MetroLaser, Inc. provides a range of high and low-speed digital holography systems for your depth imaging needs. Based on the optical configuration and imager used, each system listed below can be tailored to accommodate your field-of-view, depth resolution, magnification, standoff distance and acquisition rate.
Low-speed high-resolution digital holography
Fast, portable, sensitive, and easy to use Imaging Technology
MetroLaser’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.
Portable Projection Schlieren System is based on our patented Digital Focusing Schlieren technology (US Patent 9,232,117 B2).
MetroLaser has also licensed a galvanometer-based digital schlieren streak camera design from MetroLaser Inc. The original camera was designed by Ben Buckner and Drew L’Esperance at MetroLaser for imaging of supersonic rocket sled tests under an SBIR contract for the U.S. Air Force (Contract No. FA9201-08-C-0260) and was successfully demonstrated at the Holloman High Speed Test Track. That highly ruggedized unit was intended to operate in conditions in the desert near the track, so it is overbuilt for many applications, but the basic design can potentially be applied to many streak camera applications. The main advantage over other rotating mirror streak camera designs is that the galvanometer mirror can be synchronized with targets on the fly, so it is uniquely suited to situations where the target velocity is variable or unpredictable. These cameras are not available as a standard product, but please contact Spectabit Optics LLC regarding applications for which the design can be adapted.
Several press articles on the original system have appeared on sites such as optics.org, and it is more thoroughly described in a 2013 paper for Optical Engineering by Spectabit Optics LLC cofounders Ben Buckner and Drew L’Esperance.
Schlieren Streak Image of Shotgun Pellets in Flight
Digital focusing schlieren imaging is a patented (US 9,232,117 B2) complementary technology to the analog Schlierenscope.
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).
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 sales@spectabit.com 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.
Most basic form of the digital schlieren camera
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. 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 links below:
Very old WMV schlieren video of vapors rising from a glass of applejack (produced with a very early version of the sTube™ system and a 19 inch computer monitor on a kitchen table)
MetroLaser will continue to pursue state-of-the-art research and development as well as the commercialization of optical diagnostics systems to measure flow velocity, temperature, chemical composition, surface temperature inside gas turbine engines, and non destructive inspection of composites and other components.
