A review of recent developments in schlieren and shadowgraph techniques

Abstract

Schlieren and shadowgraph techniques are used around the world for imaging and measuring phenomena in transparent media. These optical methods originated long ago in parallel with telescopes and microscopes, and although it might seem that little new could be expected of them on the timescale of 15 years, in fact several important things have happened that are reviewed here. The digital revolution has had a transformative effect, replacing clumsy photographic film methods with excellent-though expensive-high-speed video cameras, making digital correlation and processing of shadow and schlieren images routine, and providing an entirely-new synthetic schlieren technique that has attracted a lot of attention: background-oriented schlieren or BOS. Several aspects of modern schlieren and shadowgraphy depend upon laptop-scale computer processing of images using an imagecapable language such as MATLAB ™. BOS, shock-wave tracking, schlieren velocimetry, synthetic streak-schlieren, and straightforward quantitative density measurements in 2D flows are all recent developments empowered by this digital and computational capability.

High-speed photography of shock waves with an adaptive illumination

“…The shadowgram was acquired using the MI:SF approach and the optical layout shown in Fig. 1 upon the removal of the knifeedge-type aperture [3]. The shock waves are labeled with S# and the boundaries of the cavitation bubbles with B#, where # stands for the number of the consecutive pulses.…”

“…This Letter describes an adaptive illumination system to visualize at once a combination of slow and fast phenomena in transparent media. It is intended to be used as a highly adaptable light source for high-speed movie shadowgraphy, schlieren photography/cinematography [1][2][3], a background-oriented schlieren (BOS) technique [4], and high-speed photo-elasticity [5]. The slower phenomena are visualized frame by frame, similar to the conventional continuous illumination or single-pulse-per-frame illumination techniques, while faster phenomena are captured within each frame by using multiple probing pulses that "freeze" the rapid motion of the propagating object at different locations on the sensor.…”

“…A drawback of using a typical laser source like Q-switched [13] or mode-locked lasers [11,14] as the light source is the inflexibility of the pulse modulation and the speckled, nonhomogeneous spatial profile. The illumination can be switched on and off relatively rapidly with LEDs [3,12,15], but to achieve high-brightness pulses with durations shorter than 100 ns, laser diodes are preferred, especially when unwanted light sources at other wavelengths have to be filtered out.…”

Precision public health to inhibit the contagion of disease and move toward a future in which microbes spread health

“…As a long-term aspiration, one could envisage a camera providing images of 3D moisture levels in real time, similar to temperature imaging with IR cameras. Schlieren and shadowgraph optics allow 3D imaging of air movements at distance [86]. It remains to be seen if these and/or other methods can be developed to noninvasively monitor the full range of airflows relevant to building occupants.…”

Schlieren Visualization of Shaping Air during Operation of an Electrostatic Rotary Bell Sprayer: Impact of Shaping Air on Droplet Atomization and Transport

“…The degree of the Schlieren effect is proportional to the first spatial derivative of the refractive index of the visualized gas [23]. In gases and air, a linear relationship exists between the gases' refractive index and its density [24]. In other words, factors like temperature and gas composition change the density of the gas and cause a shift in its refractive index that leads to its visualization using the Schlieren method [25][26][27].…”