This does not result in 50% light output because display gammas should be higher than 1 for our current video sources, as explained above. In this case, the gray bar in the center, marked by the vertical dots, is 50%stim or 50% stimulus. The position of the vertical dots at each darker end represents 0%stim (stimulus), and the position of the vertical dots at each brighter end represents 100%stim. Below is shown an actual graph of gamma 2.5.Ĭonsider a test image like the following: Note that the gamma lines in the graph above are hand drawn approximations and just for illustration purposes. With a source encoded with a gamma of 0.5, this would result in a final total gamma for source and display of 1.1, while leaving this CRT at 2.5 would result in a final total gamma of 1.25. Or put another way, at the midpoint of Input Voltage, lower gammas have higher Brightness (light intensity in this case), and higher gammas have lower Brightness (higher gammas have the midpoint pulled down to a dimmer level).įor an example CRT display that inherently has a gamma of 2.5 (blue line in the graph below), those desiring a lower gamma could apply a gamma correction (green line) to move the display’s final gamma close to 2.2 (black line). Gammas of less than 1 are above the red line (except at the endpoints), while gammas greater than 1 are below the red line (except at the endpoints). So, for example, in the diagram below, a theoretically perfect video system would have a total gamma (source and display) of 1 (red line). Specific advantages and disadvantages of pushing gamma toward one extreme or the other, or modifying certain sections of a gamma curve, can vary by scene makeup. The exact gammas used can have an impact on both contrast perception and shadow detail. This is done with electronic gamma correction using the digital video processor in the display electronics to achieve gammas that are generally in the 2.2 to 2.5 range (and are often within the control of the user). Non-CRT displays (e.g., DLP, LCoS, and LCD projectors) need to emulate gammas like those of CRTs in order to correct for the gammas in the sources they are fed. Encoding sources with gammas less than 1 is actually useful for spreading the information well for our eyes with a limited number of source bits in digital encodings. Current sources are commonly encoded around the 0.5 range with final gammas in the 1.0 to 1.25 range after taking gamma in displays into account. However, studies in the past have shown an advantage to gamma totals of slightly higher than 1 in dimly lit surround environments often used for watching television. It should be noted that gammas multiply and this would imply that reaching 1 would require a source gamma in the range of 0.4 to 0.45. Therefore, gamma correction needed to be applied to the sources to get to an approximate final gamma of 1. Our current system was mostly built around CRTs which have inherent gammas approximately in the range of 2.2 to 2.5, and our eyes are also non-linear in much the same way. Theoretically, the perfect gamma for a system from light entering a camera, to the display, then reaching our eyes, would be 1, since this would mean we were seeing the same ratios as the camera was receiving. There can also be a 0 IRE or 7.5 IRE base value for black, so it (IRE) is somewhat ambiguous when discussing levels in sources. I use X% stim (stimulus) instead of IRE, because IRE is based on analog values. In other words, you might send an IRE value of 30 (an input voltage) to the projector to see what it will actually output (brightness). Most measurements use IRE values to represent the input. It is the relationship between input voltage and the output light intensity, and describes the non-linearity of intensity reproduction.
Gamma is a basic, and very important factor in images and contrast ratios. Well, it is a little more complicated than that, and this article is designed to help you understand all the facets of the contrast ratio, and what it all means for your home theater enjoyment. In general, we know that it is the relationship between the black and white levels that a projector can show.
When we shop for a projector, one of the specifications that we really want to be aware of is the Contrast Ratio (CR).
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