Jonathan Thomson's web journal

Spectrometer Articles April 24, 2010

 

In this series of articles I would like to explain how you can use a few simple materials, a digital camera, and a bit of computer processing to obtain information about and plots of a light source’s spectrum. First I think it’s important that we are on the same page with respect to terminology. These are not standard definitions and elsewhere you will often find the words spectrum, spectrograph, and spectrogram used interchangeably as well as spectroscope and spectrometer. However, my definitions will make it both easier to understand and explain how to turn a simple spectroscope, which produces spectrographs (qualitative photographs), into a spectrometer that produces spectrograms (plots) which can be used to perform a quantitative analysis on a light source.

 

Definitions
diffraction grating – separates light by wavelength.
spectrograph – a photograph of the component wavelengths of a light source.
spectroscope – an instrument that uses a narrow slit and a diffraction grating to produce a spectrograph.
spectrogram – a plot of intensity vs. wavelength.
spectrometer – a device that produces spectrograms.
photosite – a photosensor often covered with a color filter, also called a pixel.
pixel – in a color image a pixel is the result of interpolating the data from one red, one blue, and two green photosites. In a Bayer raw image one pixel is the data from just one photosite. This image should help to illustrate the concept.

 

 

The cameras used in this project are the Canon PowerShot A75 and the Canon PowerShot A590 IS. The A75 is JPEG only and can be controlled with the remote capture program Cam4You. The A590 with the CHDK firmware supports raw capture and a simple USB remote control. Both cameras support lens adapter tubes.

 

How to Build A Spectroscope
This article explains how to construct a spectroscope using a cardboard box, razor blades, a diffraction grating, and a digital camera that can be remotely controlled.

 

Convert Spectrographs to Spectrograms
This article explains how to use a computer to convert spectrographs in to spectrograms that have a calibrated wavelength scale.

 

Determining the Spectrometer’s System Function
A spectrometer’s system function is a measurement of how the spectrometer responds to different wavelengths of light. If a camera outputs a spectrogram Y(λ) in response to a stimulus X(λ) then the spectrometer’s system function is H(λ) = Y(λ)/X(λ). The spectral radiance of an incandescent light bulb can be modeled using a blackbody radiation curve. Therefore if we record a spectrogram using an incandescent bulb as the stimulus then we can derive the system function. Once the system function is know we can use it to remove the distortion it causes in our spectrograms.

 

Radiometric Calibration
With the addition of a sensor from the TSL230 family of irradiance sensors we can radiometrically calibrate our spectrograms.

 

Noise Reduction
Ensemble averaging, dark frame subtraction, and flat-field correction can all be used to improve the SNR of a spectrogram.

 

Determining Camera’s Gray and Taking RGB Flats
A one-shot color camera records the intensity of three separate colors – red, green, and blue. In order to capture a flat frame without a color cast it’s necessary to use a light source that results in each channel of the flat having the same gray level.

 

Results
The accuracy of the spectral and radiometric calibrations are examined. The effects of different noise reduction techniques are demonstrated.

 

Code
Here you will find the download location for the spectrometer code and sample spectrographs as well as an index of all the code converted to HTML.

 

Links
History of spectroscopy

 

Continue Reading

>>>Next — How to Build a Spectroscope>>>

 

 

2 Responses to “Spectrometer Articles”

  1. [...] being monochromatic, as would LEDs. You can also add lamps with known emission spectra – Jonathan Thomas uses the spectrum of a fluorescant lamp; there is no particular reason that you couldn’t similarly use [...]


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