Three measuring channels in a photographic color printing and enlarging apparatus are utilized for determining the filter values of the primary colors. The exposure time is determined by adjusting and reading the amplification factor of an amplifier which amplifies the signal of a light-voltage transducer containing a photoelectric element, which obtains its input from a measurement of each of the three primary color components in the printing light. The light-sensitive element of the photoelectric element has a characteristic curve which is linearly related to the intensity of illumination and it is preferably a photodiode. The light impinging on the photodiode is modulated by an electromechanical light-chopping device and the measuring signal is controlled by a zero-regulator. The light-sensitive element, the zero-regulated device, the light-chopping device and filter controlling device are miniaturized and fitted into a light-measuring probe. The light-measuring probe is equipped with inclination setting feet and an inclination determining device.