Alternative Light Meters

[Steve Guttag]

I saw nothing in the specifications about integration time and how it would interpret 2-wing and 3-wing shutters with their 48Hz and 72Hz frequencies.

I'm also curious if it reads more accurate when the entire screen is white versus when just the spot being measured. I doubt a device like this is good at suppressing extraneous light from even its own optical path. Most meters will not be accurate if more than just the spot they are measuring are illuminated. However, if it is calibrated and always measured with a full screen, i suspect that it is accurate enough for what it is checking.

I'd think another "poor-mans" means of having a meter would be the QSC/USL LSS200. If one were to calibrate it it probably could be used reasonably track light levels as it does of a single auditorium. As a bonus, you get an SPL meter and a bit of a color meter too.

[Mark Gulbrandsen]

That has nothinmg to do with integration time, but it certainly does have to do with spectral response.

"The photovoltaic effect can be defined as being the appearance of a potential difference (voltage) between two layers of a semiconductor slice in which the conductivities are opposite, or between a semiconductor and a metal, under the effect of a light stream."

[Harold Hallikainen]

I did a fair amount of testing of lens flare on instruments. I found the PR-650 increased about 6% when changing from a small spot (within the capture area of the PR-650) to full screen.

The LSS-200 has done pretty well for stability, repeatability, etc. In one series of tests, I calibrated it with a xenon source and then measured a laser-based projector. I don't have the numbers any more, but it was pretty close. It does use an XYZ sensor to give pretty good accuracy of luminance and chromaticity with a variety of light sources.

Harold
LSS-200 designer

[Steve Guttag]

Harold, did you ever measure a Minolta? They didn't do as well between just the spot versus the full screen.

Mark, a meter's integration time, when dealing with a projector with a shutter, is key. Otherwise, it will mis-read a 48Hz flicker or a 72Hz flicker. You are welcomed to consult ANSI/SMPTE Standard 196

[Harold Hallikainen]

I probably measured a Minolta LS-100. All that stuff was on our engineering wiki which I can no longer access. I measured this by reducing the masking in the projector to minimum. The viewfinders on the meters indicated that the white rectangle was covered the sensor area. I got a reading there, then adjusted the masking back out to cover the full screen and recorded the difference. On the LSS-200 (and previous models), we added a baffle in the lens tube and glued a carbon tube right above the sensor chip. We also used black soldermask on the sensor PCB. It's amazing how much light bounces around, even when the surfaces are black.

Following up on a previous comment, Carsten suggest that there was no black time on DLP projectors. Instead, each image instantly came up. I think this would cause visible flicker or judder. It SEEMS like each frame needs to be flashed twice or more to keep the flicker down. And, if so, what is the duty cycle of the flashes? I'd SUSPECT that they are 50%, so a frame sequence would be something like this (for two flashes per frame):

0 ms - Image up
10.4 ms - black
20.8 ms - Image repeated
31.25 ms - black
41.7 ms - Next image up


Harold

[Carsten Kurz]

Flashing or black framing is one optional way to reduce judder/strobing. As a matter of fact, you can see a lot of that strobing on DLP projectors when watching cinematic 24p content. It is more important to apply flashing or motion interpolation on direct view screens because of their higher luminance and contrast and thus lower perception threshold. At the brightness and contrast levels of digital cinema, it is much less prominent, but still often visible, especially with todays scene lighting, shooting habits and HDR productions.

At these common frame rates, it would be very easy to analyse such a display sequence even with now common smartphones that offer 120 or even 240fps video capture rates. It would also create visible flicker artifacts already when watching a digital cinema screen on a camera display due to refresh rate interference. I know that many older home cinema projectors and TVs used to have menu options to enable or disable black frame insertion. Nowadays, motion interpolation is the state-of-the-art way to deal with that.

A couple of times I was tempted to create black frame insertion experimentally by using the 3D format setup on our Barco - it allows to set flashing/dark time parameters for sequential 3D. So, if you want to enable black frame insertion on a cinema DLP, you could just create a suitable 3D file.
I guess it is not in widespread use because, similar to film shutter blades, it causes light loss, and no one wants that on a cinema projector.

[Harold Hallikainen]

It would be interesting to see a high frame rate capture of a DLP projector. On luminance and flicker, I agree that increased luminance increases the flicker fusion frequency. Another interesting effect, I think, is that our peripheral vision is more sensitive to flicker than our "direct" vision. If I have an LED clock in my peripheral vision, I can see the flicker from the multiplexed display. I do not see the flicker when looking directly at the display.

Harold