Focusing schlieren

I wanted to try and make a focusing schlieren system.

Initial prototype

The first thing I needed was a cutoff grid. Something with a set of regular spaces hole in an opaque material. A stainless tea strainer happened to be the perfect donor, pack of 2 for $13! I clamped the perforated stainless sheet in between some lasercut holders and the mask was done.

For the optics, I used an old projector lens and my monitor to create the structured light and a raspberry pi camera to image through the mask.

Projector calibration

Because I didn't know the exact position of every hole in the cutoff grid, nor the lens parameters, I needed to calibrate the system end to end. To do this I wrote a scipt that displayed bands on the monitor at various spacing and widths. This allows each pixel on the camera to be mapped to an point on the monitor.

Results

This is the video straight from the camera for a mini butane torch being lite.