What kind of schlieren system do I need?
The most important questions to answer when selecting a schlieren technology are usually:
- How big is the area that we want to observe?
- How fast is the thing we want to see changing?
- Can we put a display system behind the area we want to observe?
- How knowledgeable and experienced is the expected operator?
- How sensitive do the measurements need to be?
Size
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.
Speed
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
Optical Access
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.
Ease of Operation
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.
Sensitivity
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.