Light interception is one of the most important factors for plant growth. The intercepted amount depends on the incoming radiation in the greenhouse and the percentage of interception by the crop and is directly related to the leaf area. Proper crop management requires the measurement of the most important growing factors. In case of application of crop growing models the simulation of the leaf area is one of the major uncertainties in the results of the models. Automatic calibration of the model based on radiation interception increases the accuracy of the model results. For the determination of the crop¿s light interception in a greenhouse, a radiation sensor was used which determines the ratio of the incoming radiation from the upper side and the reflection at a specific wavelength from the lower side. The ratio of reflected radiation versus intercepted radiation can be used to estimate light interception as well as leaf area. However, the measurement of this fraction can only be used after filtering out erroneous data due to technical errors, insufficient diffuseness, insufficient solar height, etc. This paper describes the boundary conditions to be taken into account for proper measurement of reflected radiation. Time series measurements of incoming, reflected and global radiation are used, to filter the data acquired by the sensor. The sensor functioned well during tests on a cucumber crop in a commercial greenhouse.