The question is not quite clear... But lenses and mirrors come to mind. A lens or a angled mirror redirects light. You can be clever and use lenses and mirrors to concentrate light together in a volume, area or point. Of course we are familiar with these types of devices, glasses, microscopes, prisims, etc.
Still not enough information, but I'll take a stab, photodetectors generally have quite a wide range of responsivity and will produce a measurable voltage when lots of different wavelengths are incident (e.g. 500 - 900 nm). You can work around this using optical filters, signle-mode fibers, or diffraction gratings, which isolate narrowband light from a broadband source. If you have e.g. 550 and 800 nm light, split it, send one through a 550 nm filter and the other through a 800 nm filter, then send each to its own photodetector with known responsivity in the given ranges.
From the two output voltages you can deduce from a single incoming light source what intensity is at 550 nm and what intensity is at 800 nm.