phy_optics said:
12V, 50W, MR16 halogen lamp spectrum is measured as shown ('B') with our newly developed mini-spectrometer having resolution ~1nm.
This problem has been bugging me for a few days now...if it is an etalon-type effect (which is the case, according to ThorLabs), then using your supplied graphs to estimate a free spectral range of 50 nm and a center wavelength of 540nm, the cavity path length is 2.9 um (air) or 1.9 um (glass). This doesn't obviously correspond with ThorLabs' claim of a 'window' causing the effect- unless the window is in part of the beam path where there is focusing or converging rays (which you mentioned in another post). Then, the cone angle of the focused beam allows for path length differences, analogous to Newton's rings. In this case, the 1.9 um path length difference is trivial- a 1mm thick window acquires this for a f/32 system. So, if it is indeed an etalon effect, it can be explained by path differences acquired as converging/diverging light passes through a window.
But, there a problem with this simple result: Czerny-Turner spectrometers are fast: f/4 or so. That's too fast to provide nice fringes like you see- they would be washed out. So, the window needs to be located at the entrance coupler rather then within the system (e.g. the CCD), which is where (I think) the ThorLabs window is located.
And, of course, there's another problem- you didn't specify if there's a window present! Your OP says it's a 'newly developed' spectrometer: developed by your group? What is the optical design? Or, is it actually the ThorLabs spectrometer?
In the end, if you guys are simply using a ThorLabs spectrometer and don't like the fringes, then correct the measured spectrum against a known ('standard') source and use that to correct other measurements.