Film has been used to detect x-rays since the early days of their discovery by Rontgen. Although nowadays superseded by other techniques, film still provides a cheap means of x-ray detection, making it attractive in high-school or undergraduate university courses. If some sort of quantitative result is required, the film's optical absorbance or optical density can be measured with the help of an optical densitometer. In this article, the authors describe the construction of a simple optical densitometer using an infrared light-emitting diode and a silicon PIN photodiode as detector. The use of an infrared photodiode with a visible light-tight case enables the operation of the apparatus without a specialized light-proof box. (Contains 6 figures.)