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Spectral Calibration


Calibrating a CCD spectrometer requires establishing the relationship between the pixel number in the CCD array and the wavelength. To acquire a spectrum suitable for calibration you’ll need a light source – a mercury vapour lamp is often recommended because mercury (Hg) vapour gives a number of spectral lines that are prominent and nicely spaced. However extreme care must be exercised when using a Hg vapour lamp as it can produce intense UV radiation - a much safer alternative is to use an inexpensive fluorescent lamp, as discussed below.

In the activity described here, a spectral data file called “mercury 0.02” will be used for calibration. The data in this file is in ascii format and consists of a single column of intensity values, with 2048 entries in all. The spectrum was obtained by recording the emission from a Hg lamp using a transmission grating spectrometer, CCD-TGS.

The steps below describe the use of a LabVIEWTM vi that determines the calibration constants in a quadratic fit relating pixel number to wavelength.

Steps

1) Start LabVIEW™ and run CalibFileSpectrum.vi
Click on the forward arrow button near the top left of the screen. A file list will appear prompting you to open a file for calibration.

2) Select and open the “mercury 0.02” data file for calibration
Examine the spectrum. You will notice that there are five peaks in the data – three on the left-hand side of the display and two on the right. These peaks are five strong emission lines in the mercury spectrum.

Wavelength Colour of Emission Line
366.0 nm Near ultraviolet
404.7 nm Violet
435.8 nm Blue
546.1 nm Green
578.0 nm* Yellow

For reference, a list of these emission lines is included under “Hg lines” on the right-hand side of the LabVIEW™ screen.

The peak at 366.0 nm is actually a compound triplet and is best left out of the calibration process for that reason.

(*) The peak at 578.0 nm is actually a doublet comprising peaks at 577.0 nm and 579.1 nm. These separate peaks are not resolved by the CCD-TGS and are best treated as a single peak with a wavelength of 578.0 nm for the purposes of calibration.

The data is displayed as a graph in the Waveform Display box with the pixel number of the CCD along the X axis and its signal strength on the Y axis.

3) Familiarise yourself with the tools for determining the values of peaks in the spectrum
In the Waveform Display box are yellow cross hairs that can be moved using the cursor to coincide with a peak in the display, allowing its pixel number and signal strength to be read from the boxes immediately under the Waveform Display box.

To allow the value of peaks to be accurately measured, sections of the Waveform Display can be enlarged and fine movements of the enlarged section made by clicking on the magnifying glass and hand icons immediately under the pixel number and signal strength boxes described above. Also, to the right of these boxes is a padlock icon. Clicking on it allows the cross hairs to be moved freely (“Free”) or made to automatically snap to a nearby peak in the waveform (“Snap to Point”), thereby allowing them to be rapidly positioned and the pixel number of the peak to be determined.

4) Determine the pixel number of each of the peaks in the spectrum
There are five peaks in the mercury vapour spectrum in the “mercury 0.02” data file. Moving from left to right, the first peak corresponds with the mercury vapour emission line at 366.0 nm and is ignored (see above for details); the remaining four peaks, in wavelength order, are emission lines at 404.66, 435.83, 546.07 and 578.00 nm.

5) Enter the values of the calibration points into the calibration table
For the “mercury 0.02” data file there are four calibration points – the peak values determined in step 4), excluding the peak corresponding to 366.0 nm. Each determined value is called a “channel” and the wavelength of the corresponding emission line in the spectrum in nanometers (nm) is called “value”. The data pair for each calibration point is entered into the calibration table. For example, for the “mercury0.02” the (channel, value) for the first calibration point is (535.0, 404.7).

6) Use the hand cursor to indicate the number of calibration points
For “mercury0.02” the number is 4.

7) Perform the calibration
Set “Type” to [Quadratic] and then click [Calibrate].
LabVIEW™ calculates the polynomial coefficients of the fit – a constant term, a linear term and a quadratic term. We obtained the following values:

constant term = 295.502
linear term = 0.224866
quadratic term = -0.000027

The calculated wavelengths of each of the entered peaks are also displayed and the wavelength error for any single peak should be small – no more than 0.3 - 0.4 nm. In the case of the test file the values we obtained (exptl, calc) were (404.66, 404.92), (435.83, 435.44), (546.07, 546.38) and 578.00, 577.82). The linear term shows that the spacing per pixel is only 0.203 nm so a calibration error of 1-2 pixels is quite acceptable.

The image below shows the LabVIEWTM front panel at this point in the procedure - the spectrum in the right hand window is the calibrated version.

8) Once you are satisfied with the fit, click the [Done] button
You will be returned to the CalbFileSpectrum.vi panel with your spectrum displayed, now with a calibrated wavelength scale. As a check you might wish to use the cursor to measure the wavelength of the near UV feature in the spectrum – it should be close to 366 nm.

At this point your options are to [Save] this spectrum, to process [Another] or quit (Done).

9) Stop and Close CalibFileSpectrum.vi
Click on the red stop button near the top left hand side of the screen. Click on the red [close] button at the top right hand side of the screen. Push the [Don’t save SubVIs] button and return to the main LabVIEW™ menu.

Calibration at Longer Wavelengths - Neon Emission Spectrum


Small neon indicator lamps are readily available and at very low cost. For example, an NE-2 neon lamp can be run from either a 110V or 240V AC mains supply providing a series resistor is used (typically 220k for 240V operation). Care must be taken at all times when working with mains wiring !!

In the spectrum at right, light from an NE-2 is coupled into an ILX511-based CCD spectrometer with a concave grating, a spectrum is then recorded and CalibFileSpectrum .vi is used for calibration, using the following prominent Ne lines :

585.249
614.306
703.241
724.517

Some further useful information on spectral calibration can be found here - http://www.astrosurf.com/buil/us/spe2/hresol4.htm

Typical Spectra


Here are some examples of typical spectra obtained using an ILX511 CCD detector with the CCD processor board.

The wavelength axis is first calibrated using known Hg wavelengths as seen in the bottom spectrum, as described above.

The cool white fluorescent lamp spectrum shows the signature, or fingerprint of the same Hg emission lines plus some other features due to rare earth elements in the phosphor coating on the lamp's envelope.

The top spectrum of a tungsten bulb demonstrates continuous, as opposed to discrete emission but also reveals some ripples, likely due to some interference fringing effects.

Emission Spectrum of a "Cool White" Fluorescent Lamp

Stacks Image 2518
Fluorescent lamps contain a small amount of mercury (Hg), and the emission from these lamps therefore shows Hg lines. The phosphors in these lamps contain rare earth elements; these also emit giving rise to a spectrum with many useful features for calibrating the wavelength scale of a spectrometer.

Checking the Spectral Characteristics of LED's

Stacks Image 2009
LED's are extremely useful light sources in low cost instruments but before using them for this purpose it desirable to record their emission spectra. Here, a concave grating spectrometer is used to record the emission from 3 LED's with nominal wavelengths of 405 nm, 525 nm and 590 nm, respectively. From left to right the colours of the LED's are violet, green and yellow.