Schlieren Instruments

Fast, portable, sensitive, and easy to use Imaging Technology

Schlieren Products

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Focusing Schlieren Products

The table below illustrates the basic matrix of focusing schlieren systems and their ranges of utility. Lower frame rates (FPS) and longer shutters correspond to more economical systems, since they require slower cameras and less light.

Digital Focusing Schlieren * ≤ 3 ft./1 m diameter * more economical Analog Projection Focusing Schlieren * larger areas * (usually) higher sensitivity
Video Rate (shutter > 1 ms, 100 > FPS) sTube™ with consumer-grade digital display Schlierenscope
Short-Exposure Video Rate (shutter ≥ 1µs, 100 > FPS) sTube™ with our FlashScreen display High-Speed Schlierenscope
High-Speed Short-Exposure Video (shutter ≥ 1µs, 100 < FPS sTube™ with our PowerBox display Custom development (contact sales@metrolaser.com)

What kind of schlieren system do I need?

The most important questions to answer when selecting a schlieren technology are usually:

The question of size is a primary determiner of the type of schlieren system that will be practical in an application.  It is rarely practical to build a classical system larger than around 1 foot (~30 cm) in diameter because of the cost and mounting complexity of the large mirrors required. In many cases, a system over 6 inch (~15 cm) in diameter may be more trouble than it’s worth, though for very small areas of a few inches (under 10 cm), they are usually the best solution.

Intermediate-sized schlieren viewing areas, up to about three feet in diameter (~ 1 meter) are fairly easily accommodated with  our patented digital focusing schlieren technology, which uses off-the-shelf digital display systems.

Large-area schlieren systems, more than 3 ft. (1 meter) in diameter, generally have to be projection focusing systems, which project a pattern onto a retroreflective screen (either a conventional projector screen or retroreflective panel). These can be scaled up as large as the largest practical projection screen, though the effective viewing area is around 50-75% of the total screen size. Spectabit offers two main types of projection system, an analog projection system, which is bulkier and requires a more sensitive alignment, and a less expensive but usually somewhat less sensitive digital projection system.

For large-scale outdoor schlieren applications, such as viewing aircraft in flight, the only realistic options are usually background-oriented schlieren (BOS) systems, including our solar limb schlieren technology. These generally have low sensitivity relative to classical and focusing schlieren systems unless the background has particularly high contrast (as with solar limb schlieren) , but they can be useful for visualizing strong schlieren features such as shock waves. BOS generally requires the presence of some kind of textured background, such as a landscape.

The speed requirement of the measurement impacts the type of camera and the type of illumination system. To capture high speed events, such as a firearm discharge, the camera exposure has to be short to prevent blurring and the illumination has to be correspondingly bright. In BOS and projection focusing schlieren, it can be difficult to obtain the required light levels for exposures of less than the millisecond range, though solar limb schlieren can be used down to microsecond exposures because of the Sun’s brightness. One should also consider whether one needs multiple frames (video) or single-shot images. In the former case, the video rate is very important in determining the required system design. Frame rates in the conventional “video-rate” range, less than 100 frames per second or so, are fairly straightforward with all of the above technologies. Frame rates over this may exclude some types, depending particularly on the exposure time. Generally, if exposure requirements are longer than a few milliseconds and frame rates less than 100 frames per second, the system does not have to be particularly specialized. We have a great deal of experience with high speed imaging applications, though, and we can offer practical solutions to a wide variety of more specialized applications with, for example, sub-microsecond exposure or 1000 frame-per-second requirements.

To decide how fast a system’s shutter speed needs to be, the main factors are (A) the required image resolution of the moving object and (B) the absolute speed of that object. The object might be an aircraft, a bullet, an exhaust plume, or an air current. It’s possible that the flow being imaged might be moving faster than the object that created it, so bear that in mind. Take the image resolution in real units, for example, maybe you need to resolve 0.1 millimeters (1E-4 m). To get this figure, you can take the the size of field of view and divide by the number of pixels across it. Then take the speed of the object, say 100 m/s and divide the absolute resolution by it, so 0.1 mm/100 m/s = 1E-6 s, or a microsecond. You could use a longer shutter, but the image will be blurred. In fact, to get good images, it’s really best to have the shutter at least a few times shorter to get a really sharp image.

It’s also important to consider how many frames are needed to see whatever process is being analyzed. In some applications, one might only need one snapshot, while in others, one might need to see a whole sequence of images, say 10 1-microsecond frames taken over the course of 1 millisecond. Both the light source and camera needed in the latter case tend to be quite a bit more expensive. An alternative which can work if the process is fairly repeatable is to run multiple experiments, taking separate 1-microsecond snapshots at different time delays. We have standard analog and digital focusing designs that can support microsecond snapshots, but due to the extreme light efficiency required, high speed video (more than a few hundred frames per second) generally requires a digital system with a high efficiency back light (the schlieren system itself is still relatively affordable but the camera can be quite expensive). For shutter speeds much under a microsecond, more specialized custom designs are required, though we’ve built and tested pulsed-laser based systems with down to 20 nanosecond exposures.

After the size and speed, one must consider the optical access to the schlieren field to be observed. There are three main options, backlit, projection, and natural background (BOS) systems. Backlit systems (including classical and backlit focusing schlieren systems, as well has some high speed BOS systems with special illumination) require the ability to place some sort of optical device (e.g. a mirror or screen) behind the schlieren target area. This might be as thin as a flat-screen television, but some applications preclude this kind of access. Projection systems are more flexible in this manner, since they only require a flat surface, which makes them attractive for such applications as wind tunnels and enclosures such as refrigerators. If the application precludes even the installation of a projection screen, imaging the existing background as-is for BOS is a possible solution.

An important point to consider is the expertise of the expected operator. Classical schlieren systems are by far the most difficult to use, since they require very precise alignment, and even experienced optical technicians may find them difficult to set up and maintain. BOS systems lie on the other end of the scale, since they require little more than operating a camera, while projection focusing schlieren systems are also largely point-and-shoot devices after the initial focusing and alignment step. BOS systems, though, tend to have considerably worse sensitivity than either focusing schlieren or classical schlieren, which is their main disadvantage. Backlit schlieren systems tend to be slightly more difficult to operate than projection systems, since the alignment between the backlit grid and the camera system must be maintained, but with our patented digital schlieren technology, the alignment and maintenance of the alignment are mostly taken care of by software. 

Schlieren sensitivity is difficult to quantify in an easily understandable way, though one can often look at the requirement based on the application. Visualizing shock waves, for example from explosions and supersonic motion, is generally fairly easy, and almost any schlieren technique can visualize them. Thermal convection is generally a more difficult target, unless the temperature differences are extremely large, and generally a focusing or classical schlieren system would be used for most thermal convection applications. Gas species contrast, such as in leak detection or exhaust visualization, also tends to require higher sensitivity, though this depends to a large degree on the types of gasses and concentrations involved. The most challenging applications include visualization of turbulence (as with boundary layer observations) and acoustic phenomena, visualization of sound. These typically require highly sensitive schlieren systems, unless very strong turbulence or intense sound is involved.

Schlieren Instruments

Analog Schlierenscope

The analog Schlierenscope is an analog projection focusing schlieren system designed for large area applications.

The apparatus is factory-aligned and comes with its own illumination system. Unlike traditional schlieren systems, the entire apparatus is contained in a single unit, so it is essentially a point-and-shoot schlieren system. This makes it massively easier to use than traditional schlieren systems, and because it is a focusing schlieren system, the system sensitivity is concentrated on a particular image plane, which can allow schlieren effects in different parts of the optical path to be distinguished.

Compared to our digital schlieren systems, it is somewhat larger, heavier, and more expensive, but it can also be more sensitive. Schlierenscopes are presently a custom order but can be fairly readily designed for ranges from 5 to 20 feet with throw ratios of around 6:1 (distance:height). Schlierenscopes have been delivered using flash illumination and can be customized for various high speed applications such as ballistics and high-speed wind tunnels (note that export outside the United States may be subject to restrictions for hypersonic wind tunnel applications).

Please see also our 2014 NASA Tech Briefs Article, “A Portable, Projection Focusing Schlieren System“. The latest brochure can be downloaded at this link. Application of a high-speed system was also described in a 2014 conference paper coauthored with NASA, “Application of a Novel Projection Focusing Schlieren System in NASA Test Facilities”.

Below, we show a few images captured with Schlierenscope systems.

Portable Projection Schlieren System

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.

Schlieren Streak Camera

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.

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Schlieren Streak Image of Shotgun Pellets in Flight

Digital Focusing Schlieren

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.

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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)

High Speed Video Schlieren

Digital focusing schlieren imaging can be extended to high speed video applications.

Digital focusing schlieren imaging can be extended to high speed video applications. To complement the sTube™ receiver, we have developed a high-intensity digital backlight system based on transparent LCD technology. This is capable of providing sufficient intensity for microsecond exposures over customer-specified durations. The frame rate is limited only by the camera capabilities. We have successfully tested it with frame rates up to 8,400 fps with a Phantom Miro m320s, though it can be readily adapted to a wide variety of cameras.

Unlike classical schlieren, which needs a point-like light source, focusing schlieren uses a large-area light source. Modern pulsed LEDs are well suited to this. They are not very effective as fast, high-power point sources, since it is difficult to squeeze more than a a few watts of radiometric power out of a single LED, but an array of closely-spaced LEDs can easily put out dozens or even hundreds of watts for short duty cycles. By matching the optical etendue of a flat LED array to the focusing schlieren optics, we can couple the output of dozens of LEDs into the system, which allows the high optical throughput needed for exposures in the range of a few hundred nanoseconds to microseconds. With classical schieren, such short exposures are really impractical without expensive and finicky arc flash sources. Focusing schlieren can also utilize large arc flash sources like studio photography flashes that produce a thousand times the energy of quasi-point-source arcs. This flexibility allows us to use a range of low-cost commercially available light sources that could never be used in a classical schlieren system. The images below, for example, used a 300 W-s xenon studio flash (300 Joule wall-plug energy, probably around 5-10 J radiometric).

Usually, an illuminating array can be as much as 1/4-1/3 the dimension of the modulator for ideal etendue matching, so since the modulator is usually around twice the size of the field of view, an illuminator about half the linear dimension of the desired field of view can be coupled into the optics. For example, a 15×15 cm LED array with a pulse output of hundreds of watts can dump nearly all of its energy into a 30×30 cm field of view, which could easily expose even a low-cost CMOS sensor in a hundred nanoseconds. This allows us to achieve schlieren light throughputs dozens of times higher than classical schlieren systems can practically achieve.

We typically use high energy xenon sources for high-speed video sequences in the multi millisecond range, but we can also use pulsed LEDs to get sub-microsecond snapshots without a specialized high-speed camera.

Below are images that were shot with this system of a candle flame interacting with an air rifle pellet and post processed with our specialized schlieren image processing technology. The original images have 1600×1200 pixels with a 5 microsecond exposure and 1,250 frame per second rate.

This innovative approach to focusing schlieren features simple alignment and a large area, up to 20 inches across with this system, without the cost and weight associated with the 20 inch mirrors that would be necessary in a comparable classical schlieren system. The backlighting system comes in at a mere 66 pounds, compared to a 20 inch mirror system which could come at a few hundred pounds with the necessary mounting system. Commercial transparent displays up to 84 inches diagonal are available, allowing a staggering 40 inches of schlieren field of view—a comparable classical Z-type schlieren system can weigh as much as 1000 pounds with the mounting.

Use the < and > keys to view the previous and next frames.

SAFRAN Schlieren for Aircraft in Flight Software

The Synthetic Limb Edge Schlieren technique is supported by our proprietary SAFRAN (Schlieren for Aircraft in Flight Reduction and Analysis) Package.

This easy-to-use software is designed to take a set of solar (or lunar) transit images and extract the schlieren information from the edge distortions. Why spend time and money developing this complex numerical and image processing code yourself when you can just license the software from the people who invented it? Need customizations? We have decades of image processing experience in scientific and engineering applications-see our image processing capabilities.

The simple linear main menu guides the user through the analysis steps+

The next step allows manual selection of the relevant image area, as well as sophisticated preprocessing options to improve the schlieren results.

Finally, the velocity adjustment dialog allows selection of the feature of interest according to its speed and angle. Image enhancement options specially designed for schlieren contrast enhancement make it possible to maximize the visibility all sorts of relevant contrast features. 

SchlierenView – the basic software that enables our digital focusing schlieren product line. This is also available separately without the DFS capability to support real-time schlieren image enhancement with any kind of schlieren system, or it can be purchased with a patent license for customers who want to build their own digital focusing schlieren systems.

Custom Schlieren Systems

As well as our standard products, MetroLaser also has the capabilities to design and manufacture both customized focusing and classical schlieren systems for a variety of applications. We can also provide fee-based consulting for guiding customers’ in-house development efforts. Please contact sales@metrolaser.com.

One design recently executed for a client is a lens-based small-scale classical schlieren system with a 20 mm diameter viewing area and pulsed flash-lamp illumination (below), specially optimized for the customer’s size, resolution, and sensitivity requirements. Custom systems are delivered with full documentation and schlieren image processing software.