|
What is Video Display Gamma?
You might think that when a video display device (like a TV or projector) was given an input signal of 50% maximum value, the display would produce 50% of the light intensity produced by a maximum value signal. This would be a nice linear relationship. Input a signal level of 25% of the maximum value; get 25% of the maximum light intensity output, etc.
|
|
|||||||||
|
Tom Schulte, CET SENCORE, Inc. Application Engineer 1.800.736.2673 or 1.605.339.0100 |
||||||||||
|
If you are just learning about video calibration, you may be unsure of what the calibration procedures are and how to perform them. The goal of this article is to bring you up to speed on what video display calibration is all about.
|
||||||||||
|
CRT Display Operation |
||||||||||
|
|
There is, however, usually a non-linear relationship between a video display’s input signal and the light intensity output. This is because CRTs inherently have a power law response to input control voltage; light intensity produced by the CRT is approximately equal to the applied voltage percentage, raised to the 2.5 power. This is due to the electrostatic interaction characteristics of the electron gun’s cathode and grid. An input signal of 50% maximum value produces |
approximately 18% of the maximum light intensity (0.5 2.5 = 0.18). An input signal of 25% maximum value produces only about 3% of the maximum light intensity (0.25 2.5 = 0.03). The numerical value of the exponent of this power function is commonly known as gamma (indicated by the Greek letter γ). This CRT 2.5 gamma function is graphed in Figures 1 and 2. The gamma of CRTs in different displays varies a little, but they are all generally assumed to be 2.5.
|
||||||||
|
|
|
|||||||||
|
Figure 1: This graph shows the nonlinear relationship between input signal voltage and light output for a CRT display. Note that a 50% input signal level (red) produces only about 18% of maximum light output level (blue). This corresponds to a gamma of 2.5.
|
Figure 2: This graph shows different gamma curves between the 2.5 CRT gamma and linear, with a gamma of 1.
|
|||||||||
|
|
||||||||||
|
This nonlinear CRT light output must be compensated to achieve correct reproduction of video image intensity. To compensate for the CRT gamma, video cameras add the inverse of the CRT gamma function (1/2.5 or 0.45) to their video output signal, to make everything come out linear in the end (Figure 3).
|
|
|||||||||
|
|
Figure 3: A gamma-compensated video signal (blue) is sent to a nonlinear CRT display with a 2.5 gamma (red). The result is a linear reproduction (black) of each of the light levels in the original scene. |
|||||||||
|
|
||||||||||
|
If a video display system doesn’t have an overall linear response from the original scene to the picture on the screen, the picture’s midrange light intensity levels will be incorrect. If the CRT gamma is under-compensated, the picture’s mid-tones will be too dark and the picture will lack detail. If the CRT gamma is over- compensated, the picture’s mid-tones will be too light and the picture will be washed out (Figure 4).
|
|
|||||||||
|
|
Figure 4: The middle picture is the result of proper signal compensation for the CRT gamma. The scene-to-screen path has an overall gamma of 1 (linear). The top picture shows the result of under-compensation, and the bottom picture shows the result of over-compensation. |
|||||||||
|
|
||||||||||
|
There is a perceptual viewing effect that also comes into play in determining the amount of gamma compensation to apply to video signals. If video images are viewed in a dim environment, with a dark surround, a subjective effect called Simultaneous Contrast causes the reproduced image to appear lacking in contrast, as illustrated in Figure 5. |
||||||||||
|
|
||||||||||
|
|
Figure 5: Simultaneous Contrast (Surround) Effect. The three gray squares surrounded by black are identical to the three gray squares surrounded by white, but the black-surround series appears to have lower contrast than the white-surround series. |
|||||||||
|
|
||||||||||
|
This dim surround effect can be overcome by slightly under-compensating the CRT’s assumed 2.5 power gamma. To provide compensation for dim viewing environment surround effect, all video cameras and video production use a gamma compensation exponent of about 1/2.2 instead of 1/2.5. So, CRT displays have a non-linear light output gamma characteristic of 2.5, and we send the displays video signals that are compensated by a gamma factor of 1/2.2. The difference between the two corrects for the dim surround effect. |
||||||||||
|
|
||||||||||
|
Fixed-Pixel Display Operation |
||||||||||
|
|
Non-CRT (fixed-pixel) displays, such as plasma or LCD flat panels, or DLP or LCD projectors, are inherently linear display devices, unlike CRTs, which are inherently nonlinear. Since these newer fixed-pixel displays must properly display the same program material that has been produced for CRTs, however, they must be designed and set up to produce the same 2.5 gamma characteristics as a CRT display. If a fixed-pixel display doesn’t emulate the 2.5 power gamma of a CRT, video program material that has been properly gamma compensated for display on a CRT won’t look right on the fixed-pixel display.
When a manufacturer designs a fixed-pixel display, they build in circuitry that changes the display’s natural linear light output characteristic into a nonlinear characteristic, which is hopefully similar to the 2.5 gamma of a CRT. Oftentimes, though, rather than having just one gamma setup, the manufacturer provides a number of selectable gamma settings (probably for marketing purposes). These selectable gamma settings may be in the user menu, or they may be in the service menu. The question then becomes: “what is the proper setting of the gamma selection switch?” The answer is: whichever gamma setting comes closest to providing a 2.5 gamma characteristic, to mimic the gamma of CRT displays.
|
|||||||||
|
Video Display Gamma Check |
||||||||||
|
|
To check a display’s gamma characteristic, we can simply check for the difference in light output between a 100 IRE test pattern and a 50 IRE test pattern, or we can do a more thorough check at every 10 IRE, from 10 to 100 IRE. Before we do this, though, we want to adjust the display’s black level (brightness control) and peak white level (contrast or picture control) as accurately as possible. Misadjustment of either the black level or white level will introduce offset errors into our measurements. The Pluge pattern from the Sencore VP300, VP400 or 802B generators works very well for precisely adjusting the display’s black level. The VP400 HiLoTrk pattern makes it easy to precisely adjust peak white level on fixed-pixel displays for no clipping. On the VP300 or 802B generators, use the Staircase pattern.
|
|||||||||
|
|
Once the black level and peak white levels are properly adjusted, you can quickly determine which gamma setting on the video display gives a 2.5 gamma: 1. Display a white window pattern at 100 IRE and measure the luminance level with a ColorPro Color Analyzer. 2. Multiply the 100 IRE luminance level from step 1 by 0.18 to find the expected luminance level for 50 IRE, for 2.5 gamma. 3. Display a white window pattern at 50 IRE and measure the luminance level. 4. Repeat step 3 for each of the gamma settings.
The gamma control setting that comes closest to producing 18% of the 100 IRE luminance at 50 IRE is the desired setting for matching CRT gamma at 2.5.
|
|||||||||
|
To perform a full gamma plot, use the CP5000 report feature. On the Report page (Figure 6), perform a Post-Calibration capture for each 10 IRE level from 10-100 IRE.
|
||||||||||
|
|
|
|||||||||
|
|
Figure 6: Use the CP5000 Post-Calibration capture to capture white window luminance levels from 10 IRE to 100 IRE. |
|||||||||
|
|
||||||||||
|
After you have captured the ten luminance level samples, preview the Gamma Graph (Figure 7). The graph should be very smoothly increasing from 10 IRE to 100 IRE. Ideally, the gamma value should be the same at every IRE level, but that will never be the case with an actual display, especially if the black level and white level aren’t set accurately. A CRT display will at least normally have a fairly smooth curve. A fixed-pixel display may have peaks or dips in the gamma curve. If you have a choice, pick the display’s gamma setting that is the best compromise between being close to 2.5 gamma (or just under) and giving a smooth light response curve.
|
||||||||||
|
|
|
|||||||||
|
|
Figure 7: In the CP5000 Report section, after you have used the Post-Calibration capture to capture luminance values, click the Graph Preview button to view the Gamma Graph. The display’s gamma is usually considered to be either the 50 IRE gamma value, or the average of gamma values across the entire luminance range.
|
|||||||||
|
|
||||||||||
|
Conclusion |
||||||||||
|
|
For more information on how your business can benefit with the Sencore multimedia video generators or color analyzers, call your Sencore sales representative 1.800.736.2673 or outside of the U.S. 1.605.339.0100.
Learn more - VP400 VideoPro family: http://www.sencore.com/vp400/index.htm
Learn more - CP5000 ColorPro: http://www.sencore.com/products/cp5000.htm
|
|||||||||
|
|
||||||||||
|
1.800.736.2673 or 1.605.339.0100 |
||||||||||
