Even the SpectraCal test showed that the fluctuation of a UHP bulb is tiny after the first couple of minutes of turning it on, hovering between 41 and 42 nits in their example.
Regarding distance the meter should be placed from the screen, this is one of the most common questions I am asked. In one sense the correct answer is, "It doesn't matter." The reason for this is that as you pull the sensor back from the screen the intensity of the light hitting the sensor decreases. However, the size of the reading area on the screen increases. These two cancel each other out. So, it really doesn't matter how far the probe is from the screen.
However, there are some practical constraints guiding meter placement. The most important one is that you don't want the size of the measurement area to be larger than the test pattern. This is especially important if you use window patterns. It makes getting a good aim point harder. For this reason, I generally recommend using full field test patterns for front projectors.
You also don't want the meter reading its own shadow, so you have to angle the meter a little. For high gain screens, such as the HP High Power, this is a problem because they color shift off axis. So you have to be very careful with screens like this to keep the angle as small as possible.
SMPTE recommends taking readings from the viewing position, but that will not be practical for most consumer probes. You need a professional tool with a good aiming system and narrow viewing angle for this. Readings from a couple of feet away should be fine in most cases.
Originally Posted by breadvan
Tom what about projectors? And how far do you place your meter off the screen?