A Toy Spectroscope and Its Operational Diagram

Version 2.3.2 of 22/4/2010-3:08 p.m.

reflection grating toy spectroscope view 1

reflection grating toy spectroscope view 2

A toy spectroscope using a CD or DVD as a grating, can be built easily. This design is based on the CD Spectrometer page by Jerry Xiaojin Zhu. The author was curious how well this spectroscope would perform, so he set forth to build one.

  1. Operational Diagram
  2. Color Spectrum Photos
  3. Improved Operational Diagram
  4. Improved Color Spectrum Photos

The technical specs of this toy spectroscope are very simple: It uses a CD/DVD as a dispersion grating and a slit made of two shaving blades, mounted on paper. The viewer is simply the human eye at an angle of ~90 degrees relative to the incoming light. The CD has roughly 600 lines per mm, so the resolving power is roughly 600 x 40mm (covered track radius on average) = 24,000. That's not bad! That's almost twice as high as the resolving power of the Phasmatron spectroscope around the area of the Sodium D lines!

Operational Diagram

operational optics diagram for the toy spectroscope
Operational Optics diagram for the toy spectroscope.

Color Spectrum Photos

Here are some spectra photographed through this toy spectroscope and a Nikon CoolPix digital camera and processed with Photoshop.

Light Type Visible Spectrum
[1]: This is the spectrum of a high pressure Mercury fluorescent lamp. Not bad! The yellow lines are almost resolved! All the lines are visible, although there are many artifacts, mainly showing as ghosts right of the spectral lines. The ghosts occur because of multiple reflections between the front and back side of the CD. Note that the dispersion is now linear, contrary to the dispersion from prisms, which is non-linear. high pressure Mercury fluorescent lamp spectrum
[2]: The spectrum of a high pressure Mercury fluorescent lamp, same as above, but with the camera zoomed to its limit. Again, all the lines are visible, but the ghosts and artifacts make it difficult to differentiate between what is real and what's not. zoom 1 high pressure Mercury fluorescent lamp spectrum
[3]: Same spectrum as above, this time with the slit closed somewhat more than in [1] and [2]. Note that yellow lines are starting to resolve, but overall brightness diminishes greatly. Note multiple artifacts of the green Mercury line. high pressure Mercury fluorescent lamp spectrum slit small 1
[4]: Same spectrum as above, this time zoomed on the red fluorescence area. The intensity of the green artifacts has diminished, but they still show pretty badly. zoom 2 high pressure Mercury fluorescent lamp spectrum
[5]: Same spectrum as above, with the slit closed even more. The two yellow Mercury lines are now clearly resolved, but the artifacts are still there. high pressure Mercury fluorescent lamp spectrum slit small 2
[6]: The spectrum of a CFL 4,000K triphosphor fluorescent. It's not bad, but the toy spectroscope still generates lots of artifacts which interfere with the actual spectrum lines. The artifacts here are of two kinds: Artifacts from internal reflections between the CD sides and artifacts from second order spectra. cfl 4,000K triphosphor fluorescent spectrum
[7]: Same spectrum as above [6] at a slightly higher magnification, obtained by zooming the camera in. cfl 4,000K triphosphor fluorescent spectrum zoom 1
[8]: Same spectrum as above [6] at a still higher magnification. The two kinds of artifacts are now easily recognizable. cfl 4,000K triphosphor fluorescent spectrum zoom 2
[9]: Same spectrum as above [6] at the highest magnification the camera allows. Unfortunately, at this magnification the camera is not able to focus properly, so this image doesn't do the toy spectroscope much justice. cfl 4,000K triphosphor fluorescent spectrum zoom 3

Ok, so this toy spectroscope can't perform up to lab standards. This was expected. A CD grating is very poor quality compared to commercial quality gratings[1]. Can we now improve the situation a bit? Yes, we can! We can transform the CD from a reflection grating to a transmission grating! That way, reflection artifacts will most likely be gone (we hope). Let us then perform a little surgery on that CD disk:

  1. Cut the CD in half, using large scissors.
  2. Attach some scotch tape against one of the cut edges.
  3. Detach the scotch tape gently, removing the aluminum coating from the CD.
  4. Repeat the above, until all traces of the CD aluminum lining are gone.
  5. Attach the clear CD half against the carton tube, so that the cut diameter is parallel to the slit.

We now have a transmission grating spectroscope! Here's what it looks like:

transmission grating toy spectroscope

Improved Operational Diagram

operational diagram of the improved transmission grating spectroscope
Operational diagram of the improved transmission grating spectroscope

As mentioned above, CD disks have roughly 600 lines/mm, so the grating constant is in this case: d=10-3/600=1.66*10-6m. DVDs have roughly 1351 lines/mm, so the DVD grating constant is d=10-3/1351=0.74*10-6m, so using a DVD is even better! Incidence is perpendicular to the grating, so we apply the formula d*sin()=k*. For the green Mercury line, =546*10-9m, and k=1 for the first order spectrum, =>=arcsin(546*10-9/1.66/10-6) ~ 19.2.

Improved Color Spectrum Photos

Light Type Visible Spectrum Spectral Distribution
[1]: The spectrum of a High Pressure Mercury Vapor fluorescent lamp, through the improved CD spectroscope. Excellent! The artifacts are gone now and the spectrum is nicely discernible. high pressure Mercury vapor lamp spectrum high pressure Mercury vapor lamp spectrum distribution
[2]: Same as above, at a slightly higher magnification. Note that not only there are no artifacts, but the weak 491nm Mercury doublet is nicely visible. high pressure Mercury vapor lamp spectrum zoom 1 high pressure Mercury vapor lamp spectrum zoom 1 distribution
[3]: Same as above, upping still the magnification a bit. Note that the yellow Mercury doublet easily resolves. high pressure Mercury vapor lamp spectrum zoom 2 high pressure Mercury vapor lamp spectrum zoom 2 distribution
[4]: The spectrum of a CFL 4,000K triphosphor fluorescent. Not bad at all! All artifacts are now gone and the terbium/europium and Mercury lines are clearly visible. 4000K compact fluorescent lamp spectrum 4000K compact fluorescent lamp spectrum distribution
[5]: Same spectrum as above [4] at a slightly higher magnification, obtained by zooming the camera in. 4000K compact fluorescent lamp spectrum zoom 1 4000K compact fluorescent lamp spectrum zoom 1 distribution
[6]: Same spectrum as above [4] at a still higher magnification, at a lower exposure to reveal the Mercury yellow doublet, which is clearly separated by thin dark space. 4000K compact fluorescent lamp spectrum zoom 2 4000K compact fluorescent lamp spectrum zoom 2 distribution
[7]: Same as above at still higher magnification. 4000K compact fluorescent lamp spectrum zoom 3 4000K compact fluorescent lamp spectrum zoom 3 distribution

[8]: Spectrum [4] at the highest magnification the camera allows using a digital zoom. At this magnification the camera is not able to focus exactly, but all the major features of the CFL spectrum are visible still. 4000K compact fluorescent lamp spectrum maximum zoom

Note that with a regular CD, the theoretical resolution of the grating would be: = /(k*N), where is the wavelength, k is the order of the spectrum and N is the total number of lines. For the CD grating we have, the grated radius is 4cm = 40mm, so N = 40*600 = 24000, which means: = 5893/(1*24000) ~= 0.24 A, for the first order spectrum. In principle then this spectroscope can resolve the sodium D line, since the difference between D1/D2 is 5.97 A. Let's see then:

[9]: The spectrum of a low pressure sodium discharge. If you look carefully, the D doublet resolves. The real acid test for that is the distribution to the right, which shows a very slight separation between D1/D2! low pressure sodium lamp spectrum low pressure sodium lamp spectrum distribution

Notes/References

  1. Look carefully at Jerry Xiaojin Zhu's CD Spectrometer page. One can see reflection artifacts in these photos as well.
  2. Spectral lines appear curved because the CD's tracks are curved.
  3. For color spectrum photographs taken with a high quality small spectroscope, click here.
  4. For color spectrum photographs taken with a high quality large spectroscope, click here.
  5. To see some of the lamps that produced these spectra, click here.

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