“Seeing” Infrared Light
A Classroom Demonstration
By Allan Treiman
Purpose. You and your students will ‘see' infrared light, normally invisible to the human eye, as a continuation of the visible rainbow spectrum.
Introduction. The existence of infrared or invisible light is difficult to demonstrate convincingly. We use invisible light (electromagnetic radiation) all over – TV/VCR remote controls, wireless computer connections, radio and TV transmissions, microwaves for communications and cooking. Our eyes can detect only a small part of this electromagnetic spectrum.
This lab relies on an oddity of the light sensors used in digital cameras, which are either CCD or CMOS detectors. Both kinds are sensitive to visible light (which is why they are used), but are even more sensitive to infrared light near the visible spectrum (see graph). So, for a CCD or CMOS camera to match what a human sees, all of the infrared light (wavelengths longer than 700 nanometers) must be removed with a filter. Removing this filter (as in the Webcam Modification web page ) restores the CCD/CMOS's sensitivity to infrared light.
Light detection efficiencies of CCD and CMOS compared to the three types of cone cells (red, green, blue) in the human eye. Rainbow spectrum for scale. CCD/CMOS are more efficient than cones, and are sensitive to infrared (invisible) light with wavelengths out to ~1000 nanometers (nm).
- Overhead projector
- Cardboard with a slit in it, ~ 2 mm wide and 5–10 cm long
- More cardboard or other opaque stuff to cover the face of the overhead projector
- Diffraction grating or prism (either glass or plastic)
- Screen (or white paper or cardboard) for viewing the spectrum
- Modified (hacked) webcam, following instructions on linked pages
- Computer and software to acquire and display webcam images
- (Optional) LCD projector to put computer display and webcam image on screen/wall
Demonstration setup. Arrange the overhead projector so it points to the side of a white screen (or posterboard or paper pad). Center the cardboard with the slit (pointed up-down) on the face of the overhead, and cover the rest of the face with more cardboard so that light can only come through the slit. Hold the diffraction grating or prism outside the angle part of the overhead projector, so that the beam of light from the slit hits the grating or prism.
Experiment with the orientations of the grating (or prism) and projector (or paper) until the rainbow projects onto the screen. With a diffraction grating, you may need to rotate it (around a horizontal axis) until the rainbow spreads out horizontally. With a prism, hold it so its faces are vertical, and rotate it (around its vertical axis) until a rainbow forms somewhere in the room. Then, rotate the prism and move the projector until the rainbow falls on the screen.
Set up the modified webcam and software following manufacturers' instructions. Place the webcam to view the screen (on the overhead projector face works). Confirm that the webcam works, and its image displays on the computer/projector).
The setup. Slit in cardboard on overhead projector lets a beam of light through to the diffraction grating (held by Tori). Light passing through the grating projects a rainbow spectrum on the white paper. Modified webcam, the round ball with cord, sits in a convenient place and is aimed at the rainbow.
The spectrum. As projected through the diffraction grating onto the white paper. Julie holds her finger at the limit of what she can see at the red end. From this photo, it is clear that the camera is sensitive to light further left (longer wavelengths) than Julie (or I) can see.
Demonstration. The point here is to compare what your students see with what the modified webcam registers. As we did in the workshop, it is nice to be able to project the webcam image on a screen next to the spectrum. Having a student place their finger at the farthest red they can see provides a reference point for showing how far the CCD/CMOS detector in the webcam is sensitive.
The Finger. Julie's finger on the rainbow spectrum, same scene as above, but viewed through the modified webcam. All the red, pink, and white to the left of her finger is infrared light, detected by the modified webcam but invisible to humans.
The Spectrum. Without Julie's finger. You can see that the IR part of the spectrum, leftward of about the middle of the red, is about the same length as the visible part of the spectrum. This shows that the invisible IR part of this rainbow is about as big a range of wavelengths (1000–700 nanometers) as the visible part (400–700 namometers).
Extensions. You can do about a million things with the IR-enabled webcam — basically any exercise that involves observing light.
- If you point a TV/VCR remote at the camera, you can see that the remote makes light when you push a button.
- Plant leaves appear very bright in infrared (remember the ALTA spectrometer lab?), and that shows up nicely.