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.