Concerning typical films that are used as agricultural covering materials, we have studied their solar light transmittance, illumination transmittance, and photon transmittance. As for the period average transmittances of different types of light during the experiment period, all films showed more or less the same vertical relationship, except the light-selective purple-colored film. Materials with higher transparency had lower variation coefficients of transmittances. The same tendency as these results was observed when materials were placed horizontally and when they were placed on a slant. As for the occurrence of abnormal transmittance values, black polyethylene films with low transmittance had a higher occurrence of abnormal solar light transmittance values. As for illumination transmittance, films with high transmittance had a higher occurrence of abnormal values, and photon transmittance produced the same result. Abnormal values did not result from the measurement principle of light, but were largely influenced by the transmittance of films and solar light conditions. From these experiments, we found that there was little difference between solar light transmittance, illumination transmittance, and photon transmittance, and that the occurrence of abnormal values was largely influenced by the time and period of measurements and light conditions.