Does perspex effectively change solar spectrum or compact fluorescent light spectrum recorded with CCDSPEC spectrometer?

I mentioned at the RAG meeting on Friday evening that I was making a portable calibration light using a compact fluorescent bulb in a desk stand and that I was making a Perspex diffuser to go in front of it.

Lee wondered whether the Perspex would change the recorded spectrum due to its absorption characteristics. I am very grateful to Lee for pointing out this possibility as it clearly needs to be checked, especially as being able to know the exact wavelengths of the peaks on the spectrum produced by the bulb is vital to the calibration process and if these change as a result of the Perspex then this will mean that I will need to be careful using this material during the calibration process.

I have this tested this hypothesis this evening. If the Perspex does change the spectrum then I will know that it will limit the use of the lamp as a calibration device.

My test below used the solar spectrum initially, followed by use of same compact fluorescent bulb as being used in my portable calibration light.

Result of these tests = Perspex sheet reduces intensity but does not effect spectrum – therefore can be used as diffuser in front of calibration light without problems.


Solar spectrum taken by pointing CCDSPEC spectrometer out of window (not attached to a telescope) – this spectrum is without anything in front of the CCDSPEC spectrometer:

The following spectrum was taken as above but this time I placed the piece of Perspex I intend to use as the diffuser on the new portable compact fluorescent bulb calibration lamp in front of the CCDSPEC touching the 2 inch eyepiece adapter so that very little light could get into the CCDSPEC without going through the Perspex (below):

In the following image, I have compared the above two spectra and added some lines to demonstrate that absorption lines are essentially unchanged between the two:

The above spectra are not calibrated on x-axis. The following is a repeat of the image with Perspex in front of the CCDSPEC but this time calibrated to give wavelength on x-axis in nanometres (nm):

The above shows that the solar spectrum as shown on CCDSPEC stretches from essentially 400nm to 750nm – this will be limited by the instrument’s response as the solar spectrum extends either side of this.

The following graph shows typical transmission characteristics of variety of plastic materials including acrylic from 200-1200nm:

From the above spectrum, it is unlikely that plastics make little difference to the spectrum of light over the range of wavelengths covered by CCDSPEC, and this appears to be born out by my solar spectra today.

Spectrum from CCDSPEC pointed at desk lamp with compact fluorescent bulb WITHOUT Perspex between them (darkened room, below):

Repeating above but this time placing Perspex over end of CCDSPEC – compact fluorescent bulb, darkened room:

Perspex can be seen to reduce intensity but does not meaningfully change the spectrum so will not adversely affect the ability of the Perspex covered light to act as calibration instrument. The reason that I want to add perspex in front of the calibration light is to act as a diffuser and give even illumination (the Perspex has appropriate surface to act as diffuser).

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