Contrast Ratio Secrets Uncovered

The contrast ratio is a display panel measure defined as the ratio of the luminance of the brightest color (white) to that of the darkest color (black). Contrast Ratio is an important factor in producing a quality image and definitely a point of consideration in the selection of any kind of display device. However, it should not be the prime determining selection criteria - particularly as the viewing environment can severely affect perceived contrast ratio.

Static vs Dynamic Contrast Ratio
Dynamic contrast ratio is another measure which takes into account time. While static contrast ratio is luminosity of the brightest and the darkest color the system is capable of producing simultaneously at any instant of time dynamic contrast ratio is the ratio of the luminosity of the brightest and the darkest color over time.

Manufacturers like to quote dynamic contrast ratios in their specifications as the ratios are much higher than static contrast ratios. Static contrast ratio is a much more meaningful measure for the viewer as it matches how they perceive contrast ratio - on an image by image or scene by scene basis.

Measuring (Static) Contrast Ratio?

Contrast Ratio is measured using a test procedure with the two most common being:

American National Standards Institute (ANSI) test where a checkerboard pattern of 16 black and white rectangles are displayed. The ANSI contrast ratio is the result of dividing the average light output of the white rectangles by the average light output of the black squares.
The Full On/Full Off contrast test which measures the ratio of the light output for a white to the light output of an all black (full off) image for the display being tested.

Full On/Full Off tests yield higher results than an ANSI tests. Manufacturers generally quote the Full On/Full off (higher) test results for their products but, mostly do not specify which test was used. It is commonly argued that the ANSI test is more indicative of contrast ratio due to its checkerboard pattern being closer to actual viewing conditions.

Factors Affecting Perceived Contrast Ratio

The Human Eye

The human eye, at any particular instant, can perceive contrast ratio over a range of 400:1 to 10,000:1 according to numerous references. However, the human eye is a dynamic organ and can adjust, both chemically and via iris movement, over some 30 minutes in steady light conditions to perceive higher contrast ratios of up to between 1,000,000:1and 10,000,000:1. One can think of the eye as an adaptive detector. Age is a factor influencing what contrast ratio might be perceived by the particular individual.

The images displayed can affect the perceived contrast ratio with the human eye being able to detect higher contrast ratios for static images as opposed to moving images.

Ambient Light Conditions

Even small amounts of ambient light (one LUX – a candle flame) significantly reduces the ability of display devices to render higher contrast ratios (in the 1,000’s). A black area can only be as black as whatever incident light is reflecting from it. You cannot “project” the color black. Doubly detrimental is the rapid drop off of the eyes contrast sensitivity with higher ambient light levels.

Spatial Frequency

The eye is sensitive to the spatial frequency which is a factor related to the distance between the contrasting image elements in relation to each degree of field of vision at the eye. Consider that the frequency is the count of the number of alternating vertical stripes of black and white across an arc of one degree with the eye as its origin. Increasing the number of vertical stripes across the arc (making them thinner) will, beyond a certain point, reduce the eyes ability to perceive contrast ratio – even though the actual contrast between the white and black stripes remains the same. For a practical demonstration of this concept try this dynamic demonstration of the eyes variation in contrast sensitivity with spatial resolution by the USD Internet Psychology Laboratory.

Conclusions

Differences in contrast ratios in the multi-thousands should only be a consideration in choosing between display devices when you are going to place them in a strictly controlled, very low ambient light environment – something like a windowless home theatre room with dark finishes on all walls and surfaces.

For higher ambient light viewing environments brightness should be a much higher rated selection criteria than contrast ratio.

There is no substitute for judging between the image quality of displays than viewing them side by side with your own eyes in a viewing environment with an ambient light level similar to where they would be located.

[...] There seems to be a lot of competition lately from manufacturers to have the television with the highest contrast ratio. Although important, there are factors such as the amount of ambient light which quickly reduce the difference between televisions with contrast ratios in the order of thousands. For more background see Presentation Technology Reviews technology guide on contrast ratio. [...]

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Thank You

Comment by Alex — April 23, 2007 @ 10:14 am

[...] Consumers should be careful not to base television choice on just the highest dynamic contrast ration which seems to be a competition point between manufacturers. It’s much like the mega-pixel count with cameras at the moment. Read PTek’s technology guide on contrast ratio for more information. [...]

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[...] high contrast ratio is a very important factor for a sharp, clear image. Mitsubishi have applied a couple of [...]

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