An optical assembly is disclosed including two laterally variable bandpass optical filters stacked at a fixed distance from each other, so that the upstream filter functions as a spatial filter for the downstream filter. The lateral displacement may cause a suppression of the oblique beam when transmission passbands at impinging locations of the oblique beam onto the upstream and downstream filters do not overlap. A photodetector array may be disposed downstream of the downstream filter. The optical assembly may be coupled via a variety of optical conduits or optical fibers for spectroscopic measurements of a flowing sample.