Digital focusing schlieren imaging is a patented (US 9,232,117 B2) complementary technology to the analog Schlierenscope. Unlike the Schlierenscope, DFS systems can be self-aligning which reduces the bulk of the instrument and the expense of production. They can be either projection-based or digital-display based. In the former case, the DFS system projects its own grid pattern onto a background such as a projection screen or even a blank wall. In this version, it is very similar to a Schlierenscope in application. In the latter case, the computer controlling the DFS system is connected to an external digital display device, such as a computer monitor, digital television, or digital projector, and this device produces the grid pattern. This allows the schlieren camera system to be reduced to an unprecedented compactness, not much larger than a high end digital camera (below). You can download a copy of our 2015 paper "Digital focusing schlieren imaging" (with minor corrections) presented at SPIE in San Diego in 2015 for details of how it works (or see http://dx.doi.org/10.1117/12.2189533).

Digitial schlieren camera
Most basic form of the digital schlieren camera

The core camera-grid-lens assembly, the sTube™ system, can be combined with a computer and our SchlierenView software with virtually any sort of digital display to produce a functional live-video schlieren system. sTube™ models will be available standard items for purchase in the near future, and special orders are currently being taken (contact This email address is being protected from spambots. You need JavaScript enabled to view it. for details). The sTube™ system can be used as-is with a digital projection system, though in some situations the positioning of the camera relative to the projection beam can be awkward. The flexibility of the software control allows calibration in under a minute to get schlieren images in situations where it would be almost unthinkable with previous schlieren technologies.

More recently, we have developed high-speed versions which use a back-illuminated panel driven with a high speed flash. These systems have been demonstrated with exposures down to a few microseconds. We have also successfully performed digital (and analog) projection schlieren imaging with 10 ns laser pulses, but laser driven systems are still experimental.

 

High-speed digital schlieren display panel
High-speed (microsecond exposure) digital schlieren display panel

 

Mach diamonds from a high pressure nozzle

Mach diamonds from a 3 mm air nozzle, 830 kPa, 1 µs exposure

Schlieren image of fan blowing air from a soldering iron
sTube Pro image of fan blowing air from a soldering iron
Schlieren image of a fan drawing a thermal updraft off of a teacup
Digital focusing schlieren image of a fan drawing a thermal updraft off of a teacup

The highest-quality digital schlieren systems with ultra-high-def pixel count (4k) can reach sensitivity comparable to classical schlieren systems.

We are in the process of developing an assembled single-unit digital schlieren projection system with the receiver and projection units in line. These systems will be somewhat larger and more expensive than the basic sTube™ system but will offer greater ease of use in projection applications, greater flexibility with projection light sources, and are expected to offer greater sensitivity.

Sample videos acquired with sTube™ systems are available from the lnks below:

WMV schlieren video of vapors rising from a glass of applejack (produced with an sTube™ system and a 19 inch computer monitor on a kitchen table)

WMV schlieren video of a hairdryer (produced with an sTube™ system and a commercial digital projector through a beam splitter - the weaker convection originating from the bottom of the image is from the projector exhaust)