To see the flow images and the changing process of unsteady flow field and high-speed flow field of high-speed in every moment, photography technique is an indispensable means. With high speed photography technique of "quick capture & slow play “, we can slow the flow of rapid changes to a degree of which can be distinguished by the human eye's persistence of vision (0.1 s).
What is high-speed photography? The resolution of Human eyes on time is 1/24 second. And the frequency of film photography and play is set at 24 frames / second, and with this very characteristic, showing discontinuous projection makes people feel continuous. But for many high-speed moving objects and transient phenomenon, is limited by the time rate of change of human time points, we can't see the process. And high-speed photography, can put the high-speed movement, the phenomena of development and moving law clearly show in front of people.
Definition of high-speed photography in the information theory:
For a high-speed passage of a process which human eye cannot follow, high-speed photography provides a coupled information of space and time series, and the spatial information is represented by images; the time information is represented by capture frequency. Held in the United States in 2004, the 26th international high-speed photography and photonics conference ", the definition of high-speed photography is amended as: photography with speed greater than 128 fps, and can continuously last at least 3 frames. But now, the meaning of a high-speed photography is far beyond the general confine of photography, the limit of shortest exposure must be constantly revised. Ten years ago, the limit is for a fraction of hundreds of millions of a second, now it is a fraction of hundreds of billion of a second or less, and continuous shooting can be 600 million frames per second.
High-speed photography machines is called high speed camera .
There are many different kinds of cameras but its working principle is the same: turning optical image signals into electrical signals, so that it can be storied or transmitted. When we shoot an object, the reflected light of object is collected by lens, the lens makes the reflected light focus on the receives of the camera device (such as target surface of photography tube). And the camera device converts light into electrical energy, then we get "video signal. Photoelectric signal is very weak, it needs to be amplified by pre-amplify circuit, and then processed and adjusted by various circuits, at last ,the standard signal can be sent to video and other record medias to record , or transmitted by through the transmission ,or sent to monitor to display. As the name suggests, the "high-speed photography" is a photographic method with a very quick shoot speed and very short exposure time.
Principle of high-speed photography
Spatial resolution, spatial resolution of images is also called the resolution ability, it is a factor representing the degree of the image clarity, which is the ability to analyze image details. Spatial resolution generally refers to this: the minimum distance d between two black lines of the images with resolution which can be measured on the image plane. the inverse of this N is defined as the spatial resolution.
Time resolution: an index representing how quick exposure time of the camera can be, it refers to the minimum time interval of recognizing moving objects. The smaller time resolution is; the faster objects it can shoot. Time resolution has two kinds of measurement methods, that is, picture photography and scan photography.
Video frequency: the frequency of ordinary movie camera is 24 fps, and the frequency of high speed camera photography is much higher, it is about 102 ~ 109 fps, the frequency of ultra high speed camera is even higher.
The true high speed photography for the first time in history is performed by British chemist, linguists and photography pioneer Henry Talbot (Henry Talbot). In 1851, Talbot sticked a small piece of the London times page to a wheel, made the wheels spin in a dark room. When the wheel rotated, Talbot use flash (at the speed of 1/2000 of a second) of Leiden battery (it is a container that can collect charge, Leiden battery is the precursor of the capacitor), took a few square centimeters of the original pages, and obtained clear images. It looks like a static entity, but actually a moving entity. The father of Flash photography - Harold Mr Jie: if one can control the flash, make it keep pace with the rotation of the machine, it is possible to create an unprecedented miracle, marking moving moment curdled so as to research high speed generator. The journal of electrical engineering issued in 1931, Mr Jie formally announced this finding. In 1940, as of an urgent request from MGM studios, he proved the minimum speed degree of film photography should reach, making "within blink” produced by the company win the Oscar awards. In The Second World War, he had directed U.S. squadron how to use flash bang to do aerial photographic reconnaissance at night. Later on The Allied "D day”, 1944(June 6) ,the technology has played a great role in raids. He also worked out a method of blast shooting for the United States atomic energy commission and capture distance can be as close as ten miles.
The application fields of high-speed photography
1:Science field: high-speed photography is widely applied in the field of science， it is often used to record the change of the analysis of the movement of objects, and even biological organs, microbiology, molecular movement.
2:sports competition: people use high-speed photography technology, analysis skill characteristics of excellent athletes, find their athlete's shortcomings, improve training methods specifically, and increase their competitive level rapidly.
3:Military field: firstly ,it can test, inspect performance of reconnaissance weapon ,secondly, it can analyze explosion effect. Thirdly, study the damage effect of different bullet speed, the faster the bullets, the smoother the cutting plane. the killing effect of different material object can also be worked out through research of high-speed photography.
4:Pure art photography: in addition to the listed specific use above, high-speed photography is also a photography method of getting an amazing artistic effect. High-speed camera system: initially two sequence flash high-speed camera system structure is put forward. It uses quadratic sequences flash laser light source to send two ultra short laser pulses within the CCD camera exposure time, realizing capturing images of high-speed moving target at two different locations. This system can realize imaging twice in a photo station, this is equivalent to the function of two camera stand function. In the modern ballistic measurement it has a broad application prospects. Ancient optical is still constantly moving forward, the mainly new achievements in recent years are in light, optics and quantum oscillator amplifier, high-speed photography, fiber optics, etc.
With the development of the society, the advance of science and technology. High-speed photography technology will have a better development and improvement, it will bring our scientific career, life, society, nation more benefits. High speed photography technology is also an indispensable part in the development of science and technology.
