# Learning about spectrometers (curvy tungsten spectrum)

I have performed an experiment in which I used a spectrometer (CCS200 from Thorlabs) to measure the spectrum of a tungsten source (it was connected to the spectrometer with an optical fiber). The spectrum is shown in the figure:

I would like to know why I get that curvy spectrum, when the tungsten source should emit a spectrum that looks like a black body emission.

I have read this website about spectrometers, but it doesn't have references to check some of the things written there, and there are some things that come without explanation (for instance, symbols whose meaning are not explained). Is there any "short" (maybe up to 30 or 40 pages) text or chapter or source where the working of a spectrometer is explained with a bit of detail so that I could explain the curviness of my spectrum?

• The regularity suggests that there are reflecting surfaces between the tungsten and the detector that are causing interference. Look for flat reflecting surfaces and tilt them so that reflections don't go into the detector. See if that helps. – garyp Sep 19 '16 at 14:42

Many CCDs have problems with interference fringing in the red and near infrared. This results from interference due to multiple reflections between the front and back surfaces of the CCD. It is a particular problem for "back thinned" CCDs, where in exchange for an enhanced quantum efficiency, there has to be an additional coating on the top surface where the interference is generated. See for example this description that explains back illuminated CCDs and discusses interference patterns towards the end.

The amplitude and pattern of fringing depends on the wavelength of the light and the thickness of the detector and the spectral resolution. Often you would use exposures ofa continuum lamp like tungsten to estimate a "flat field" for a stable fringing pattern.

The example below is a near infrared (0.9 micron) flat field for the WFC3 CCD camera on the Hubble space telescope.

I chased up a bit of the documentation for your device and couldn't work out whether it is back illuminated or not.

• Hi Rob, Thanks for your answer. I didn't find if it's back or front illuminated, but I found a very small note saying that there is a glass window protecting the CCD, maybe that film of glass is producing the interference. What do you think? – Vladimir Vargas Sep 21 '16 at 1:03
• Depends how thick it is - see John Rennie's answer. However, I doubt it would be this thin. – Rob Jeffries Sep 21 '16 at 6:41
• @VladimirVargas A little late to the party here, but Thorlabs states on the product page: Note: A glass window in front of the CCD causes etalon effects which vary from unit to unit. If you require more information to determine if these spectrometers are suitable for your application, please contact Tech Support. If you look at the example spectrum of a tungsten lamp (in the fibre bundle section), the ringing is clearly visible. What calibration information did you get with the spectrometer? I'd guess just px to wavelength and linearity correction. – Josh Feb 13 '17 at 1:36

I think those are interfernce fringes. I have attempted to measure the peak positions from your graph and I find they are all integer divisors of about $6.8 \mu$m. That suggests to me you have something in your spectrometer acting as an etalon with a spacing of $6.8\mu$m.

Peak λ (/nm)   6.8μm/λ
448        15.1
481        14.1
518        13.1
568        11.9
620        10.9
681         9.9
754         9
849         8
963         7

• Wouldn't the spacing of that "something" be related with the angle of incidence of the beam and with the index of refraction of that "something"? Cheers John :) – Vladimir Vargas Sep 21 '16 at 1:01
• @VladimirVargas: yes, there will be a factor of $\cos\theta$ in there as well as a factor of $n$. – John Rennie Sep 21 '16 at 4:55