Spectrometer Calibration Lab
By: Makenzie Thieme
Purpose:
The purpose of this lab was to focus on characterizing and calibrating a UV/Vis spectrometer
with a 2048 pixel back-thinned CCD detector. We did this by utilizing a specific set of code on
Jupyter notebook in order to plot dark current at various temperatures and describe their
relationship, and to calibrate the spectrometer using a spectrum of a Hg lamp.
Theory:
Dark current is the small electric current that flows through photosensitive devices, such as CCD
detectors. It is one of the main sources for noise in image sensors and is used to determine any
defects by monitoring the peaks in the dark current histograms based on temperature. Dark
current is due to the thermal excitation of electrons into the conduction band and collection in
the CCD wells. The generation of dark current is heavily dependent on temperature because it
is a thermally activated process. Cooling of the CCD to a low temperature would result in a
smaller amount of dark current. The second part of this lab dealt with the calibration of the
CCD detector using a Hg lamp. Wavelength calibration is used for accurate and repeatable
analysis of spectral data. By using a Hg lamp, two or more emission lines are identified along
with the pixel positions on the CCD and then inserts the data to fill in any gaps between the
spectral lines1.
Results and Discussion:
Graph 1: Dark current spectra at various temperatures
1Case, M. Spectrometers: Spectroscopic calibration uses LEDs and nonlinear optimization, 02/01/2016
Graph 2: Average Dark Current vs. Temperature
Graph 3: Hg Spectrum at 10°C
Graph 4: Calibrated Hg Spectrum at 10°C
For the first graph, dark current is being observed at different temperatures in the CCD
detector. Each color line is representative of a different temperature and this is plotted based
on the intensity of the dark current and the number of pixels in the detector. The blue line
represents dark current at 10°C and the purple line represents the highest temperature at 30°C.
Based on what we know about the relationship between dark current and temperature, we
would expect the dark current at a higher temperature to have a higher intensity than the rest
of the lower temperatures. Graph 2 takes the average dark current at each temperature and it
is plotted based on the average intensity and the spectrometer temperature. Again, we can see
that the spectrometer temperature at 30°C has the highest intensity, and the lowest
temperature have the lowest intensity. Graph 3 is Hg spectra taken at a specific temperature,
10°C. This was plotted against average intensity and the number of pixels in the detector.
Determining the number of wavelengths should correspond to the number of peaks observed
on the graph. In graph 3 we see seven peaks, corresponding with the number of Hg
wavelengths identified at 302.150 nm, 313.155 nm, 334.148 nm, 365.015 nm, 404.656 nm,
407.783 nm, and 435.833 nm. Graph 4 is the calibrated Hg spectrum at 10°C, plotted based on
intensity and the corresponding wavelength of the peaks identified in graph 3. We then
calculated the difference between our calculated and actual peak wavelengths. We
determined the differences to be 0.26781881, -0.14568902, 0.04721093, 0.27341129,
-0.02014176, 0.04622084, and 0.11235273. By these numbers we can see that there is a small
difference between the calculated peak wavelengths of the Hg spectrum and the actual
wavelengths.
Conclusion:
In this lab we focused on characterizing dark current and calibrating the CCD detector using a
Hg lamp. We generated a few graphs, analyzing dark current based on temperature and their
corresponding intensities. We saw that the actual peak wavelengths determined were pretty
accurate to the ones that were calculated. The process of running the code was a little difficult
but eventually we were able to generate a calibrated spectrum of the dark current data at 10°C.
There weren’t many errors in this lab because we weren’t able to move on to the next part of
the code until the last part was correct.