True, but photographs work just fine as long as you have a reference beside your plasma, and you take the pictures in total darkness with the same camera settings. The whole point is take a pic of your plasma at x number of hours and then several hundred hrs later to see if it has lightened up. I fully agree, just a picture of your plasma by itself with no kind of reference for comparison reveals very little.
If you can't set an LCD/CRT monitor beside your plasma, lay a black piece of felt in front of the screen, so you have some contrast between the glowing screen and the dark black felt. Then repeat the process several hundred hours later, with the same felt in front of the screen. If the TV's blacks have lightened considerably, you ought to be able to easily tell by comparing the two photos and how much the screen glows compared to the black felt. If there has been a major lightening, the black felt will stand out against the screen much more prominently.
Of course you must do this in total darkness and you have to be careful to record the exact same picture/camera settings each time.
Also, download the free AVS test disc and use a 0 IRE, which is better than just using a blank input. You can even make a 0 IRE jpeg from the test disc and probably just use the SD card slot on the side.