Last weekend, a friend and I spent some time photographing laser beams.
Laser beams are not directly visible in air, and so talcum powder was used to scatter light. The first such image (of two laser beams) was generated by taking a long exposure shot in a darkened room. Talcum powder was spread by a person standing in front of the screen, and the long exposure resulted in the image of the moving person being almost completely “averaged out”.
Initial Method of viewing a laser beam.
Admittedly, this particular photo has a number of salient flaws: the non-uniform background; the lack of contrast between the laser and the screen; the slight shake in the image from the pressing of the shutter release.
The set-up was then reworked to use a single laser device. Two mirrors were used to bounce a beam back and forth in front of the screen. The screen itself was moved towards the camera, and a ladder was placed behind the screen, allowing a person to spread talcum powder easily while not appearing in the image. The camera itself was tethered to a computer, and the shutter release was triggered over USB, eliminating camera shake.
Subsequent images showed substantial improvement:
Long exposure shot of laser beam.
The ray from the laser source is the bottom-most one, with the source placed on the right. The lowest ray is much more well-defined than the highest one, suggesting that either the laser source is not particularly good (the beam divergence is high) or the mirrors are imperfect (either with dust on the surface or an imperfect reflective surface). The gap between lasers decreases upwards, once again due to the mirror being slightly warped.
The red “glow” around the laser beam is probably due to secondary scattering by talcum powder between the camera lens and the plane of the lasers. This effect was especially pronounced towards the end of this experimentation, when a large amount of powder was present in the air.
Compare the above image to one that has a (relatively) short exposure time:
Same setup, shorter exposure.
You can see the contribution a single particle makes towards the entire image by examining the left side of the image. The glow is also substantially less intense in this image.
Here is a photo of the set-up with the screen removed:
Behind the screen.
Notice the bottle of talcum powder?
Future expansion for this includes using a laser “slice” to track the flow of particles within a moving fluid.