USB Spectrometer

Principle of operation

Principle of operation of the spectrometer.

The spectrometer uses a diffraction grating to split the different spectral components of the incident light.

The incident light goes through a narrow slit located in the focal plane of a converging lens which produces a beam of parallel rays for each point of the slit.

The produced passes through a diffraction grating which deviates each wavelength differently. Each wavelength produces a parallel beam in a different direction

Finally the webcam lens converges each spectral component on a particular point of the sensor located in its focal plane (the webcam is focused at infinity)

In the first order, the angles \(i\) and \(r\) are related by :

$$ \sin r = \sin i + \lambda/a $$

The angle \(i\) is fixed by the spectrometer geometry and the angle \(r\) gives the position \(x\) on the webcam sensor corresponding to the wavelength \(\lambda\) with the relation \(x = f \tan r\). Practically, one measures the position on the sensor of two known wavelengths to find \(f\) and \(i\), the grating used has 1000 lines/mm hence \(a\)=1µm.

Entrance slit and lens

Entrance slit of the spectrometer

The entrance slit of the spectrometer is made of two pieces of brass sheet fixed on an aluminium plate by screws. This system makes the slit width adjustable (but not really easily). The borders of the brass sheet have to be well polished to make the width as regular as possible.

Entrance optics of the spectrometer

The entrance lens of the spectrometer was scavenged from the objective of an old camcorder. It has the advantage of already containing fixation screws and allows to adjust the slit-lens distance.

The grating and webcam

View of the webcam and the grating in front of it.

The grating is an holographic grating containing 1000 lines/mm bought on eBay.

The webcam is a Logitech Quickcam Pro 4000 which has a CCD sensor and is (was) widely used by amateur astronomers.

The software

Screenshot of the software (with the solar spectrum)

In order to control the spectrometer, I created a small piece of software which retrieves the pictures captured by the webcam and processes them to extract the spectrum. It is still full of bugs but allows for example :

To calibrate the spectrometer from lights of known wavelengths

To record spectra by computing the average of a large number of acquisitions (to reduce noise)

To apply mathematical operation on spectra (addition, multiplication, subtraction, division)

To select the sensor area to be used. In order to reduce the noise, the software sums up the values of the pixels of one column

View of the webcam sensor

Example spectra

Solar spectrum view through atmosphere (presence of absorption lines : Fraunhofer lines)

Solar spectrum with absorption bands of the atmosphere

Transmission spectrum of the infrared spectrum of a digital camera or webcam . It blocks IR where the sensor in very sensitive.

Transmission spectrum of the IR spectrum of a webcam

Transmission spectrum of the filter located in front of infrared remote control receivers. It blocks the visible light to prevent interference with ambient light.

Transmission spectrum of an infrared remote control filter.

Spectrum of a Compact fluorescent lamp (energy saving). It presents emission lines of atomic elements, especially mercury (details)

Spectrum of a compact fluorescent lamp

Transmission spectrum of Grenadine syrup. It mainly absorbs green light which produces its pink-red colour.

Transmission spectrum of Grenadine syrup

Pictures