Accurate estimates of cumulative light exposure are an important prerequisite for the assessment and limitation of photochemical damage to museum objects on display. The task is complicated because spotlights used to highlight particular features illuminate objects’ surfaces unevenly, and also because indirect light sources, for example diffuse sunlight within exhibition spaces, result in changing total illumination levels throughout the day and seasonally. This paper presents a methodology for determining the annual light exposure of 2-D objects by combining the results of continuous light readings adjacent to the object and one-off point measurements over its illuminated surface, a method that allows a more accurate estimate of total exposure than either monitoring method alone. Two pieces of information are required to calculate cumulative exposure: first, the ratio of direct to indirect lighting, which is arrived at by quantifying the amount of visible light falling on the object relative to that received by its surroundings; and, second, the diurnal and seasonal variation in illuminance of indirect light sources, particularly diffuse daylight. Two paintings in different galleries exposed to different ratios of diffuse sunlight to direct artificial light – one low and the other high – were used to refine and test the method.