Radiation is transformed into other types of energy partially or totally while passing through matter, especially into heat; i.e. this radiation is absorbed. Optical filters change the spectral composition of the incident radiation by absorption and reflection. The spectral transmission ratio of optical filters gives the ratio of transmitted radiation to incident radiation at a certain wavelength. The monochromatic light necessary for the determination of the spectral transmission ratio is generated by a prism monochromator. A photomultiplier serves as detector. That measurement setup enables measurements in the wavelength range from 240 nm to 900 nm.