A light meter is used to measure visible light in units of lux or footcandles. A separate UV meter is required to measure UV energy in microwatts per lumen. Some UV meters can also measure visible light by changing detectors within the device. A 35mm camera with a manual light meter can also be used to measure visible light levels, as described in Protection from Light Damage.
Remember that artificial light does not change significantly, but monitoring natural light can be problematic because it differs according to the time of day, the weather, and the season. Readings should be taken at times when the light is brightest to be sure that the highest light levels are being recorded. Another useful tool is the Blue Wool standard, cards containing samples of blue-dyed wool that fade at a known rate relative to each other (the first few samples on the card correspond to light-sensitive materials such as paper). Use of these cards can demonstrate the cumulative degree of fading caused by the intensity of light in a particular location