Display contrast

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 luminances of the stimuli considered, for example: ΔL/L near the luminance threshold (known as Weber contrast[1]), or LH/LL for much higher luminances.[2]

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 displays.

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).

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 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).

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 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.

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-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.

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.

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.

Repetitive-Impulse-Response
repetitive impulse response of an LCD-monitor

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.[3] 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.[4] High-dynamic-range (HDR) LCDs are using that technique in order to realize (static) contrast values in the range of CR > 100.000.[5]

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 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.[6]

"Ambient contrast"

The contrast that can be experienced or measured in the presence of ambient illumination is shortly called "ambient contrast".[7] 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".[8]

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-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.

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 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.[9] 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.[10]

See also

References

  1. ^ Charles Poynton on Weber contrast
  2. ^ IEC(50)845-02-47
  3. ^ T. Shiga and S. Mikoshiba: "Reduction of LCTV Backlight Power and Enhancement of Gray Scale Capability by Using an Adaptive Dimming Technique", SID03 Digest, pp. 1364-1367
  4. ^ H. Chen, et al.: "Locally pixel-compensated backlight dimming on LED-backlit LCD TV", JSID 15/12(2007), pp. 981-988
  5. ^ H. Seetzen, et al.: "A High Dynamic Range Display System Using Low and High Resolution Modulators", SID03 Digest
  6. ^ STOP Specsmanship
  7. ^ E. F. Kelley: "Diffuse Reflectance and Ambient Contrast Measurements Using a Sampling Sphere", SID ADEAC06 Digest, pp. 1-5
  8. ^ E. F. Kelley, et al.: "Display Daylight Ambient Contrast Measurement Methods and Daylight Readability", JSID 14, 11, pp. 1019-1030
  9. ^ ANSI IT7.215-1992: Data Projection Equipment and Large Screen Data Displays -- Test Methods and Performance Characteristics
  10. ^ M. E. Becker: "Viewing-cone Analysis of LCDs: a Comparison of Measuring Methods", Proc. SID'96, pp. 199

External links

Active shutter 3D system

An active shutter 3D system (a.k.a. alternate frame sequencing, alternate image, AI, alternating field, field sequential or eclipse method) is a technique of displaying stereoscopic 3D images. It works by only presenting the image intended for the left eye while blocking the right eye's view, then presenting the right-eye image while blocking the left eye, and repeating this so rapidly that the interruptions do not interfere with the perceived fusion of the two images into a single 3D image.

Modern active shutter 3D systems generally use liquid crystal shutter glasses (also called "LC shutter glasses" or "active shutter glasses"). Each eye's glass contains a liquid crystal layer which has the property of becoming opaque when voltage is applied, being otherwise transparent. The glasses are controlled by a timing signal that allows the glasses to alternately block one eye, and then the other, in synchronization with the refresh rate of the screen. The timing synchronization to the video equipment may be achieved via a wired signal, or wirelessly by either an infrared or radio frequency (e.g. Bluetooth, DLP link) transmitter. Historic systems also used spinning discs, for example the Teleview system.

Active shutter 3D systems are used to present 3D films in some theaters, and they can be used to present 3D images on CRT, plasma, LCD, projectors and other types of video displays.

Amazon Kindle

The Amazon Kindle is a series of e-readers designed and marketed by Amazon. Amazon Kindle devices enable users to browse, buy, download, and read e-books, newspapers, magazines and other digital media via wireless networking to the Kindle Store. The hardware platform, developed by Amazon subsidiary Lab126, began as a single device in 2007 and now comprises a range of devices, including e-readers with E Ink electronic paper displays and Kindle applications on all major computing platforms. All Kindle devices integrate with Kindle Store content, and as of March 2018, the store has over six million e-books available in the United States.

Contrast ratio

The contrast ratio is a property of a display system, defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (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. Manufacturers have traditionally favored measurement methods that isolate the device from the system, whereas other designers have more often taken the effect of the room into account. An ideal room would absorb all the light reflecting from a 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 color 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 color 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.

Cyber-shot

Cyber-shot is Sony's line of point-and-shoot digital cameras introduced in 1996. Cyber-shot model names use a DSC prefix, which is an initialism for "Digital Still Camera". Many Cyber-shot models feature Carl Zeiss trademarked lenses, while others use Sony, or Sony G lenses.

All Cyber-shot cameras accept Sony's proprietary Memory Stick or Memory Stick PRO Duo flash memory. Select models have also supported CompactFlash. Current Cyber-shot cameras support Memory Stick PRO Duo, SD, SDHC, and SDXC. From 2006 to 2009, Sony Ericsson used the Cyber-shot brand in a line of mobile phones.

High-dynamic-range imaging

High-dynamic-range imaging (HDRI) is a high dynamic range (HDR) technique used in imaging and photography to reproduce a greater dynamic range of luminosity than is possible with standard digital imaging or photographic techniques. The aim is to present a similar range of luminance to that experienced through the human visual system. The human eye, through adaptation of the iris and other methods, adjusts constantly to adapt to a broad range of luminance present in the environment. The brain continuously interprets this information so that a viewer can see in a wide range of light conditions.

HDR images can represent a greater range of luminance levels than can be achieved using more traditional methods, such as many real-world scenes containing very bright, direct sunlight to extreme shade, or very faint nebulae. This is often achieved by capturing and then combining several different, narrower range, exposures of the same subject matter. Non-HDR cameras take photographs with a limited exposure range, referred to as LDR, resulting in the loss of detail in highlights or shadows.

The two primary types of HDR images are computer renderings and images resulting from merging multiple low-dynamic-range (LDR) or standard-dynamic-range (SDR) photographs. HDR images can also be acquired using special image sensors, such as an oversampled binary image sensor.

Due to the limitations of printing and display contrast, the extended luminosity range of an HDR image has to be compressed to be made visible. The method of rendering an HDR image to a standard monitor or printing device is called tone mapping. This method reduces the overall contrast of an HDR image to facilitate display on devices or printouts with lower dynamic range, and can be applied to produce images with preserved local contrast (or exaggerated for artistic effect).

Nokia E65

The Nokia E65 is a smartphone in the Eseries range, a S60 platform third edition device with slide action. It shared many of the features of the N95 (quad band GSM, 3G, wifi, bluetooth) released around the same time, but thinner, lighter and without the GPS.

It was followed fairly quickly by the E66, which was very similar but gained an FM radio, a newer release of S60, A2DP bluetooth, GPS and 3.2 mpixel camera.

ThinkPad X series

The ThinkPad X series is a line of notebook computers and convertible tablets originally produced by IBM and now marketed by Lenovo.

IBM announced the ThinkPad X series (initially the X20) in September 2000 with the intention of providing “workers on the move with a better experience in extra-thin and extra-light mobile computing.” The ThinkPad X series replaced both the 240 and 570 series during IBM's transition from numbered series to letter series during the early 2000s. The first X Series laptops were "slimmer than a deck of cards" and "lighter than a half-gallon of milk", despite the presence of a 12.1-inch Thin-film transistor (TFT LCD) display. These design values – thin and light – continued to be a part of the ThinkPad X-series laptops even after the purchase of IBM’s Personal Computing Division by Lenovo. The first X Series ThinkPad released by Lenovo was the X41 in 2005.The ThinkPad X-series laptops from Lenovo were described by Trusted Reviews as combining an ultraportable's weight and form factor with a durable design. The X-series laptops include traditional ultraportables, as well as convertible tablet designs. According to Lenovo, the ThinkPad X-series laptops include low power processors, offer long battery life, and offer several durability features such as a Roll Cage, magnesium alloy covers, and a spill-resistant keyboard, but currently lacks a replacealble battery and ram slots.

Timex Datalink

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