The blue gun is usually the first color to run out of range as light output demands increase. Defocusing the blue gun allows the gun to produce considerably more light before it runs out of range. This allows the projector to achieve a higher total output. Fortuantely, human vision is not very sensitive to the blurring caused by blue defocus. We see most of our detail in red and green so overall this is usually a good compromise.
If you're running the projector way below max output, the blue would not have to be defocussed. That's also why modern HD ready rear screen TV's don't need the blue tube defocussed, because they can put out much more light than is really required. But with front CRT projection, that's not usually the case. If you didn't defocus the blue tube, the 100 IRE output when projected wouyld have a decidedly yellow tint. If you increse the blue CRT drive, you might be able to get an acceptable white, but your mid grays would be way too blue.
When I ISF calibrate a front CRT, I trade off between blue drive and blue defocus to get a reasonable gray scale, but not too much defocus.
There is a right and wrong direction of defocus. On your Sony, as I recall, the correct direction of defocus on the blue tube is clockwise away from best focus. Display a dot pattern and crank up the contrast. Focus blue . then as you turn the focus control clockwise, the blue dot should become uniformly fatter (it should be symmetrical, too, or your 2/4 pole magnet assembly is out of alignent). In the counterclockwise direction, you should see a blue dot with a halo around it (again, if the dot isn't in the middle of the halo, check your magnet alignment).
Put up a 10 bar gray scale and set yoour contrast and brightness to where they normally are, and rock the blue focus back ands forth. You'll see the white bar go from yellowish to white to bluish.
If you are running the output low enough that the blue gun has sufficient output even when focused then yes, the image could be better with it focused. However, the focused state is usually about 20 to 30% lower in light output than defocused. The way I approach it is to try hitting a 100 IRE window pattern with an effective 13 to 14 Foot Lamberts. That includes screen gain so you actually target a FL reading of 13/screen gain at the screen surface with the meter. Translates to just above 10 FL for a 1.3 gain screen. If grayscale won't track correctly then I defocus blue in the underfocus direction (uniform blob) slightly and try again. The usual symptom of insufficient blue output is a blue hump in the middle IRE windows when the upper and lower ends are correct. At low output levels, all three guns can track together well even with blue focused. For most size screens that people want, one has to defocus blue to get enough light output to hit the 13 to 14 effective FL output level.
BTW, I set up my probe on a microphone stand in front of the screen looking back at the projector. I usually don't take the scrren surface into account, but if the customer insists, I turn the probe around to get reflected light off of the screen and get a correction factor. Do you agree witrh this? If you have a better idea, and I'm sure you do, I'd love to hear it.
Chuck, I think that's a good and reasonable approach. The only other variant to consider would be setting the top end visually with the comparator and then getting the correction factor with the sensor pointed at the projector. That would also correct for the screen material and allow work with non-CRT projectors. That assumes you trust your color vision at the high end better than the meter. At the low end, I believe the meter more than my eyes by a wide margin. At the top end, my vision is pretty good at picking up differences that the meter just barely notices.
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