There are several factors affecting the response of radiochromic dye dosimeters to ionizing radiation. Some of them are avoidable or controllable, but can not be corrected in practice such as effect of light. Others are not avoidable or controllable, but can be corrected, such as effect of temperature. Spectral characteristics of incandescent light, daylight, UV-light and sunlight have been determined in this paper. This is helpful for the explanation of the effect. The changes in illumination of different light sources with distances are shown. The relationships between the change in optical density per unit thickness and time of exposure to incandescent light, daylight, UV-light, and sunlight have been measured. It comes to a conclusion that radiochromic dye dosimeters are not so sensitive as photoplate film to visible and incandescent light but the background and post-irradiation optical densities will be rapidly increased when they are exposed to daylight, sunlight, or UV-light for a while. Therefore, they should be kept or stored in dark places.