Backlight for high-speed video whisker tracking

Here’s a simple recipe for a very bright uniform background for high-speed videography. This approach will work well for any applications where the outline of a small objects needs to be measured at high frame rates. For vibrissa tracking, or calibrating piezo stimulators I currently only use a single such backlight and no other light sources – this avoids any reflections on the tracked object and usually gives the cleanest, most interpretable data.

Obligatory warning:  Do not look at LEDs with unprotected eyes – these things get extremely bright and you might damage your retina. 

backlight on

The basic design has three components, bottom to top:

  1. A customizable array of LEDs, attached to a heat-sink, with a power supply and current regulator.
  2. A spacer/reflector made from mirrored acrylic.
  3. A glass diffuser.

LED arraybacklight LED array

For whisker tracking I use either deep red, or NIR leds, in the common hexagonal packaging , in a grid pattern of one led every ~2cm or so, with a sufficiently powerful driver. The density of the LEDs could easily be increased to yield an even more powerful backlight.

I just superglue the leds to a aluminium sheet with a thin layer of insulator in between (here I just used lab tape – not ideal but good enough). The aluminum sheet is then just clamped to the optical breadboard so the whole table works as heatsink. If this is not an option, big CPU coolers are fairly cheap and can remove a lot of heat. As a current regulator I use a BuckPuck from ledsupply.com driven from a sufficiently powerful DC supply (old laptop power supplies work well, or even ATX supplies if 12V are sufficient for your LEDs). Alternatively, a current limited bench supply would also work.

Spacer/Reflector

To get a uniform backlight, I use a square box (just 4 sides) out of _mirrored_ acrylic (from mcmaster). It just sits on the led array in a couple of guides so its always in the same place. Ideally, this reflector should be measured so that the resulting apparent/virtual pattern of LEDs seen by the diffuser is totally uniform, but in practice I found that this does not matter a whole lot.

Diffuserbacklight_LED_diffuserOn top of the spacer, I use a home made diffuser, made from two sheets of cheap frosted glass, glued together (here I just use kapton tape), held at ~5mm from each other with spacers. I just cut the glass myself with one of these. This double diffuser works better than much more high quality single-sheet ones, and is ~10x cheaper. make sure that the construction of the diffuser allows for easy cleaning, so avoid tape or glue that can’t tolerate ethanol.

The trick is to play with the spacing of the mirrored box, and the LEDs until the light is very uniform. This calibration only really works when using a camera, because eyes are surprisingly bad at detecting brightness gradients – also, the brightness of this light can reach unsafe levels so avoid staring directly at the light even with the diffuser.

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