Portable Projection Schlieren

Schlieren imaging is a popular technique for visualizing distortions in transparent media like air and water. These distortions can be related to such important physical parameters as temperature, density, pressure, and composition, so the schlieren effect can be used to create images of heat flow, gas flow, shock waves, intense sound, and dozens of other phenomena that are critical in science and engineering. Schlieren imaging in general was developed in the 19th century and relies on forming an image with rays that pass by a sharp cutoff filter, which is arranged so that the ray intensity has a steep derivative along the edge of the filter. This arrangement allows small deviations in the path of the rays to produce a large change in the image intensity from the undeviated level. Classical schlieren systems though use light collimated by optics such as mirrors or lenses, which effectively limits the area under test to the size of the optics and makes the technique very difficult and expensive to use for larger imaging areas.

Spectabit’s Portable Projection Schlieren System is based on our patented Digital Focusing Schlieren technology (US Patent 9,232,117 B2). It employs a digital projector to project a pattern of lines onto a screen behind the object. The screen reflects the light back into the sTubeTM optical system where it passes through a cutoff filter before it reaches the camera sensor. The camera is focused on the schlieren object (a heat flow, for example), not the screen or cutoff filter, and the depth of faield is shallow enough that the screen pattern is largely or completely defocused. In the absence of density gradients, light rays travel straight from the screen to the cutoff filter, which blocks approximately half of the light rays. Advanced image processing algorithms in the included SchlierenViewTM software are applied to remove background noise and enhance contrast, resulting in a background of uniform intensity. Hot and cold air currents, gas vapors, and shock waves create zones of varying density that bend light rays, thereby distorting the line pattern projected on the background screen. The distortion alters the amount of light that passes through the cutoff filter to the image sensor. In the resulting image, air currents and shock waves appear as sharply focused light and dark objects etched in a gray background. The system is so sensitive that it is possible to see warm air rising from the palm of an outstretched hand.