Other pages on this website use the spectra of light sources to analyze the quality or kind of light emitted by various sources of light. The following article shows how the page author obtained these spectra.

The page author has used three different methods to obtain spectra. They are categorized in this table according to how well they work and what can be seen with them.

Diffraction grating glasses
Direct View MemoryMultiple light sources clutter spectra,
Size of source
Diffraction grating glasses
and wrapping paper tube
Direct View MemoryOther light sources shine inside tube,
Size of source
Diffraction grating glasses,
wrapping paper tube, and slit
Direct View MemoryOther light hits tube
Diffraction grating glasses
and black cardboard with hole
Direct View MemoryLight shining on cardboard
Compact Disc
Direct View MemoryMultiple light sources clutter spectra
Compact Disc, slit,
and housing
Direct View MemoryDifficulty of aligning spectroscope parts
Diffraction grating
and camera
Photography Cut and pasteMultiple light sources clutter spectra
Size of source
Diffraction grating, camera,
and wrapping paper tube
Photography Cut and pasteOther light sources shine inside tube
Alignment difficulties
Diffraction grating, camera,
wrapping paper tube, and slit
Photography cut and pasteLight shining on cardboard
Alignment is very difficult

CAUTION! Do not use any of these spectroscopes to look at or photograph the sun.

The light from the sun is too intense, and will damage your eyes or a camera. Put a small white object in the sun on a dark background, and observe the spectrum of the light coming from this white object.

Obtaining the spectrum

There are several easy to find ways to obtain a spectrum:

  1. Diffraction Grating Glasses

    These can be obtained at any education supply store, and at many novelty, toy, or party stores. Most of them make 4 spectra in a cross around the light, with more at other angles.

    This is the easiest to use device. Put on the glasses. One lens can also be placed over the lens of a camera.

    These are cheap. Obtain several pair, as you might want to let others use them, or to cut up a pair to build something else.

  2. Other diffraction grating sheeting

    This is harder to get. Some education supply stores have it, and it can be ordered from Edmund Scientific.

    Look through it, or place it in front of a camera lens

  3. Compact Disc

    This is easily obtained. One suggestion is to use a CD that comes in an advertising packet, or as a computer supplement of a book you don't plan to use. This way, you don't tie up a music or installation CD.

    This is harder to use and align, because the spectrum is a reflection from the CD surface. It occurs at an odd angle. It is hard to determine which source is being observed if multiple light sources are in the area. Also, some CDs exhibit multiple overlapping spectra. Avoid those CDs.

  4. Prism

    This can be found at Edmund Scientific or at many education supply stores. Be sure to get an equilateral prism, not a 45-45-90 prism.

    The chief problem with a prism is aligning it. The light comes in at an odd angle to the prism, and the spectrum leaves it at multiple angles. It will not be further covered here.

Using Diffraction Grating Glasses

There are several easy to observe a spectrum with diffraction grating glasses:

Using a Sheet of Diffraction Grating Material

Transparent diffraction material can be used in the same way the diffraction grating glasses are used. Hold it near the lens of the eye or the camera.

Reflective diffraction grating is used in the same way the CD is used (see below). It has the same limitations, except that there is no curvature making the red wider than the blue. It has the additional limitation that the sheet must be kept flat.

Most sheets of diffraction grating produce only two spectra in opposite directions, rather than four in a cross. There are no spectra at other odd angles. This makes it easier to sort the spectra of different light sources seen at the same time.

Using a Compact Disc

A compact disc can be used to obtain a quick look at the spectrum of a light source:

  1. Turn so the source is behind you and to one side.
  2. Hold the CD near your face, or the entire spectrum will not be visible.
  3. Close the eye away from the CD.
  4. Look at the source reflected in the shiny side of the cd.
  5. Turn the CD so the image of the desired source is at the rim on the side closest to your nose.
  6. The spectrum of the source will be on the side of the center hole opposite from the image of the source.
  7. Adjust the distance of the CD from your eye and the angle of the CD until you can see the entire spectrum.

This would be very hard to photograph, because the alignment is critical.

The red part of the spectrum will be wider than the blue part because the recording is made in a circular direction around the disc.

Using a Prism

A prism is much harder to use than any of the other devices mentioned above. A device must be used to keep the light source, the prism, and the observer or camera lined up.

This page will not further cover the prism.

Tricks in the Eye

Unusual characteristics of the eye:

Tricks in the Film Camera

Unusual characteristics of film:

Tricks in the Digital Camera

Unusual characteristics of digital cameras:

Light Source Seen on a Monitor Screen

Do not try to find the spectrum of a light source that is shown on a TV or monitor screen. You are looking at the spectrum of the monitor screen, not the spectrum of the source. The light from the source has already been color-separated into RGB signals, so the only variation in the monitor spectrum will be the strengths of the red, green, and blue phosphors.

You can NOT match colors under different lights using either RGB representations of them or CMY printouts approximating them. They will have totally different color properties.

Never use a swatchbook printed on a color printer or mass-produced using three-color (CMY) or four-color (CMYK) printing to match pigment colors or select paint colors for a room. Anything other than a swatch of the actual paint or pigment used is useless for this purpose. The actual pigment will be affected by different lights in ways different from the way the inks in the swatchbook are affected.

Spectrum of a Monitor Screen

white spectroscope target Use the image at right to view the spectrum of your monitor.

To view it in isolation, right click on it and choose View Image. Then use the View menu to select Full Screen. This makes everything black except the white marks. Turn off the room lights. Now use the spectroscope.

Look to the right or the left of the marks to see the spectra. Three widths are provided for spectroscopes of different efficiency.

The three colored marks allow you to see the spectra of the single red, green, and blue phosphors.

The spectra of phosphor (CRT) screens, LCD (liquid crystal) screens, LED (light emitting diode) screens, and plasma screens should be different, but not too much different. All of them need the three primary colors of light: red, green, and blue.

To leave full screen mode, move the mouse to the top of the screen and select the Restore icon. Then use the Back button to return to this web page.