grading on monitor for projection

I was grading a video on a monitor. The video will be made into a DCP and projected in a dark theatre.
So I tried to make the monitor viewing emulate the theatre viewing. I fully darkened the room. I tried reducing the monitor white to 60 nits but this reduced contrast to just 500:1. The monitor achieves 750:1 at 140 nits. I tried viewing the 140 nit monitor through 0.4 ND filters. I think I can make a DCP that will put the same luminances on the screen as my monitor shows except near black where the 750:1 is a limitation.

Then I looked at SMPTE RP 431-2:2011. For the Reference Projector the frame-to-frame, white-to-black contrast should be at least 2000:1. But the intra-frame white-to-black contrast need only be 150:1. For the intra-frame test half the screen area is white. So there's how monitors differ significantly from projectors. On a monitor the intra-frame contrast is essentially the same as the frame-to-frame contrast. On a Reference Projector the former is 13 times less.

Some of the contrast reduction in the intraframe test is due to light leaving the screen and being reflected by the room back onto the screen. This hardly happens to the monitor because it is not reflective like the movie screen. The rest of the contrast reduction in the intraframe test is due to scatter in the projector's optical path. Room scatter and projector scatter can be lumped together and the large drop in contrast with the Reference Projector can be attributed to a 1.2% total flare factor. This means that 1.2% of the light projected onto the screen can be regarded as spread uniformly over the screen. The reason for this calculation is to estimate other intra-frame contrasts than the one in the half-white half-black frame test.

For example, when the Reference Projector projects an image with no white, but with half the frame at 18% grey and the other half black, the black is now 1/700 of white -- similar to the monitor. With 1/10 of the screen at 50% grey and the remaining 9/10 of the screen black, the black is now 1/900 of white -- again similar to the monitor. Etc.

To the degree that presence or absence deep black is important for grading, projected and monitored images are in general incomparable. A solution would be to start with a high contrast OLED monitor and to add a contrast-degrading function based on the total light within the frame. This could be done real time in hardware, or it could be pre-computed as a filter.

Dennis Couzin
Berlin, Germany