Abstract: Optical measuring apparatus for determining chromaticity of thin films on a substrate includes a light source for illuminating the substrate and a measuring apparatus for dispersing light into various wavelengths and making wavelength dependent intensity measurements. Radiation from the light source is reflected or transmitted by the substrate to the measuring apparatus along a first beam path having a first diaphragm for cutting off the radiation from the substrate in a leak-tight manner. Radiation from the light source is also transmitted to the measuring apparatus directly along a second beam path having a second diaphragm for cutting off radiation from the light source in a leak-tight manner. The light source (6a) consists of a globe photometer (6a), in which a lamp (4) is provided. A steadily burning light source, especially a halogen lamp, is used as the lamp (4).