While printing postcards for the recent APUG exchange, I noted that exposure across postcards was not even. At first I thought this was due to developer exhaustion but this was not the cause. When I make several prints back to back (as with postcards) it appears that the enlarger bulb produces a decreasing light output. After about 6 or 7 consecutive exposures of about 7 seconds each (with time enough to change paper between exposures), the last 1 or 2 prints in the sequence are very underexposed. I understand that with short exposures any difference in light output across prints is more likely to show. However, should the output vary this much? Should I let the bulb cool down before making the next exposure in a sequence? Mabe this is the first sign that the bulb is about to go??