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. Generally a picture with a higher contrast ratio is perceived as being sharper. However, it should not be the prime determining selection criteria – particularly as the viewing environment can severely affect perceived contrast ratio. Reducing ambient light and the use of darker walls and finishings can both improve contrast ratio.

There are two types of contrast ratio commonly quoted in specifications – Dynamic and Static contrast ratio.

Measuring Dynamic Contrast Ratio

A method of measuring Dynamic Contrast Ratio is 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.

Measuring Static Contrast Ratio

Contrast Ratio can be measured using the 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.

Static vs Dynamic Contrast Ratio

Full On/Full Off (Dynamic) tests yield higher results than an ANSI (Static) tests. Manufacturers generally quote the Dynamic Contrast Ratio most probably because of the higher figures for the test results. 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.

Other Contrast Ratio Tests

A problem with contrast ratio determination is the number of organizations recommending numerous and varying methods and standard practices. Organizations include IEC/ANSI, VESA and FPDM.

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.

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.

Further Reading

For an in depth analysis of contrast ratio of for home theater read The Contrast Ratio Game on the Practical Home Theater Guide website.

Siemens Display Technologies believe the human eyes maximum contrast ratio detection is only 300:1. Their paper Ensuring Image Quality for Diagnosis and Surgery also discusses the detrimental effect of ambient light on contrast ratio perception.

Vision and Contrast from Packer Engineering.

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