NOVELTY - This method produces a beam simulating solar radiation from a known high intensity source, e.g. a pulsed xenon tube. The beam is incident on solar cells, for testing purposes. The method of beam path guidance in the dispersion unit acts in effect, as a dynamic spectral mask. A number of mirrors lying close together can be moved. In accordance with the instantaneous location of each mirror, a fraction of the spectrally-dispersed beam is reflected into an exit beam path, for further use (i.e. incidence on the solar cell under test).    USE - A solar simulator used in measuring solar cell opto-electronic characteristics.    ADVANTAGE - Precise measurements of opto-electric characteristics is achieved from solar cells and modules. By variation of the spectral beam intensity of the solar simulator, in conjunction with measurement synchronization, a new measurement method is obtained for spectral sensitivity. Any time-dependent spectral distribution can be generated by selective dynamic masking as described. Wide variations in intensity can be superimposed.    DETAILED DESCRIPTION - The moving mirror system resembles a known digital light processor (DLP) in function. It is, however, suitable for the extended spectral range and intensities appropriate for solar cell testing. It withstands the very high beam intensity of the solar simulator. The mirror(s) are moved at such a velocity or rotary speed, that the desired spectral masking is achieved over time. This movement velocity is related to an e.g. 5 ms irradiation interval from the solar simulator. In this way the spectral irradiation is varied in a desired time-resolved manner. Dynamic spectrometric measurements are taken simultaneously with solar cell output measurements. The output is e.g. the solar cell short circuit current. All data is sent for further processing. Reference cells are used in conjunction with the multiplexing methods. AN INDEPENDENT CLAIM IS INCLUDED FOR the corresponding method.    DESCRIPTION OF DRAWING(S) - The beam path and optical elements are represented in a schematic diagram.    beam source (e.g. pulsed xenon lamp (1)    collimators (2)    pre-filter (3)    beam-splitting and spectral dispersion section (4, 5)    dispersion unit (detailed) (6)    recombiner (7)    solar cell positioning- and measurement surface (9)