Ocean Optics S2000 - Manual (Page 103)

Appendix C: Calibrating the Wavelength of the
Spectrometer
The following describes how to calibrate the wavelength of your spectrometer. Though each spectrometer is
calibrated before it leaves Ocean Optics, the wavelength for all spectrometers will drift slightly as a function of
time and environmental conditions.
You are going to be solving the following equation, which shows that the relationship between pixel number and
wavelength is a third-order polynomial . . .
λ
p
= I + C
1
p + C
2
p 2 + C
3
p 3
. . . where
λ
is the wavelength of pixel p, I is the wavelength of pixel 0, C
1
is the first coefficient (nm/pixel), C
2
is
the second coefficient (nm/pixel 2 ), and C
3
is the third coefficient (nm/pixel 3 ). You will be calculating the value for
I and the three Cs.
Setting Up
To re-calibrate the wavelength of your spectrometer, you will need the following:
" A light source that produces spectral lines. Ocean Optics’ HG-1 Mercury-Argon lamp is ideal for this purpose.
If you do not have an HG-1, you will need a spectral line source that produces several (at least 4-6) spectral
lines in the wavelength region of your spectrometer.
" Your spectrometer.
" An optical fiber (for spectrometers without a built-in slit, a 50-
µ
m fiber works best).
" Either a spreadsheet program (Excel or Quattro Pro, for example) or a calculator that performs third-order
linear regressions. If you are using Microsoft Excel, choose Tools | Add-Ins and check AnalysisToolPak and
AnalysisToolPak-VBA.
Calibrating the Wavelength
of your Spectrometer
1. After placing OOIBase32 into Scope Mode, take a spectrum of your light source. Adjust the integration time
(or the A/D conversion frequency) until there are several peaks on the screen that are not off-scale.
2. Move the cursor to one of the peaks and carefully position it so that it is at the point of maximum intensity.
Record the pixel number that is displayed in the status bar (located beneath the graph). Repeat this step for all
of the peaks in your spectrum.
3. Using your spreadsheet, create a table like the one shown on the next page. In the first column, place the exact
or true wavelength of the spectral lines that you used. Most calibration line sources come with a wavelength
calibration sheet. If you do not have a wavelength calibration sheet for your light source, you can probably
find the wavelengths for your spectral lines (assuming that they are being produced by pure elements) in a
Chemistry or Physics textbook or handbook, such as the CRC Handbook of Chemistry and Physics. In the
second column of this worksheet, place the observed pixel number. In the third column, calculate the pixel
number squared, and in the fourth column, calculate the pixel number cubed.
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