The invention relates to a measuring device for determining a vegetation index value (REIP) of plants. The measuring device comprises a plurality of light emitting elements, each of which emits substantially monochromatic light at a predetermined wavelength, a light receiving element which receives light from the light emitting elements reflected by the plants and generates a signal indicating the respective intensity of the received light, and a control means which successively activates the light emitting elements in a cyclical sequence, determines the respective intensity of the reflected light based on the output signal of the light receiving element, and calculates the vegetation index value based on the determined intensities of the overall measurement cycle. According to the invention, a light frequency converter is provided as the light receiving element.