The direction and diffuseness of the light source can interact with the three-dimensional shape of the target, and with the “shinyness” or “glossiness” of its surfaces. A point-source of light (such as the sun, or a small light bulb) is highly directional, so the reflectance measured from a point relative to the grey standard will be highly dependent on its angle relative to the light source as well as the surface reflectance. Therefore, under highly directional lighting the reflectance measured from a surface by your camera will only be accurate if that surface has perfect Lambertian reflectance (is very diffuse), and the angle of the surface is exactly equal to that of the grey standard. Clearly these conditions are almost never true, so care must be taken with directional point light sources. Direct sunlight is arguably a different case because measurements made by a camera in the field will be qualitatively similar to the reflectance measured by an eye, and the lighting conditions are ecologically relevant. However, as long as uniform lighting angles relative to the target are being used, point sources are suitable for comparing reflectance values within a given study.
The effect of the surface angle (i.e. the shape of the target) and its diffuseness can be minimised by using a diffuse light source. Shiny objects and complex 3D objects will therefore benefit most from diffuse lighting conditions. Artificial light sources can be made more diffuse with standard photography umbrellas or diffusers in the human-visible range. However, particular care must be taken when diffusing UV light sources as the diffuser must be UV reflective. White plastic umbrellas and diffusers should therefore not be used for UV sources. Metal-coated umbrellas or sheets of natural white PTFE (Polytetrafluoroethylene, easily bought online from plastic stockists) are suitable for diffusing UV light sources.
In the field, lighting angle and diffuseness can be difficult to control for. If possible, attempt to photograph only under sunny or overcast conditions, not both. If this is not practical, or you are interested in the variation in natural lighting conditions then take note of the lighting for each photograph and take multiple photographs of each sample under all ecologically relevant lighting conditions. Then lighting conditions can be entered into the statistical model during analysis.