Display contrast
Encyclopedia
Contrast in visual perception
is the 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 luminance
s of the stimuli considered, for example: ΔL/L near the luminance threshold (known as Weber contrast ), or LH/LL for much higher luminances .
A contrast can also be due to differences of chromaticity
specified by colorimetric characteristics (e.g. the color difference
ΔE CIE 1976 UCS).
Visual information
is always contained in some kind of visual contrast, thus contrast is an essential performance feature of electronic visual display
s.
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
).
In the field of electronic visual displays the following forms of contrast can be distinguished:
----
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).
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 sometimes found in the electronic displays field, K, is:
with 0 ≤ K ≤ 1.
K = 0 means no contrast, the maximum possible contrast, Kmax equals one.
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.
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 PDP
s. 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.
(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.
1 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,
2 measure the luminance and/or the chromaticity of the first test pattern and record the result,
3 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,
4 measure the luminance and/or the chromaticity of the second test pattern and record the result,
5 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.
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 . 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 . High-dynamic-range (HDR) LCDs are using that technique in order to realize (static) contrast values in the range of CR > 100.000 .
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 luminanc 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.
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 . 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".
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
.
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
.
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
is the 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
Physics
Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
is a quantity intended to correlate with the perceived brightness
Brightness
Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target...
contrast, usually defined by one of a number of formulae (see below) which involve e.g. the luminance
Luminance
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 or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
s of the stimuli considered, for example: ΔL/L near the luminance threshold (known as Weber contrast ), or LH/LL for much higher luminances .
A contrast can also be due to differences of chromaticity
Chromaticity
Chromaticity is an objective specification of the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness ....
specified by colorimetric characteristics (e.g. the color difference
Color difference
The difference or distance between two colors is a metric of interest in color science. It allows people to quantify a notion that would otherwise be described with adjectives, to the detriment of anyone whose work is color critical...
ΔE CIE 1976 UCS).
Visual information
Information
Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...
is always contained in some kind of visual contrast, thus contrast is an essential performance feature of electronic visual display
Electronic visual display
An electronic visual display is display technology which incorporates flat panel displays, performs as a video display, output device for presentation of images transmitted electronically, for visual reception, without producing a permanent record....
s.
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
Viewing cone
-Viewing direction:When a visual display with non-vanishing size is seen by an observer, every point of the display area is seen from a different direction as illustrated in fig. 1. No two spots on the display are seen from the same direction...
).
In the field of electronic visual displays the following forms of contrast can be distinguished:
- luminance contrast,
- color contrast,
- full-screen contrast (time sequential),
- full-swing contrast (maximum and minimum luminance),
- static contrast (optical response settled),
- transient contrast (optical response not settled),
- dynamic contrast (technique for improving the sequential contrast of LCD-screens by backlight modulation)
- dark-room contrast (no ambient illumination),
- "ambient contrast" (short for: contrast in the presence of ambient illumination),
- concurrent contrast (dark and light areas presented at the same time, simultaneously),
- successive contrast (contrast between visual stimuli presented in sequence).
----
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 luminanceLuminance
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 or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...
), is often used for high luminances and for specification of the contrast
Contrast ratio
The contrast ratio is a property of a display system, defined as the ratio of the luminance of the brightest color to that of the darkest color that the system is capable of producing...
of electronic visual display
Electronic visual display
An electronic visual display is display technology which incorporates flat panel displays, performs as a video display, output device for presentation of images transmitted electronically, for visual reception, without producing a permanent record....
devices. The luminance contrast (ratio), CR, is a dimensionless number
Dimensionless quantity
In dimensional analysis, a dimensionless quantity or quantity of dimension one is a quantity without an associated physical dimension. It is thus a "pure" number, and as such always has a dimension of 1. Dimensionless quantities are widely used in mathematics, physics, engineering, economics, and...
, 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 sometimes found in the electronic displays field, K, is:
with 0 ≤ K ≤ 1.K = 0 means no contrast, the maximum possible contrast, Kmax equals one.
Color contrast
Two parts of a visual fieldVisual field
The term visual field is sometimes used as a synonym to field of view, though they do not designate the same thing. The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments", while 'field of view' "refers to the physical...
can be of equal luminance, but their color (chromaticity
Chromaticity
Chromaticity is an objective specification of the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness ....
) is different. Such a color contrast can be described by a distance in a suitable chromaticity system (e.g. CIE 1976 UCS, CIELAB
Lab color space
A Lab color space is a color-opponent space with dimension L for lightness and a and b for the color-opponent dimensions, based on nonlinearly compressed CIE XYZ color space coordinates....
, CIELUV).
A metric for color contrast often used in the electronic displays field is the color difference
Color difference
The difference or distance between two colors is a metric of interest in color science. It allows people to quantify a notion that would otherwise be described with adjectives, to the detriment of anyone whose work is color critical...
ΔE*uv or ΔE*ab.
Full-screen contrast
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
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
-displays and in PDP
Plasma display
A plasma display panel is a type of flat panel display common to large TV displays or larger. They are called "plasma" displays because the technology utilizes small cells containing electrically charged ionized gases, or what are in essence chambers more commonly known as fluorescent...
s. A small test pattern
Test Pattern
Test Pattern may refer to:* Test Pattern , a Canadian game show* Test Pattern , an album by Sonia Dada* Test pattern or test card, a television test signal...
(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.
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 stateDark state
The term dark state refers to state of an atom or molecule which can no longer absorb photons and therefore appears dark. All atoms and molecules are described by quantum states -- different states can have different energies and a system can make a transition from one energy level to another by,...
(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.
Static contrast
The standard procedure for contrast evaluation is as follows:1 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,
2 measure the luminance and/or the chromaticity of the first test pattern and record the result,
3 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,
4 measure the luminance and/or the chromaticity of the second test pattern and record the result,
5 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.
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. |
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 . 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
Digital signal processing
Digital signal processing is concerned with the representation of discrete time signals by a sequence of numbers or symbols and the processing of these signals. Digital signal processing and analog signal processing are subfields of signal processing...
required for implementation of the dynamic contrast control technique in a way that is pleasing to the human visual system
Visual system
The visual system is the part of the central nervous system which enables organisms to process visual detail, as well as enabling several non-image forming photoresponse functions. It interprets information from visible light to build a representation of the surrounding world...
(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 . High-dynamic-range (HDR) LCDs are using that technique in order to realize (static) contrast values in the range of CR > 100.000 .
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).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
Low-key lighting
Low-key lighting is a style of lighting for photography, film or television. It is a necessary element in creating a chiaroscuro effect. Traditional photographic lighting, three-point lighting uses a key light, a fill light, and a back light for illumination...
reflected from the display surface into the light measuring device, the luminanc 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 .
"Ambient contrast"
The contrast that can be experienced or measured in the presence of ambient illumination is shortly called "ambient contrast" . 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" .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.
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-scree 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.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.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
Projection screen
A projection screen is an installation consisting of a surface and a support structure used for displaying a projected image for the view of an audience. Projection screens may be permanently installed, as in a movie theater; painted on the wall; or semi-permanent or mobile, as in a conference room...
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 . The standard ANSI IT7.215-1992 defines test-patterns and measurement locations, and a way to obtain the luminous flux
Luminous flux
In photometry, luminous flux or luminous power is the measure of the perceived power of light. It differs from radiant flux, the measure of the total power of light emitted, in that luminous flux is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of...
from illuminance measurements, it does not define however a quantity named "ANSI lumen".
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
Viewing cone
-Viewing direction:When a visual display with non-vanishing size is seen by an observer, every point of the display area is seen from a different direction as illustrated in fig. 1. No two spots on the display are seen from the same direction...
.
Luminance
Luminance
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 or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square...
, contrast and chromaticity
Chromaticity
Chromaticity is an objective specification of the quality of a color regardless of its luminance, that is, as determined by its hue and colorfulness ....
of LCD-screens
Liquid crystal display
A liquid crystal display is a flat panel display, electronic visual display, or video display that uses the light modulating properties of liquid crystals . LCs do not emit light directly....
is usually varying with the direction of observation
Viewing cone
-Viewing direction:When a visual display with non-vanishing size is seen by an observer, every point of the display area is seen from a different direction as illustrated in fig. 1. No two spots on the display are seen from the same direction...
(i.e. viewing direction
Viewing cone
-Viewing direction:When a visual display with non-vanishing size is seen by an observer, every point of the display area is seen from a different direction as illustrated in fig. 1. No two spots on the display are seen from the same direction...
). The variation of electro-optical characteristics with viewing direction
Viewing cone
-Viewing direction:When a visual display with non-vanishing size is seen by an observer, every point of the display area is seen from a different direction as illustrated in fig. 1. No two spots on the display are seen from the same direction...
can be measured sequentially by mechanical scanning of the viewing cone
Viewing cone
-Viewing direction:When a visual display with non-vanishing size is seen by an observer, every point of the display area is seen from a different direction as illustrated in fig. 1. No two spots on the display are seen from the same direction...
(gonioscopic approach) or by simultaneous measurements based on conoscopy
Conoscopy
Conoscopy is an optical technique to make observations of a transparent specimen in a cone of converging rays of light...
.
External links
- Charles Poynton: Reducing eyestrain from video and computer monitors
- Contrast of Sonys XEL-1 OLED-TV-screen with ambient illumination http://techon.nikkeibp.co.jp/english/NEWS_EN/20071226/144779/