High-speed camera

High-speed cameras are used to record transactions that are either extremely short time or run extremely fast or both conditions satisfy ( slow motion). High-speed cameras are in use everywhere, where movements or material behavior must be analyzed, which are not recognized by the human eye or conventional cameras. Normal cinema cameras expose 24 frames per second in television films to use either 25 (PAL, SECAM) or 29.97 ( NTSC). Through high-speed cameras may one seconds of recording time will be extended to several minutes of playback time.

• 4.1 Applications in the film (similar to television)
• 4.2 high-speed camera footage of crash tests 4.2.1 Mechanical resistance of high-speed cameras in crash tests
• 4.2.2 Synchronization of multiple Hochgeschwindigkeitskameras/3D-Aufnahmen
• 4.2.3 Lenses for high-speed digital cameras in crash tests

Method

There are five different types of high-speed cameras:

Film-based method

• High-speed film cameras: 360 fps
• Rotary prism cameras: 10.000 fps. Such high-speed cameras expose by running the film through a prism, instead of using a shutter.
• Drum cameras: 20.000 fps
• Rotating mirror cameras: up to 25 million fps

Internal construction of Hitachi 16HM, clearly the silvery transport gear can be seen behind the lens.

Digital processes

Be used CCD and CMOS sensors. Today's high-resolution camera systems usually have CMOS sensors that enable high resolution with small size and low power consumption and heat generation. Almost every high-speed digital camera can deliver the full resolution only up to a certain recording speed, usually 500 or 1000 frames per second, newer camera systems up to 7000 frames per second. Exception is the (I ) S- CCD sensor which retains its full resolution up to a speed of 1,000,000 fps.

If a specific recording speed is exceeded, the resolution must be usually reduced, because the microprocessor of camera can not only handle the same amount of data per time. In this case (high speed video ) frame rates of around 1 million frames per second are possible with today's HS cameras. The resolution is then, depending on the camera manufacturer, at best, 312 × 260 pixels.

High speed digital cameras usually have a limited internal memory in which ( depending on resolution and recording speed), only a limited number of images or frames can be stored ( 100 frames in the case of IS- CCD sensor). "Long Time Recorder Systems" around this limitation, in which they store the data directly to external storage media.

In the impact analysis is currently a resolution of 1024 × 768 pixels (dots ) are common. Recent high-resolution camera systems to achieve a resolution of 2048 × 2048 pixels ( at 1000 fps) or 1504 × 1128 pixels. At present it is not possible to realize this high resolution (1504 × 1128 px) together at very high speeds (1 MFPs) and a high number of images ( 100 images ).

Storage in digital high-speed cameras

High-speed cameras have usually have an internal or external ring buffer. If a camera is started, so take this endlessly with the set parameters on until the camera is communicated via a trigger signal that the male process has now taken place or will take place soon. After receipt of the trigger signal of the still any remaining ring buffer is filled with images, and terminates the recording process. Following the image data of the ring memory for other purposes are available.

For long-term record systems, however, the data is not written in the above-described ring-storage method, but sequentially to external storage media. Thus, in these, the capacity is directly dependent on the size of the storage medium. Usually here a RAID disk system is used that allows depending on data rate between a few minutes to a few hours recording time.

In addition to the electrically supplied trigger signals, it is also possible to feed a trigger signal on the captured image or the position of the camera in modern cameras. Some high-speed cameras have image trigger. In these cameras, a trigger signal is triggered by certain actions in the image. The movement of objects in the image is registered as an action by the firmware (software ) of the camera and replaces the actual recording ( trigger). However, other camera systems also have GPS receiver, trigger a recording when the camera is at a certain position or passes.

After the successful recording, the recorded data are processed and archived. The camera software reads out the pictures from the camera and adds them together if needed to make a video.

Electronic procedures

• Image converter cameras: 20 million fps
• High-speed framing cameras: 500 million fps. With the help of highly specialized high-speed framing cameras can count frequencies up to 500 million frames per second reach. A full second is not included here; the recorded events usually run from within a few microseconds.

One-dimensional shots per streak. With this technique, the end It 2011 images at a rate of 600 billion frames per second to implement.

Trigger

A problem when shooting with high-speed cameras is to start recording at the right moment, as when shooting events are very short and are often already over before they are perceived by the human eye. Therefore, each high-speed camera has at least one so-called trigger option. Mostly this is a externally supplied electrical signal.

Exposure

An important factor in all camera images and photographs, the exposure. In the field of high-speed imaging, it is even more important than in other areas of the image creation. While the commercial video cameras and camcorders with exposure times in the millisecond range [ms ] work are the exposure times of high-speed cameras, depending on the recording speed in the microsecond range. The exposure time for each frame is thus very short ( ≤ 1/15.000 s ), why with increasing image number is always stronger light sources are needed. Since such high frame rates are usually used for extremely short operations, often come strong flash or very strong continuous light sources ( several kilowatts of light power ) are used. In general, high-speed cameras need because of the very short exposure times, a lot of light in order to achieve a meaningful dynamic brightness range and depth of field. To this end, when shooting objects are very strongly illuminated. Sometimes it is so that the correct illumination of about shooting objects caused more effort than the actual film process and the subsequent image processing. Even the intense light for high -speed imaging often leads to the fact that when shooting objects so hot during the film process that they may melt or catch fire.

Should be mentioned in connection with the exposure also that black white ( monochrome) functioning high-speed cameras at the same exposure time up to three times more sensitive than color cameras of the same type. Thus, three longer exposure times or according to stronger light sources must be used than in black and white cameras with color cameras partially by a factor.

Applications

Apply this cameras among others in the following areas:

• In the film (primarily for effects shots in advertising and for extreme slow motion in sports with up to 4000 fps)
• In basic scientific research, for example, theories of turbulence to verify empirically, Particle Image Velocimetry ( PIV)
• In the automotive industry, such as in crash tests
• To analyze in military technology, for example, deformation of material under fire
• In medicine, for example, to record vocal fold vibrations
• In production lines, for example, when troubleshooting automated packaging operations
• In machinery and equipment
• In welding, laser welding
• In the laboratory simulation of meteorites, micrometeorites and space debris impact processes on planets or satellites

Applications in the film (similar to television)

With film, high-speed cameras are used to image Dramaturgische emphasis on certain procedural sections. By the time stretching operations illustrated act such as explosions bigger and more powerful than the original design. Accidents look painful. At the same time many processes gain by recording with high-speed cameras to aesthetic quality.

Due to the high material costs for footage to be practically applied only digital method for high- speed shooting in film today. Because the recordings made will be demonstrated usually on HD monitors or in the movies, it is necessary that the used cameras even at the highest recording speed still provides an image that is appropriate to the best possible playback medium. In the cinema, for example, a resolution of at least 2K or Full HD would be necessary. Modern high-speed cameras provide a resolution of up to 4K, which still leaves enough room for the post-processing of the filmed material.

High-speed camera footage of crash tests

In the automotive industry, high-speed cameras are used for the analysis of crash tests. This so-called acceleration fixed cameras are mostly ( crash- fixed or fixed - HighG ), which can fulfill their tasks due to their robustness to strong impacts or shocks also onboard ( in the car or rig).

The automotive industry uses is now predominantly digital camera systems, but also high-speed film cameras are still occasionally in use. In the area of ​​crash analysis, the high-speed imaging are performed with 500 or 1000 frames per second, 1000 frames per second are standard. At a take-up speed of 1000 frames per second, the distance between two consecutive images ( period ) is 1 millisecond long.

Higher recording speeds than 1000 frames per second are rarely required in standard crash tests and are usually only used for the recording of Airbagausfaltungen or even faster operations. Since the memory of a digital high-speed camera is limited, such a camera can also record limited long. If a camera, for example, can store 1500 images in a particular image resolution, at 1000 frames per second after 1.5 seconds has ended. If one were to make a recording at 10000 frames per second, the recording is finished already after 150 milliseconds. If you want to record and analyze a process over a long period, thus resulting in very fast processes and recording speeds big problems when a camera with the classic sensor → RAM memory principle is used. Modern long-term systems to address this problem.

Mechanical resistance of high-speed cameras in crash tests

In crash tests conducted by automotive industry high requirements in terms of mechanical loading on high-speed cameras. These crash- resistant high speed cameras are used, which ( 100 times the Erdfallbeschleunigung ) can withstand in all axes for a period of up to 25 ms high acceleration up to 100 g. Moreover, this acceleration fixed cameras must provide a solid connection possibility to the surrounding structures. Of course, a crash- resistant high -speed camera must also have a robust casing against impacts. In addition, chemical resistance of the housing and protection play an important role from dust and other foreign bodies. The lenses to be used must be able to withstand high loads. Very important is also insensitive to the ambient temperature and the ambient humidity. Most digital high-speed cameras have a temperature sensor in the housing, which turns off the camera for self-protection when it is too hot.

Synchronization of multiple Hochgeschwindigkeitskameras/3D-Aufnahmen

Increasingly also accident situations a 3D analysis is carried out. To create a 3D high- speed recording, two or more high-speed cameras of the same type (the same type of camera guarantees equal processing speed of the synchronization signals) from several perspectives on the object to be filmed or directed to the filming process. The said procedure is recorded synchronously with all cameras. Is then calculated with a graphic processing software on the computer from the plurality of 2D images a 3D image. For the calculation of the 3D recording the synchronous expiration of all camera systems involved is essential. Even deviations of synchronicity in the range of a few microseconds can distort greatly the result of the 3D image.

From modern high-speed cameras, for example in the crash analysis, besides the high recording speed and high image synchronicity between multiple cameras expected. For a clear analysis of the collision process from multiple perspectives must be held. Synchronous recording from different perspectives is essential. Therefore, all modern high-speed cameras of the crash analysis diverse synchronization options - for example by an external frequency generator, which supplies all the cameras at the same time a highly stable signal. A further possibility is the use of the GPS time signal as a common constant. As a prerequisite, of course, an exact recording speed with minimal deviation of the period between two successive images is essential. A high-speed camera works very precisely and must for high synchronicity regularly undergo a calibration.

Lenses for high-speed digital cameras in crash tests

High-speed cameras require appropriate lenses. In crash tests for the so-called onboard recordings ( ride-along in the experimental set-up or in a vehicle ) used not only crash- resistant high -speed cameras, but also crash- resistant lenses. In general, there are standard lenses known manufacturers which have been approved by the manufacturers of high-speed cameras and declared suitable.

Zoom lenses or lenses with variable focal length can not be used onboard, as this principle can not be crash- resistant. Zoom lenses are built against much more complicated lenses with fixed focal length and have inside a delicate precision mechanics for adjusting the distances between the individual segments optics ( lenses) of the objective. The said fine mechanics inside a zoom lens can not withstand the high acceleration of a crash test in the rule. In addition, zoom lenses are much larger and heavier than fixed focal length lenses, so the weight and size of the lens can damage its attachment to the camera by higher stall torque and higher lateral force. For stationary use zoom lenses are used, however, preferred because they provide a high degree of flexibility in the adjustment of the frame.

Very important in terms of lenses is the light intensity of a lens. With high-speed cameras in crash tests, the larger the better. The light intensity is indirectly again, the light transmittance of a lens. Because you have to work at high-speed cameras with low shutter speeds and high lighting expenses, lenses are preferable with high light transmission. In general, lenses are used with light intensities of 1:1.2 1:2.8 to 1:4 over. With zoom lenses, make sure that the aperture set on the light intensity over the entire zoom range remains the same. In addition, zoom lenses have lower light levels compared to lenses with a fixed focal length due to the higher number of lenses. Unfortunately, the price of a lens depends very strongly of the light intensity. Partial every little increase in light intensity is paid for by doubling or tripling of the price. Zoom lenses with throughout the entire zoom range constant light intensities cost up to five times more than the same zoom lenses with variable ( changing ) light intensity. Often, a compromise must be made.

As regards the value of the focal length, you should be based on the given requirements and the desired image sections. It should simply be that lenses with short focal length ( ≤ 16 mm), so wide angle lenses that distort the image at the edges strong and so complicates analysis of the recording. Lenses with large focal lengths ( ≥ 200 mm), so telephoto lenses can also be used only conditionally, as the light intensity decreases with increasing distance greatly and thus an exposure with high-speed cameras is difficult. In crash tests lenses are used with focal length ranges from 4 mm to about 100 mm, so that distances of 0.3 m to about 15 m to the covered object to filming without any problems.