Contrast Ratios
Dear Gary: I have a question relating to how the various companies determine the contrast ratio for any particular front projector. I have a Runco CRT front projector. The contrast ratio number is relatively low, yet my black level and contrast is superb. DLP, LCD, and LCoS manufacturers are all over the place when it comes to their contrast ratio claims. I see, 800:1, 1000:1, 2000:1, and now there are claims of 3000:1. What I don’t see are the blacks achieved by my CRT. How do they determine their contrast ratios? Are they taking the measurement from the screen or internally? (And what’s the difference?) I would really love a frame of reference. What is the magic number for a DLP, LCD, and LCoS projector to equal a good CRT projector?
Bob Brown, AmateurHomeTheater.com
Video Technical Editor Greg Rogers Comments:
Your eyes aren’t deceiving you––your CRT projector should produce the best black level. But the contrast ratio values also make sense when you realize that the “relatively low” contrast ratio of the CRT projector is a measure of intra-field image contrast, and the contrast ratio values for the fixed-pixel projectors refer to on/off contrast ratios. I’ll explain these terms and why both are important attributes of display performance. The on/off contrast ratio is an indirect way of specifying the black level of a projector (or other display). A projector’s black level can’t be specified directly because it depends on screen size, screen gain, lamp power, iris setting, and so forth. Instead, it’s more convenient to specify and measure the on/off contrast ratio, which is equal to the luminance (brightness) of an all-white (100-IRE) field divided by the luminance of an all-black field. CRT projectors can be adjusted to produce virtually no light when they display an all-black field, so the on/off contrast ratio of a carefully calibrated CRT projector is nearly infinite. But fixed-pixel projectors use a projection lamp that produces a constant brightness that can’t be totally blocked or deflected when displaying a black field. A small amount of residual (leakage) light always reaches the screen. As you noted, there is a wide variation in the on/off contrast ratios of fixed-pixel projectors, and recent HD2+ DLP™ projectors have on/off contrast ratios greater than 3,400:1. The on/off contrast ratio is critically important to picture quality because it determines the minimum (absolute) black level in the darkest scenes. A tiny increase in black level brightness severely reduces shadow detail discrimination and casts a haze over predominantly dark scenes. I also find it objectionable when the “black” in an image is noticeably brighter than the black surrounding the screen. The Society of Motion Picture and Television Engineers (SMPTE) recommends a peak brightness of 12 foot-Lamberts (fL) for digital cinema, and I think that is also about optimum for home theatre. Although no current fixed-pixel projector technology can produce black levels as dark as a properly adjusted CRT projector, HD2+ DLP projectors can produce 12 fL of brightness with black levels at 0.0035 fL. I think many critical viewers will be satisfied with a black level of 0.005 fL, and most will be satisfied with 0.0035 fL. Nevertheless, manufacturers should and will continue to improve on/off contrast ratios, because CRT projectors demonstrate that even lower black levels will continue to improve picture quality in dark scenes. The contrast ratio specified for a CRT projector is always an intra-field contrast ratio. It’s intended to describe a projector’s ability to display contrast differences when there are bright objects in a scene. Intra-field contrast ratios are much less than on/off contrast ratios because light from the bright objects is scattered across the image, which obscures darker objects. The light scattering occurs within the lens and the optical system of the projector, but it may also occur within your theatre as light reflects around the room and back onto the screen. There is no single measurement that can precisely describe intra-field contrast for normal images because it depends on the size, brightness, and location of the objects within an image. Hence, we can only define a figure-of-merit based on an artificial test pattern. One such figure-of-merit is the ANSI contrast ratio, which is computed from the average luminance of the 8 white and 8 black blocks of a 4 x 4 checkerboard image. This is a laborious process and fraught with practical measurement uncertainties. I use a modified “ANSI” measurement that is the average contrast ratio of the four central blocks in the 4 x 4 checkerboard. The contrast ratio for each block is measured separately by fixing the position and angle of a light meter a few feet in front of the projector, and then electronically reversing the black and white blocks of the checkerboard pattern. This process increases the magnitude of the “black” luminance values to minimize the influence of room reflections and to improve measurement precision. It also eliminates other issues related to making measurements off the screen. CRT front projectors have intra-field contrast ratios that range from roughly 50 to 150, while the HD2+ DLP projectors that I have measured range from 390:1 to 730:1. All on/off and intra-field contrast ratio measurements should be made with the projector calibrated exactly as it will be used for critical viewing, which is the way we make these measurements in reviews at Widescreen Review.
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