Optical measurement method and system

A system for optical measurement comprising:

• a light source for generating input light;
• a beamsplitter downstream of the light source for splitting the input light into first light and second light;
• a sample holder downstream of a first path of the beamsplitter for receiving the first light, resulting in output light;
• an output photodetector for measuring output light emitted from the sample;
• a flattening filter in a second path of the beamsplitter for filtering the second light, resulting in filtered light;
• a correction photodetector downstream of the flattening filter for receiving the filtered light;
• wherein the beamsplitter is not dichroic;
• one of the first light and the second light comprises reflected light and the other of the first light and the second light comprises transmitted light;
• the correction photodetector is configured for providing control signals to the light source for maintaining an intensity of the input light;
• the correction photodetector has a sensitivity function that varies with wavelength;
• and
• the flattening filter has a transmission function adapted for normalizing the spectrum of the second light with respect to the sensitivity function to mitigate wavelength-dependent photosensitivity of the photodetector.