Invisible Strobe Flash

By Jerry Reed
Category: Photography & Video
Difficulty: Moderate

A few years ago, I began documenting the bats flying around my backyard bat house, using inexpensive, monochrome security cameras and a DVD recorder. The little mammals streaked impressively through the frame, but I was disappointed when I started single-framing through to see what they looked like. In each frame showing a bat, it was a mere blur.

I decided to use a xenon strobe to freeze the bats in the images. It wouldn’t be synched to the video frame rate, but the combination would be statistically likely to capture at least a few usable stills.

The problem was, some bat experts I consulted thought the strobe would delay their activity and might even drive them away. So I needed to make a subtle strobe, which sounds like an oxymoron.

I knew that xenon tubes produce a wide spectrum, and many tinted plastics block visible light but pass near-infrared (NIR) wavelengths that monochrome cameras are sensitive to. After some experimentation, I found that filtering the flash through 5 layers of “Limo Black” window tint film darkens it to a dim purple spark, but illuminates the field of a sensitive video camera from 10 feet or more.

Steps

Step #1: Set up the strobe and filter.

For an automotive strobe, just wrap 4-5 layers of film around the face and duct-tape them together in back. Leave the leads hanging out so you can connect a battery or other 12V DC power source.
For plug-in strobes, make a filter by stacking 4–5 squares of film (there’s nothing magic about 5 layers; you should experiment). Don’t remove the film’s transparent backing, which makes it easier to handle. If the strobe has a slot for a colored filter, slide it in there. Failing this, just stick the film over the strobe with duct tape. I mounted my camera and strobe to a board to make a single unit. It’s ugly, but we’re using this in the dark, right?