When permitted, I usually take a non flash photograph of each painting and the information signage. It is surprising the number of "hot spots" there are on irreplaceable works of art.
The in camera light meter, although it is a reflectance meter, can give an estimate of how much additional light is falling on a specific part of the painting. If there is an area in the painting with same color, the readings will be more accurate. Zooming in for a reading with the camera set for single segment spot metering will give even more accuracy. The painting on the left recorded almost 4 times as much light on the upper part of the portrait within the portrait compared to the lower half. The portrait on the right has dark background in both the hot spot and properly illuminated background. The meter readings is 3 times as much light in the hot spot. A more accurate assessment would require an incident-light meter. Exhibition rooms with high ceilings make it easier to protect the upper part of large vertically oriented paintings. Most of the following examples are not as severe as the above two portraits.
