Wednesday, June 13, 2012

monitor MTF? sure!

Okay, so I need to know the spatial frequency transfer function of the monitor I've used to do most of my experiments over the past couple of years. I've never done this before, so I go around asking if anyone else has done it. I expected that at least B** would have measured a monitor MTF before, but he hadn't. I was surprised.

Still, B**'s lab has lots of nice tools, so I go in there to look around, and lo, T** is working on something using exactly what I need, a high speed photometer with a slit aperture. So today I borrowed it and set to work doing something I had never done and didn't know how to do. It was great fun.

D** helped me get the photometer head fixed in position. We strapped it with rubber bands to an adjustable headrest. I've started by just measuring along the raster. The slit is (T** says) 1mm, which is about 2.67 pixels on my display. I drifted (slowly) squarewave gratings with different wavelengths past the aperture - this was more complicated than it sounds. The monitor is run at 100Hz, and CRTs flash frames very rapidly, just a millisecond, so getting the photometer settings just right (it runs at 18Khz) took a bit of adjustment, and figuring out good settings for the gratings, slow-enough speed to drift them at (I'm limited by the 10 second block limit imposed by the photometer)..

Anyways, I got back temporal waveforms which I treat as identical to the spatial waveforms. As expected, the power of these waveforms drops off as the gratings get finer. But, I know that it drops off too fast, because of the aperture. If the aperture were exactly 1 pixel across, and if it were aligned precisely with the raster, and if a bunch of other things were true, then I could know that each epoch recorded by the photometer reflected the luminance of a pixel, and my measurements would reflect the monitor MTF. But, like I said, the aperture is 1mm, so each 10ms epoch is an aliased average over >2 pixels. I'm not even thinking about the reflections from the photometer head (there's a metal rim to the aperture T** had taped on there).

My solution: code an ideal monitor, record from it with the same sized aperture, and divide it out of the measurements. I can then guess a blur function - Gaussian - and fit that to my (4) data points. That's what I did: here is my first estimate of the vertical MTF of my Dell p1130 Trinitron:

The Nyquist limit for this display, at the distance modeled here, is about 23cpd, so I guess this Gaussian is in about the right place. It's hard to believe, though, because horizontal 1-pixel gratings look so sharp on this display. I feel like these must be underestimates of the transfer. I am nervous about how awful the vertical will be...

It wasn't too bad, just a bit blurrier than the horizontal. Still makes me suspicious that I'm underestimating the horizontal. Not going to bother putting up plots, but here's my estimate of the pixel spread function (you can just see that it's a little broader left-right, that's the vertical blur):

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