Spectabit’s Portable Projection Schlieren System (see also our brochure) 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 sTube™ 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 field 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 SchlierenView™ 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.
This innovative system layout has an enormous practical advantage over most schlieren systems. The components are lightweight and self-aligning and the focusing schlieren grid is projected using an off-the-shelf digital projector, making this one of the few realistic options for truly portable and large-scale schlieren imaging in the field. The two optical components, the sTube and the digital projector can be mounted on simple tripods. The only other equipment needed is a stand for the laptop computer that controls the system. Even the projection screen is optional. In many cases, a flat, reasonably brightly painted wall is sufficient even for a 5-foot tall schlieren field of view. A screen only becomes necessary in the presence of high levels of background light or extremely long working distances (usually more than 15 feet), and even then any conventional projection screen works.
A video taken with one of these systems (10 fps, 1200×1920) can be seen at this link.