I love learning things when I least expect it!..
Last week I learned something interesting about the efficiency of reflective gratings.. I learned they behave quite differently depending on how the light is polarized!
If the polarization of light is parallel or perpendicular to the grooves, a grating can be more or less efficient in reflecting it depending on wavelength!
Optometrics taught me this, with their efficiency curves! (The interesting detail being that roughly on the blaze wavelength, it behaves the same for both polarizations).
Yey! All good up to here!
Well today I played with an LCD monitor and a polarization filter! I discovered the light coming from this monitor was actually polarized in one direction!
One or two seconds after discovering this,
I formulated in my head the same thought you are probably having now: "Wow! This means I can have problems in making spectroscope flats with LCD monitors!" 
Meanwhile I have heard that OLEDs do not produce light polarized in a specific direction.. What about the panels used for taking flats?
I know the importance of a flat is just to get the slow variations out of the image, and the camera response should be determined from a known star.. But I think it is strange to take a flat with the panel in one orientation, and the images comes out darker than if we rotate the spectrograph 90ยบ compared to the panel... In some gratings, in wavelengths distant from the blaze wavelength the difference can be quite big!
If we are working in low resolution (where the spectral range includes before and after the blaze wavelength), the orientation of the spectrograph (grating) with the LCD monitor should probably be taken into account to be able to reuse the flat!...
I have used an LCD monitor as an illumination source to make a flat in spectroscopy once.. But I was unaware of any of this..
