It is measured in candela per square meter (cd/m2 or nits). A higher cd/m2 or nit value indicates a higher onscreen brightness.[10] Luminance is defined as the ‘absolute quantity of radiation emitted from a given source of visible electromagnetic radiation.’[10] Luminance ratio (LR), the ratio of maximum luminance to minimum luminance (Lmax / Lmin), is also important.

Let us first examine the luminance of viewboxes. Low-intensity viewboxes have luminance values of 1000–2000 cd/m2, while the luminance of high-intensity viewboxes is 2000–3000 cd/m2. The viewboxes for viewing mammograms have higher luminance, ranging from 3500 to 5000 cd/m2.[11] With regard to monitor displays, CRT monitors generally have a maximum phosphor luminance of about 450 cd/m2 and an LR of about 900:1. These values are higher in the case of LCD monitors, with a maximum luminance of about 1000 cd/m2 and an LR of about 1200:1.[5]

Medical-grade display systems have certain unique features that are essential for achieving accuracy and efficiency in the diagnosis of lesions. Medical displays maintain consistent contrast over a much larger angle and reduce the deleterious effects of ‘noise’ caused by luminance variations and color variations. Medical-grade displays can automatically calibrate themselves, thereby matching the contrast to every level of brightness.

It is defined as the ratio of the white luminance to black luminance. Simply put, it describes the ‘ability to produce bright whites and the dark blacks.’[3] Consumer-grade displays have a contrast ratio that is usually in the range of 300:1; in comparison, medical displays are substantially better, with values of nearly 1000:1.[12]

In the case of LCD displays, monochrome and color displays are available for the diagnostic viewing of medical images. When currently available monochrome and color LCD displays are compared, there are a few important differences. The native brightness and contrast of monochrome displays are higher than those of color LCD displays.[13]

The contrast ratio in CRT displays remains constant over a wide viewing angle.[4] In comparison, the contrast ratio in LCD displays drops rapidly from the maximum to about 10:1 for a viewing angle of 85°[4] [Figure 3]. It is therefore recommended that the person viewing an image on a monitor should be seated directly, perpendicularly in front of the monitor; a viewing angle close to 90° eliminates ‘off-angle’ viewing. On-axis viewing is considered acceptable with both CRT and LCD monitors, but off-axis distortions can occur with LCD monitors.[15]

The importance of the viewing angle. A CRT display shown directly in front of the viewer (A) and off-angle (B). An LCD display shown directly in front of the viewer (C) and off-angle (D). The contrast ratio in CRT displays remains constant over a wide viewing angle (B) unlike LCD displays (D) where it drops rapidly. An incorrect viewing angle for LCDs may make images dim or disappear or misrepresented with off-axis distortion, all of which can result in the missing of subtle lesions. A CRT monitor has a larger viewing angle that advantageously allows multiple users to review images simultaneously

In a CRT monitor, the electron beam hits phosphor-coated dots, causing pixels of an image to flicker. Individually, each pixel is hit 60–80 times a second.[16] Flicker is annoying to the human visual system.[1] It is avoided in a CRT monitor by an adequate refresh rate, denoted by the number of times a display monitor is illuminated. The refresh rate is measured in Hertz (Hz). A refresh rate of at least 70 Hz is necessary in case of diagnostic CRT monitors.[17] For larger CRT monitors a refresh rate of 85 or even higher may be needed. In case of LCD monitors, there is no flicker due to the slow response time.[1]

This term is specific for LCD monitors; it describes the amount of time taken for a pixel to change from active (black) to inactive (white).[18] It is measured in milliseconds, with a lower number implying faster transitions and fewer visible artifacts like the ‘ghosting’ effect when the image moves across the screen. In color LCDs, response rate denotes the speed with which the monitor"s pixels change colors.

Consistent presentation of digital radiographic images at all locations within a medical center can help ensure a high level of patient care. Currently, liquid crystal displays (LCDs) are the electronic display technology of choice for viewing medical images. As the inherent luminance (and thereby perceived contrast) properties of different LCDs can vary substantially, calibration of the luminance response of these displays is required to ensure that observer perception of an image is consistent on all displays. The digital imaging and communication in medicine (DICOM) grayscale standard display function (GSDF) defines the luminance response of a display such that an observer’s perception of image contrast is consistent throughout the pixel value range of a displayed image. The main purpose of this work is to review the theoretical and practical aspects of calibration of LCDs to the GSDF. Included herein is a review of LCD technology, principles of calibration, and other practical aspects related to calibration and observer perception of images presented on LCDs. Both grayscale and color displays are considered, and the influence of ambient light on calibration and perception is discussed.

The purpose of this work is to provide an overview of DICOM grayscale calibration of medical imaging grade liquid crystal displays (LCDs). In the context of this work, a “medical imaging grade” display is a high-quality LCD, which is appropriate for medical image viewing. It has the general characteristics of minimal visual noise, an appropriately wide viewing angle range, minimal reflection of ambient light, and a grayscale luminance response that is calibrated to a known and accepted standard. Each of these display properties will be discussed, with greater detail and emphasis placed on grayscale calibration. Much of the information provided here has been presented previously by other authors and groups, and references to those works are provided. Successful calibration and use of LCDs in medical imaging requires an understanding of LCD technology. Therefore, a “Technology Overview” is provided. A combination of the GSDF and LCD technology provides the “Foundations of DICOM Calibration.” There are several “Practical Aspects of Calibration,” some of which will be discussed here. Grayscale calibration is evolving from inherently grayscale displays to grayscale calibration of color displays. The “Influence of Color” on the calibration of LCDs is underrepresented in the literature and will be discussed briefly. The light source of most LCDs is a combination of fluorescent lamps and light guides. Because the luminance output of the lamps changes over time, “Maintenance of Calibration” is required to ensure quality image presentation over time. Finally, there are several “Confounding Factors” to consider when calibrating and implementing medical imaging grade LCDs into clinical practice. These include the influence of viewing angle and ambient light. Whereas the influence of ambient light on the physical properties of a displayed image is well known, the influence of ambient light on the adaptation (and perception) of the human visual system is underrepresented in the medical imaging display literature and will be discussed briefly.

Whereas the theory of grayscale calibration of medical imaging grade LCDs is based on the known response of the human visual system to contrast stimuli, practical calibration is implemented to both accommodate and utilize the specifics of LCD technology. A basic understanding of LCD technology is essential to appreciate the methods and limitations of calibration.

An active matrix liquid crystal display is often described as an optical “stack” containing a light source, light diffusers, polarizers, color filters (especially in case of a color LCD), and thin-film transistor liquid crystal matrix. An example of an optical stack is shown in Figure 1. The backlight is a combination of a light source and optical light guides and diffusers that serve to create a uniform luminance across the LCD surface. The light source is commonly a cold-cathode fluorescent lamp and may be implemented truly as backlights or commonly as “side” lights that are positioned along the outer edge(s) of the display. Other light sources, such as light emitting diodes or xenon lamps may also be used. The amount of light transmitted through the panel is a function of the relative orientation of the rear and front polarizers and the influence of the liquid crystal on the polarized light. The liquid crystal is a thin fluid layer of elongated organic molecules between glass plates. Linear “etching” of the inner surfaces of the glass plates preferentially orients the molecules near these surfaces parallel to the direction of the etching. In a resting state, the orientation of the molecules is determined by the direction of the etched grooves. As voltage is applied across the crystal, the molecules tend to preferentially align their direction in relation to the electric field. Orthogonal direction etching of the top and bottom surfaces of the liquid crystal layer provides an inherent “twist” in the orientation of the elongated molecules when a voltage is applied across the crystal. As the polarized light interacts with the twisted crystal, the plane of polarization tends to twist with the crystal. If the front and rear polarizers are oriented perpendicular to one another and the preferential plane of the polarized light is unaffected by the crystal, then very little light will be transmitted through the panel. If the molecules are highly aligned to the orthogonally oriented etching (caused by an applied voltage), then the polarized light is twisted 90° to align with the orientation of the front polarizer and a large fraction of light is transmitted. The applied voltage level influences to what extent the elongated molecules twist, thereby influencing the orientation of the plane of propagation of the once polarized light, which ultimately determines the amount of light that is transmitted by a small portion of the liquid crystal display. This description of LCD construction and operation is provided an as example. Details of the design and function of a specific LCD may well vary from the example provided there. However, the basic interactions between light, the polarizers, and the liquid crystal are common for all LCDs.

Each addressable LCD pixel has three subpixel elements, regardless of whether the display is inherently a color or a grayscale LCD. The three subelements correspond to the standard red, green, and blue color channels (RGB). In a color LCD, the colors are created by the addition of RGB filters. The color filters are omitted in inherently grayscale LCDs. Example color and grayscale pixel elements are shown in Figure 2. The “chevron” shape on the left in Figure 2 and the linear subpixels shown on the right are typical of many types of displays. As will be discussed later, the three subpixels will prove to be useful for grayscale calibration.

Grayscale calibration of LCDs is especially important because there is generally poor consistency in the inherent response of these displays. For example, the luminance (and perceived) contrast of an uncalibrated LCD can vary greatly throughout the input digital driving level (DDL) range of a single display.

The intensity of light emitted by a display, or luminance, is typically specified to have units of candela per square meter (cd/m2). The relationship between cd/m2 and the older luminance unit foot-lamberts (fL) is given by 1 cd/m2 = 0.292 fL. Because LCDs are non-Lambertian surfaces, luminance is typically measured with a photometer, which has a narrow light acceptance angle (≤5°). Some photometers have a display to provide luminance measurements to the operator. Others interface with a computer, which can provide electronic data capture and display the luminance values. Often, medical imaging grade LCDs utilize a calibration software package that supports a limited number of photometers. For calibrating medical imaging grade displays, a photometer should be accurate to within 5% except for very low luminance levels (0.5 cd/m2) where an accuracy of 10% may suffice. The accuracy of any photometer used for calibrating medical imaging grade displays should be specified by the manufacturer. Note that photometers used for calibrating color LCDs have independent RGB measurement channels. This provides for calibration of the color tone as well as the luminance of color displays.

A photometer is an appropriate device to measure the luminance emitted (or reflected) from a display. Environmental lighting surrounding a display produces ambient light, or illuminance, which, by diffuse or specular reflection into the observer’s eye, may influence the physical contrast and the perceived contrast of a display. The typical unit of illuminance is the lux (lx). As opposed to the directionally oriented light emitted from a display, ambient light incident upon any object may come from any direction. Therefore, it cannot be measured directly with a standard narrow-angle photometer, but is rather measured using a photoreceptor behind an optical cosine diffuser. Whereas ambient light cannot be measured directly using a narrow-angle photometer, the ambient light reflected from a display can be measured with a narrow-angle photometer.

The goal of grayscale calibration of electronic displays is to ensure that images presented to an observer have equal perceived contrast appearance on all displays, independent of the luminance range of the display. To achieve this goal requires that, given a specific luminance scene, the contrast perceived by the human visual system (HVS) is well known. There are several experimental models of the perceived contrast of the human visual system. There are also multiple grayscale display function models, which have been and continue to be used to calibrate displays. These include the CIELAB display function (Commission Internationale de l’Eclariage), the log-luminance linear function,

Experience has shown that the overall perceived contrast of an image presented on two GSDF calibrated LCDs of similar contrast ratio but different luminance ranges exhibit similar (although not exactly the same) overall contrast appearance. Another recognized limitation of the Barten model (and thereby the GSDF) is that, for a particular uniform background luminance, the luminance change required to perceive the visual targets was determined with the HVS adapted to that background luminance. In radiographic image viewing the HVS adapts to an overall scene luminance, but is required to perceive anatomical detail over the relatively wide luminance range of a displayed image. Because the HVS perception is optimized for a relatively narrow luminance range near the adaptation luminance, perception of detail in the light and dark portions of an image may be somewhat compromised.

The DICOM grayscale standard display function describes a relationship between luminance and monitor input values such that the slope of the curve at any point, ie the contrast, is proportional (but not necessarily equal) to the contrast associated with a JND from the Barten model. The DICOM grayscale display function is often plotted graphically as luminance versus JND index (j), where the JND index values range from 0 to 1023. The DICOM grayscale display function is presented in Figure 3 this way. For a given luminance, the luminance change required to perceive a difference corresponds to a single JND. The model data have been fit to a polynomial, which provides a convenient mathematical transform to convert back and forth from units of luminance (cd/m2) to JNDj. The transform to convert from luminance (L) to JNDjj) is provided in Eq. j to luminance,,j transforms are available in tabular format.

To perform a DICOM calibration of a display, the desired postcalibration minimum (Lmin) and maximum (Lmax) luminance values are first identified. Because the luminance output of the backlights decreases over time, it may be reasonable to expect that the calibrated maximum luminance intensity selected may adversely affect the life expectancy of a display. The desired Lmax may be less than the maximum luminance that the device is capable of producing, Ldmax. Diagnostic monochrome monitors are now commonly calibrated for an Lmax of 400 to 600 cd/cm2, whereas color monitors used for clinical review will have an Lmax from 250 to 450 cd/cm2. Similarly, Lmin, corrected for ambient light effects, is often set to be larger than the black level of the display, Ldmin. The minimum possible Ldmin value is determined by the ability of an LCD to block the backlight.

Typical contrast ratios are in the range of 250 to 400 for color displays and up to 600 (or more) for inherently grayscale displays. Commonly, Lmin is specified to be slightly higher than the minimum possible value. This allows similar displays with different maximum luminance levels to be calibrated such that the luminance ratio Lmax/Lmin is the same and images appear with similar contrast.

After the Lmin and Lmax values have been specified, the corresponding JND index values (JNDmin and JNDmax) are determined using Eq. max and JNDmin defines the total number of JNDs (JNDtot) that the calibrated display can produce. For example, a display with Lmin = 0.5 cd/m2 and Lmax = 200 cd/m2 has corresponding JNDmin = 46.6 and JNDmax = 572.2. In this example, JNDtot = 525.6. Most computers, video display driver cards, and displays are capable of 8-bit graphics. Regardless of whether the display is color or grayscale, an 8-bit grayscale palette is typical. Therefore, there are typically N = 256 grayscale digital driving levels available to a display system. The purpose of calibration is to set the luminance output for each input DDL such that throughout the entire DDL range, a change in DDL results in a JND index change that is constant. To accomplish this, the total number of JNDs is divided by the number of DDLs, minus one. This defines the average number of JNDs per DDL (JNDave) of the calibrated display. Continuing with the example started above, JNDave = 525.6 / 255 = 2.06. The JNDDDL of each DDL is next calculated as

Just as the DICOM GSDF with JND indices is useful for calibrating displays, it is also useful for characterizing the contrast properties of both calibrated and uncalibrated displays. To characterize a display, luminance measurements can be converted to JND index values using Eq. 4 and ​and55 for both an uncalibrated and a calibrated display. In Figures 4 and ​and5,5, note the lack of contrast for DDL values less than 8 and the relatively increased contrast for DDL values in the range 8 to 96 compared to the GSDF. For values greater than 128, the uncalibrated display has less contrast than the calibrated display. As might be expected, the JND space of a calibrated display is linear.

It is worth noting that calibrating displays to the GSDF with a similar luminance ratio can provide a similar image appearance on all displays, but this does not guarantee that the images necessarily look as desired. The overall appearance of digital medical images is dependent upon many variables including image acquisition parameters (peak kilovoltage [kVp] and mAs in x-ray, for example), image processing, and the various linear and nonlinear grayscale transforms provided for by DICOM Part 14.

As shown in Figures 4 and ​and5,5, a luminance output that corresponds to the GSDF is not inherent to LCDs. Calibration requires creation of a look-up table (LUT) such that for each DDL, the LUT specifies a new DDL value that corresponds to a displayed luminance that matches (as closely as possible) that of an ideal DICOM display.6 shows the JND/DDL contrast of an ideal GSDF calibrated display and a display that has been calibrated using an 8-bit LUT.

With the 8-bit-to-8-bit calibration, the net result is that the overall contrast of the display has the visual appearance of being calibrated. The overall perceptual contrast of the display is as desired. However, adjacent input pixel values may have output DDLs that are exactly the same, resulting in no displayed contrast between adjacent input levels. This effect is represented by the JND/DDL values in Figure 6, which have a value of zero. Also, adjacent input values may have output DDLs that differ by more than 1 (of 256) and the displayed contrast between adjacent input levels is excessive. This is also shown in Figure 6. Note that, for this example, the 8-bit-to-8-bit conversion resulted in 218 of a possible 256 unique gray levels in the calibrated grayscale function. The grayscale discontinuity caused by the 8-bit-to-8-bit LUT may result in “scalloping” or grayscale “banding” artifacts in a displayed image. Given that 8-bit-to-8-bit calibration generally results in loss of local contrast combined with excessive local contrast, it is generally not a satisfactory calibration method.

Use of an expanded grayscale palette, achieved by addressing the subpixels independently, provides the foundation for a more effective grayscale calibration. With the expanded palette, the output luminance can be made to more closely match the ideal luminance indicated by the GSDF. In contrast to the 8-bit palette, the same luminance value need not be used multiple times and the potential for very large and small contrast changes for adjacent DDLs is reduced. This results in a calibrated display, which more closely approximates that of an ideal display, as shown in Figure 6. Along with spatial dithering, the use of temporal modulation or temporal dithering of subpixels has been implemented by some manufacturers to further refine the luminance difference between LUT palette entries.

“Theory of Calibration” describes how a calibration LUT is derived once the desired Lmin and Lmax have been specified. For grayscale displays, typical calibrated Lmax values are in the range of 400 to 600 cd/m2. These displays generally have a contrast ratio of approximately 600:1. Therefore, Lmin could be specified as low as 0.66 to 1 cd/m2 to achieve a displayed luminance ratio equal to the device contrast ratio. Whereas the HVS can adapt to a large range of luminance levels, the luminance range of an adapted HVS is relatively limited.,Lmax = 525 and Lmin = 1.5 will have a luminance ratio of 350. The Lmax of color displays is generally less than that of grayscale displays, and may range between 250 and 450 cd/m2. For the same luminance ratio, a color monitor thus has a lower Lmin, making them more susceptible to the adverse effects of ambient light.

It is important to note that GSDF calibration of different displays does not necessarily result in displays which are exactly perceptually matched. The GSDF ensures that the perceived contrast is consistent throughout the grayscale range of a given display, but calibrated displays that have substantially different maximum luminance values or contrast ratios will not provide an exact perceptual match. For example, a brighter display will still look brighter after calibration and a higher contrast display will still have higher contrast. Because the luminance and contrast properties of medical imaging grade LCDs is not widely variable, GSDF calibration of these displays will ensure that an image presented on each display will have a similar appearance.

It might be considered that calibration based on initial measurement of the luminance at every palette LUT value provides the best grayscale calibration. For cathode ray tube (CRT) displays, luminance output is well behaved with driving level and most calibration software coarsely sampled the palette and interpolated the intervening values. Therefore, the typical number of luminance measurement points used for CRT calibration is often in the range of 18 to 32. Note that American Associati