What You Need to Know About Gamma

Ideally, images shot by a video camera would be reproduced on a display device exactly as it was recorded. Unfortunately, this is very rarely the case, as the technologies of cameras and display devices sometimes have their own nuances that slightly, and sometimes significantly, change the performance of imagery. In such cases, you need tools to correct for these abnormalities.

What is Gamma?

Gamma is a function to correct nonlinear performance inherent in CRT display devices. The CRT does not perform in a linear fashion against voltage; therefore, it might display an image not exactly as it was intended.

Voltage is the medium in which the CRT is fed the video signal. Another way to look at it is the relationship of video signal input to the CRT versus light out is not linear.

To compensate for this, a gamma "curve" is applied to the signal at the video camera during recording. This curve is defined based on the known inherent performance of a CRT. When the signal is applied to the CRT, it is already corrected for the nonlinear performance; hence, it correctly outputs the images recorded by the camera.

Since DLP devices have a natural linear function, the gamma correction needs to be removed. So to be correct in our terms, we apply a degamma function to the signal within DLP products.

What gamma do I need in my application?

Since all display devices are well documented concerning their performance characteristics, we know what gamma corrections are normally applied to a signal that was intended for use with such a device. Until recently, CRT has been the dominant display technology of display devices.

Signal types such as NTSC and PAL have gamma curves based on CRT displays. Film, on the other hand, has a gamma that is based on celluloid reproduction and film-to-video transfer. NTSC, the video standard adhered to in the U.S., has a gamma of 2.2.

Gammas also are determined based on the conditions of the environment that the display device may be used, such as a dark room (cinema, home theater), a well-lit room (conference room, sanctuary) or outdoor environment. To account for a host of conditions, gammas are created for low-light, or bright-light situations. A film-based video source in a controlled theater environment with very low lighting might use a gamma known as Film Low.

Other gammas have occurred based on nothing other than an artistic approach to the reproduction of an image. Sometimes, people just like to see images in a certain way.

Summary

Although gamma correction was born to fill the need of correcting nonlinear performance of video recording and display devices, it has also become a subjective tool for those who want a certain look, or just like the effect of one gamma curve over another. It is your choice to be objective and use the gamma, or degamma function that is intended for a certain signal type and display device, or you can be artistic and pick something you find pleasing. Different gamma curves may appeal to you for different environmental lighting conditions as well. Understand the foundation of gamma, but use as you see fit.

Tech Tip Archive

Demo Program: Commercial AV - Full Line and Precision Line Dealers:

• Dealers and Special Contractors may purchase up to two demo units per calendar year
  
• Demo units may be purchased at 5% off of DPI's published end column Commercial AV dealer pricing
  (End Column x .95)
  
• Excluding iVision, which already comes standard with a lens, the 5% demo discount may also be applied
  to (1) projection lens for each demo projector purchased
  
• Dealers in good credit standing receive standard payment terms - 2% cash with order, 1% 10 days, net
  30 days

Home Cinema Dealers:

• Home Cinema Dealers may purchase (1) demonstration iVision or dVision projection system per calendar
  year at 50% off DPI's published list price. So long as that unit is maintained in the Dealer's demonstration inventory through the end of the calendar year it was purchased, and the dealer retains in their employ at least one technical associate who has received Digital Projection training certification, the Dealer qualifies
  for Elite Home Cinema end column pricing on all additional iVision and dVision projectors purchased through
  the end of the same calendar year.
• Home Cinema Dealers may purchase (1) demonstration Mercury or HIGHlite projection system per calendar year at 50% off DPI's published list price. So long as that unit is maintained in the Dealer's demonstration inventory through the end of the calendar year it was purchased, and the dealer retains in their employ at least one technical associate who has received Digital Projection training certification, the Dealer qualifies
  for Elite Home Cinema end column pricing on all additional Mercury and HIGHlite projectors purchased
  through the end of the same calendar year.
• Home Cinema Dealers may purchase (1) demonstration Reference Display projection system per calendar year at 40% off DPI's published list price. So long as that unit is maintained in the Dealer's demonstration inventory through the end of the calendar year it was purchased, and the dealer retains in their employ at least one technical associate who has received Digital Projection training certification, the Dealer qualifies
  for Elite Home Cinema end column pricing on all additional Reference Display projectors purchased through
  the end of the same calendar year.

Manufacturer's Representatives:

Rich Jackson
ANSR
8430 Enterprise Way, Oakland, CA 94621
510-638-3800

Joe Blacker
Audio Associates
9160 Red Branch Road, Ste.E-5, Columbia, MD 21045
410-964-1212

Eric Leicht
AV Partners
3272 Windflower Circle, Thousand Oaks, CA 91360
805-492-7055

Sharon Culotta
Culotta Communication
247 D Street, Suite 108, San Rafael, CA 94901
408-836-5671

Larry O'Neill
Dobbs Stanford Corporation
2715 Electronic Lane, Dallas, TX 75220
214-350-9009

Kenneth Colson
E.D.A. Sales, Inc.
5815 Live Oak Parkway, Suite 2G, Norcross, GA 30093
770-300-9484

Steve Johnson
HP Marketing Company
7340 South Alton Way, # G, Englewood, CO 80112
303-804-9566

Dan Muckle
Muckle Sales, Inc.
7150 SW Hampton Street, Suite 115, Tigard, OR 97223
503-620-0444

Marla Suttenberg
Sapphire Marketing LLC
1 Kalisa Way, Suite 108, Paramus, NJ 07652
201-576-9490

Frank Weese
Visitec
2020 Dean St - L2A, St. Charles, IL 60174
630-762-0300

November 2004
December 2004
January 2005