A look at lightsabers and diffusive media

To recap, we are trying to make a lightsaber by projecting a lightfield into a diffusive medium. Today we’ll take a quick look at what diffusion does to the beams of light that come out of the fog-saber.

So far, I have been testing optical set-ups using a tank of water with droplets of cream mixed in. The fat-cells in the cream act as diffusors, and with a little stirring, it’s easy to disperse them to make a uniform diffusive medium. That means the light will be scattered the same in all directions, with more scattering happening the longer the light travels in the medium. Here, I increased the concentration of cream in 0.1mL increments and took pictures of the result.

Without further ado, on the left you can see the result with a focused flashlight, and on the right, you can see the result of the “fog-saber v0”.

gif_focused-torch gif_v0

We can see that at first, when the concentration is little to none, the light penetrates a long distance. The result is better with the fog-saber (right) than with the focused flashlight, since the rays diverge faster. As the concentration of diffusor increases, we can see that both beams of light shrink until they are short beams. The result actually looks very pleasing (both in the camera and in real-life).

The experiment isn’t complete, however, since there are many artifacts in the images due to capturing this with a cell-phone camera. Hopefully I can fix those so I can better show that the fog-saber design is better than an ordinary flashlight for this task.

flash fogsaber

Artifacts:

  • Image saturation: due to the low-dynamic range of the cell-phone camera. That means the center of the saber is a constant bright white. In real-life, that’s not the case. It’s actually very visible to the eye (which has a greater dynamic range than a cell-phone camera) how the fog-saber does a better job than a flash-light.
  • Rolling-shutter: The LED that runs the flashlight (left GIF) is strobed. The rolling shutter of the cell-phone camera captures each row sequentially, and since the LED is strobed, parts of the image are on while parts are off. The effect is those horizontal streaks throughout the image, which are pretty cool looking.
  • Not putting a Black background under the water-tank: There is a wood floor underneath the water-tank, which means there are brightness patterns which don’t go away. Since the camera is never perfectly still, it’s hard to background subtract. This artifact shows up when trying to numerically plot the brightness at each pixel, since the brightness is a mix of both the light-beam and the background. You can see the result as jagged peaks in at the end of the plot.

 

 

 

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