Contrast, in physics and digital imaging, is a quantifiable property used to describe the difference in appearance between elements within a visual field. It is closely linked with the perceived brightness of objects and is typically defined by specific formulas that involve the luminances of the stimuli. For example, contrast can be quantified as ΔL/L near the luminance threshold, known as Weber contrast,[1] or as LH/LL at much higher luminances.[2] Further, contrast can result from differences in chromaticity, which are specified by colorimetric characteristics such as the color difference ΔE in the CIE 1976 UCS (Uniform Colour Space).
Understanding contrast is crucial in fields such as imaging and display technologies, where it significantly affects the quality of visual content rendering. The contrast of electronic visual displays is influenced by the type of signal driving mechanism used, which can be either analog or digital. This mechanism directly influences how well the display renders images under varying conditions. Additionally, the contrast is affected by ambient illumination and the viewer's direction of observation, which can alter perceived brightness and color accuracy.
Luminance contrast
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 luminanceratio), 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).
with 1 ≤ CR ≤
A "contrast ratio" of CR = 1 means no contrast.
The contrast can also be specified by the contrast modulation (or Michelson contrast), CM, defined as:
with 0 ≤ CM ≤ 1.
CM = 0 means no contrast.
Another contrast definition is a practical application of Weber contrast,[3] sometimes found in the electronic displays field, K or CW, is:
with 0 ≤ CW ≤ 1.
CW = 0 means no contrast, while maximum contrast, CWmax equals one, or more commonly described as a percentage like Michelson, 100%.
A modification of Weber by Hwaung/Peli adds a glare offset to the denominator to more accurately model computer displays.[4] Thus the modified Weber is:
This more accurately models the loss of contrast that occurs on darker display luminance due to ambient light conditions.
Color contrast
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).
A metric for color contrast often used in the electronic displays field is the color difference ΔE*uv or ΔE*ab.
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).
This way of proceeding is suitable only when the display device does not exhibit loading effects, which means that the luminance of the test pattern is varying with the size of the test pattern. Such loading effects can be found in CRT-displays and in PDPs. A small test pattern (e.g. 4% window pattern) displayed on these devices can have significantly higher luminance than the corresponding full-screen pattern because the supply current may be limited by special electronic circuits.[citation needed]
Full-swing contrast
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.[citation needed]
Static contrast
The standard procedure for contrast evaluation is as follows:[citation needed]
Apply the first test pattern to the electrical interface of the display under test and wait until the optical response has settled to a stable steady state,
Measure the luminance and/or the chromaticity of the first test pattern and record the result,
Apply the second test pattern to the electrical interface of the display under test and wait until the optical response has settled to a stable steady state,
Measure the luminance and/or the chromaticity of the second test pattern and record the result,
Calculate the resulting static contrast for the two test patterns using one of the metrics listed above (CR,CM or K).
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.[citation needed]
Transient 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.[citation needed]
Repetitive impulse response of an LCD-monitor
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.
Dynamic contrast
This is a technique for expanding the contrast of LCD-screens.
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.[5] Also the contrast within one frame can be expanded intentionally depending on the histogram of the image (some sporadic highlights in an image may be cut or suppressed). There is quite some 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.[6] High-dynamic-range (HDR) LCDs are using that technique in order to realize (static) contrast values in the range of CR > 100.000.[7]
Dark-room contrast
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).[citation needed]
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.[citation needed]
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.[citation needed]
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.[8]
"Ambient contrast"
The contrast that can be experienced or measured in the presence of ambient illumination is shortly called "ambient contrast".[9] A special kind of "ambient contrast" is the contrast under outdoor illumination conditions when the illuminance can be very intense (up to 100.000 lx). The contrast apparent under such conditions is called "daylight contrast".[10]
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.[citation needed]
Concurrent contrast
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 simultaneous contrast is already taken for a different effect). Contrast values obtained from two subsequently displayed full-screen patterns may be different from the values evaluated from a checkerboard pattern with the same optical states. That discrepancy may be due to non-ideal properties of the display-screen (e.g. crosstalk, halation, etc.) and/or due to straylight problems in the light measuring device.[citation needed]
Successive contrast
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.[citation needed]
Methods of measurement
contrast of direct-view displays
contrast of projection displays
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.[11] The standard ANSI IT7.215-1992 defines test-patterns and measurement locations, and a way to obtain the luminous flux from illuminance measurements, it does not define however a quantity named "ANSI lumen".[citation needed]
If the reflective properties of the projection screen (usually depending on direction) are included in the measurement, the luminance reflected from the centers of the rectangles has to be measured for a (set of) specific directions of observation.
A computer monitor is an output device that displays information in pictorial or textual form. A discrete monitor comprises a visual display, support electronics, power supply, housing, electrical connectors, and external user controls.
A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but instead use a backlight or reflector to produce images in color or monochrome.
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls within a given solid angle.
Gamma correction or gamma is a nonlinear operation used to encode and decode luminance or tristimulus values in video or still image systems. Gamma correction is, in the simplest cases, defined by the following power-law expression:
A light meter is a device used to measure the amount of light. In photography, an exposure meter is a light meter coupled to either a digital or analog calculator which displays the correct shutter speed and f-number for optimum exposure, given a certain lighting situation and film speed. Similarly, exposure meters are also used in the fields of cinematography and scenic design, in order to determine the optimum light level for a scene.
A plasma display panel (PDP) is a type of flat panel display that uses small cells containing plasma: ionized gas that responds to electric fields. Plasma televisions were the first large flat panel displays to be released to the public.
In photography, exposure value (EV) is a number that represents a combination of a camera's shutter speed and f-number, such that all combinations that yield the same exposure have the same EV. Exposure value is also used to indicate an interval on the photographic exposure scale, with a difference of 1 EV corresponding to a standard power-of-2 exposure step, commonly referred to as a stop.
In color reproduction and colorimetry, a gamut, or color gamut, is a convex set containing the colors that can be accurately represented, i.e. reproduced by an output device or measured by an input device. Devices with a larger gamut can represent more colors. Similarly, gamut may also refer to the colors within a defined color space, which is not linked to a specific device. A trichromatic gamut is often visualized as a color triangle. A less common usage defines gamut as the subset of colors contained within an image, scene or video.
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.
A backlight is a form of illumination used in liquid-crystal displays (LCDs) that provides illumination from the back or side of a display panel. LCDs do not produce light by themselves, so they need illumination to produce a visible image. Backlights are often used in smartphones, computer monitors, and LCD televisions. They are used in small displays to increase readability in low light conditions such as in wristwatches. Typical sources of light for backlights include light-emitting diodes (LEDs) and cold cathode fluorescent lamps (CCFLs).
A foot-lambert or footlambert is a unit of luminance in United States customary units and some other unit systems. A foot-lambert equals 1/π or 0.3183 candela per square foot, or 3.426 candela per square meter. The foot-lambert is named after Johann Heinrich Lambert (1728–1777), a Swiss-German mathematician, physicist and astronomer. It is rarely used by electrical and lighting engineers, who prefer the candela per square foot or candela per square meter units.
Screen burn-in, image burn-in, ghost image, or shadow image, is a permanent discoloration of areas on an electronic display such as a cathode ray tube (CRT) in an old computer monitor or television set. It is caused by cumulative non-uniform use of the screen.
High-dynamic-range rendering, also known as high-dynamic-range lighting, is the rendering of computer graphics scenes by using lighting calculations done in high dynamic range (HDR). This allows preservation of details that may be lost due to limiting contrast ratios. Video games and computer-generated movies and special effects benefit from this as it creates more realistic scenes than with more simplistic lighting models.
Contrast is the difference in luminance or color that makes an object visible against a background of different luminance or color. The human visual system is more sensitive to contrast than to absolute luminance; thus, we can perceive the world similarly despite significant changes in illumination throughout the day or across different locations.
A transflective liquid-crystal display is a liquid-crystal display (LCD) with an optical layer that reflects and transmits light. Under bright illumination the display acts mainly as a reflective display with the contrast being constant with illuminance. However, under dim and dark ambient situations the light from a backlight is transmitted through the transflective layer to provide light for the display. The transflective layer is called a transflector. It is typically made from a sheet polymer. It is similar to a one-way mirror but is not specular.
Large-screen television technology developed rapidly in the late 1990s and 2000s. Prior to the development of thin-screen technologies, rear-projection television was standard for larger displays, and jumbotron, a non-projection video display technology, was used at stadiums and concerts. Various thin-screen technologies are being developed, but only liquid crystal display (LCD), plasma display (PDP) and Digital Light Processing (DLP) have been publicly released. Recent technologies like organic light-emitting diode (OLED) as well as not-yet-released technologies like surface-conduction electron-emitter display (SED) or field emission display (FED) are in development to supercede earlier flat-screen technologies in picture quality.
The viewing cone refers to the effective viewing directions of an LCD display, as seen from the eye. This collection of angles resembles a cone. The concept has been introduced as an international standard ISO 13406-2, which defines it as the range of viewing directions that can safely be used for the intended task without "reduced visual performance". This standard describes a complex procedure which evaluates the viewing cone from measurements of luminance and chromaticity versus direction of observation. ISO 13406-2 introduces 4 viewing direction range classes, from a wide viewing cone, for many simultaneous observers, to the so-called "privacy display", with a severely limited viewing cone. Compliance routes for different display applications can now be found in the successor standard ISO 9241-300.
Conoscopy is an optical technique to make observations of a transparent specimen in a cone of converging rays of light. The various directions of light propagation are observable simultaneously.
An LED-backlit LCD is a liquid-crystal display that uses LEDs for backlighting instead of traditional cold cathode fluorescent (CCFL) backlighting. LED-backlit displays use the same TFT LCD technologies as CCFL-backlit LCDs, but offer a variety of advantages over them.
An always-on display (AOD) is a smartphone feature that has the device continue to show limited information while the phone is asleep. It is widely available on Android handsets, and is available on Apple iPhones since the iPhone 14 Pro. On some Android devices, the feature is sometimes called Ambient Display or Active Display, depending on its implementation and behavior. Depending on the phone's design, it may be a replacement or complementary to another feature, such as the notification LED.
↑ Shiga, T.; Mikoshiba, S. (2003). "49.2: Reduction of LCTV Backlight Power and Enhancement of Gray Scale Capability by Using an Adaptive Dimming Technique". SID Symposium Digest of Technical Papers. 34 (1): 1364. doi:10.1889/1.1832539. S2CID62588415.
↑ 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. S2CID62621574.
↑ Seetzen, Helge; Whitehead, Lorne A.; Ward, Greg (2003). "54.2: A High Dynamic Range Display Using Low and High Resolution Modulators". SID Symposium Digest of Technical Papers. 34 (1): 1450. doi:10.1889/1.1832558. S2CID15359222.
↑ E. F. Kelley: "Diffuse Reflectance and Ambient Contrast Measurements Using a Sampling Sphere", SID ADEAC06 Digest, pp. 1-5
↑ 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. S2CID61094696.
↑ ANSI IT7.215-1992: Data Projection Equipment and Large Screen Data Displays -- Test Methods and Performance Characteristics
↑ M. E. Becker: "Viewing-cone Analysis of LCDs: a Comparison of Measuring Methods", Proc. SID'96, pp. 199
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