Was that me or the doobie Art?
post #751 of 754
12/17/05 at 10:20pm
Originally Posted by darinp2
he left all the statements I referenced in other threads as is without my comments about them there. Doing your suggestion wouldn't achieve Clarence's goal of protecting Mike Parker from answering the questions in this post
Originally Posted by darinp2
he left all the statements I referenced in other threads as is without my comments about them there. Doing your suggestion wouldn't achieve Clarence's goal of protecting Mike Parker from answering the questions
|The ANSI contrast measurement was developed to help standardize data on CRT displays. It solved several problems with measurements and allowed a more meaningful number to be presented. Prior to that full on/off ratios or peak on/off numbers were cited. Those were misleading because CRT projectors had some interesting characteristics.
1. The off state is very very dark. So dark that it is difficult to measure with accuracy. Figures from about 10,000:1 to 30,000:1 were typical and CRT's really didn't vary that much from model to model on this measure. Full on/off was almost meaningless because everybody did it well and it was tough to actually measure because everyone did black so well.
2. A CRT projector doesn't make 100 IRE white the same intensity independent of how much area of the phosphor is to be lit up. If the entire phosphor surface is lit up, the measured light output intensity is lower than if a very small portion of phosphor is lit. Pretend you are a manufacturer. What do you choose as a test pattern for measuring your white intensity? You use one with as small an area as possible so you could get a nice high peak contrast ratio.
This ability of a CRT to make small, bright objects extra intense if the rest of the background is dim is an important difference between CRT projection and digital projection. We think about the gamma curve as being constant for the image, but (GASP) CRT's get to cheat a bit. They can deliver a big punch if the scene is dark. The digital projector humbly holds white at the same intensity independent of overall scene illumination. In other words, a CRT can look brighter and punchier on many scenes even though it has the same total lumens output of a digital. This means a digital needs to have its peak white set a bit brighter than a CRT to achieve the same punch. This isn't a problem for modern digitals with their high light output. But, you should keep this in mind if you set a digital and a CRT to both make a 100 IRE window (1/4 area coverage) the same intensity. They actually might not be evenly matched in scene to scene illumination despite that maneuver. This effect is also useful for understanding why a low lumens CRT seems to keep up with an equal lumens fixed panel display.
But, I digressed. The ANSI contrast ratio helped level the playing field between CRT measurements by making the load 50% white coverage for the test. A 4 x 4 checkerboard pattern is used. Measurements of light intensity are taken in the middle of each rectangle. The sum of the white / sum of the black yields the ANSI contrast ratio. Because the area load is standardized, playing games by measuring just a small spot intensity is removed. Also, the simultaneous presence of both black and white in the test allows estimation of how well the projector optics prevented light scatter - a characteristic that actually did vary from model to model. So, overall the ANSI contrast ratio was a step forward in evaluating CRT projectors.
It has problems. The test requires multiple measurements and this takes considerable time. You need to do some math. The room backwash needs to be eliminated. Because, of our need to get results similar to formal ANSI contrast, some modified checkerboard patterns are available in Avia PRO to obtain ANSI style data without moving the sensor probe. Single position measurement doesn't take into account uniformity issues, but it is much faster and gives useful information which might otherwise not be gathered.
Along came digital projection and suddenly ANSI contrast fell out of favor. Why? First the optical scatter of modern digital projectors is very low, far better than on CRT projectors. This reduces optical clarity differences as a differentiating factor in the measurement. Also, the elevated black level of older digital displays made the black floor much higher. This characteristic DID vary from digital display to digital display. Thus on/off contrast became not only easier to measure (since black was no longer black), but also a big difference between models. Optical scatter wasn't important because it was much smaller a problem than the elevated black level. Finally, the need to equalize percentage of illuminated pattern was no longer an issue because digitals hold white the same whether the spot be large or the entire screen. Another reason for the difficult to measure ANSI went away.
So for a while, and probably today, ANSI contrast has fallen out of favor because it is difficult to measure and was thought irrelevant for comparing digitals because the differences could be culled out using just on/off measures. That's why you see more on/off measurements than ANSI these days.
Digitals are now pushing the black level down further and further. As on/off contrast ratio approaches that of the CRT's, the contribution of optical light scatter once more becomes important and may help differentiate between machines. It's still hard to measure, but at least we can get ANSI style measurements with appropriate test pattern sets such as in Avia PRO.
How does this all come together? You need both on/off and ANSI style measurements to characterize a display. The on/off measure tells you how well a display can render black. The ANSI contrast tells you how cleanly a display prevents contamination of dark areas during brighter scenes. Ideally, a display would have both on/off and ANSI contrast ratios high. That would allow fidelity throughout the entire range of scenes form very dark to very bright. The current situation is that CRT's have great on/off but relatively poor ANSI contrast. Digitals have relatively poor on/off and good ANSI contrast. Newer digitals can achieve good on/off and great ANSI contrast. Nothing has the best of both worlds - yet. What is amazing is that a high end DLP now has low enough a black level that its superior resistance to light scatter is enough to make the scatter on a CRT look foggy in comparison.
Director - Imaging Science Foundation Research Lab
Video Test Design - Ovation Multimedia / Home of OpticONE Colorimeter, AVIA and Avia PRO