Secrets Q & A

Understanding Contrast Ratios in Video Display Devices

ARTICLE INDEX

Brightness and Contrast Settings

Long ago, somebody decided to call the setting that mostly controls black level the Brightness setting, and to use Contrast for the name of the setting that mostly controls the white level. We've been stuck with those for the most part since then. With respect to proper calibration, it is important to know that the Brightness setting with a digital projector has maybe one or two settings that would be considered correct for a particular source. Basically, this is around the point where the projector will try to put out the minimum amount of light it can for video black and below, and put out more light than this for things encoded above video black. Raising the Brightness above there can make some images look better, but at the expense of raising the absolute black level, lowering the On/Off CR, and generally making some images look worse. Lowering the Brightness below the ideal range can crush detail that is encoded just above video black. So, this is one control to be careful modifying too much. I suggest using test patterns to set it correctly or hire a professional.

I believe that the Contrast setting is less important to get just right, but still important with digital projectors. Too high and white detail will be crushed. Lowering it below the point at which the digital projector is putting out close to the maximum amount of light it can for white (could be Peak White), while still maintaining color balance, means dimmer images, but also lower On/Off CR than is possible. For this reason, if images are way brighter than a person desires overall even after calibration, then using a neutral density filter, darker screen, or closing an adjustable iris are generally better ways to dim the images than just lowering the Contrast setting with digital projectors with their limited On/Off CR range (and non-zero absolute black levels). Each of those alternatives decreases both white and black levels and not just white levels.

Conclusions

We can experience huge differences in light intensity across various images and large separations of intensity within individual images in real life. Reproducing realistic blacks on a projection screen, while having reasonable whites across various images, requires a low absolute black floor as well as low washout effect from bright parts of images to darker parts of images. With standard specifications, these characteristics of a display are best indicated by the On/Off contrast ratio and the ANSI contrast ratio. Both play roles in the performance of dark material across various images and are therefore both important display characteristics.

In the future, I will talk about dynamic irises and give my take on the question, "Are dynamic irises cheating?" My short answer is that they aren't cheating, much like human vision using dynamic irises in our eyes to improve our vision range and fidelity isn't cheating, but current dynamic iris implementations are weak compared to what our eyes can perceive.

References:

http://www.onlineconversion.com/luminance.htm
http://www.electro-optical.com/unitconv/convertcalcs/energy/lumina.htm
http://msis.jsc.nasa.gov/sections/section04.htm
http://www.au.af.mil/au/awc/awcgate/sab/sabaef3.pdf - Page 239
http://www.usd.edu/coglab/CSFIntro.htm
http://www.poynton.com/notes/colour_and_gamma/GammaFAQ.html