High speed camera
High-speed cameras are used to record processes that are either extremely short-term or extremely fast or that meet both conditions ( slow motion ). High-speed cameras are used wherever movements or material behavior have to be analyzed that cannot be detected by the human eye or conventional cameras. Normal cinema cameras expose 24 frames per second , for television films either 25 ( PAL , SECAM ) or 29.97 ( NTSC ) are used. With high-speed cameras, one second of recording time can be extended to several minutes or even hours of playback time.
Procedure
There are four different types of high-speed cameras:
Film-based procedures
- High speed film cameras : 360 fps
- Rotating prism cameras : 10,000 fps. Such high-speed cameras expose by running the film over a prism instead of using a shutter .
- Drum cameras : 20,000 fps
- Rotating mirror cameras : up to 25 million fps
Hitachi 16HM rotating prism camera with max. 10,000 frames / second for 16mm film, built around 1970.
Internal structure of a Hitachi 16HM, the shiny silver transport gear behind the lens can be clearly seen.
Digital procedures
Use CCD - and CMOS sensors. Today's high-resolution camera systems usually have CMOS sensors that enable high resolution with a small size as well as low power consumption and heat generation. Almost every digital high-speed camera can only provide full resolution up to a certain recording speed, usually 500 or 1000 images per second, newer camera systems up to 7000 images per second. The exception here is the (I) S-CCD sensor, which maintains its full resolution up to a speed of 1,000,000 fps.
If a certain recording speed is exceeded, the resolution usually has to be reduced because the camera's microprocessor can only handle the same amount of data at a time. With today's HS cameras (high-speed video), frame rates of around 1 million images per second are possible. The resolution is then, depending on the camera manufacturer, at best 312 × 260 pixels .
Digital high-speed cameras usually have a limited internal memory in which (depending on the resolution and recording speed) only a limited number of images or frames can be stored (100 frames in the case of the IS-CCD sensor). "Long-term recorder systems" circumvent this limitation by storing the data directly on external storage media.
A resolution of 1024 × 768 pixels (picture elements) is currently common in impact analysis. Newer high-resolution camera systems achieve a resolution of 2048 × 2048 pixels (at 1000 fps) or 1504 × 1128 pixels. It is currently not possible to achieve this high resolution (1504 × 1128 px) together with very high speeds (1 Mfps) and a high number of images (100 images).
Storage in high-speed digital cameras
High-speed cameras usually have an internal or external ring buffer . If a camera is started, it records continuously with the set parameters until the camera is informed via a trigger signal that the process to be recorded has now taken place or will take place soon. After receiving the trigger signal, any remaining ring buffer is filled with recordings and the recording process is ended. The image data of the ring buffer is then available for other purposes.
In the case of long-term record systems, on the other hand, the data is not written to external storage media using the ring buffer method described above, but sequentially. The recording capacity of these is therefore directly dependent on the size of the storage medium. Usually a RAID hard disk system is used here , which, depending on the data rate, enables between a few minutes and a few hours of recording time.
In addition to the electrically fed trigger signals, modern cameras also have the option of feeding in a trigger signal via the recorded image or via the position of the camera. Some high-speed cameras have image triggers. With 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 triggers the actual recording (trigger). However, other camera systems also have GPS receivers that trigger a recording when the camera is at a certain position or passes it.
After the successful recording, the recorded data are further processed and archived. The camera software reads the individual images from the camera and, if desired, combines them into 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, frequencies of up to 500 million images per second can be achieved. However, a full second is not recorded here; the processes to be recorded usually take place within a few microseconds .
One-dimensional recordings are possible with streak cameras . With this technology, at the end of 2011, images with a frequency of 600 billion images per second could be implemented.
trigger
A problem with recordings with high-speed cameras is to start the recording at the right moment, since the processes to be filmed are very short and often over before they can be seen with the human eye. Every high-speed camera therefore has at least one so-called trigger option. Mostly this is an externally fed electrical signal.
exposure
Exposure is an important factor in all camera shots and photographs. It is even more important in high-speed imaging than in other imaging areas. While commercially available cameras and camcorders work with exposure times in the millisecond range [ms], the exposure times of high-speed cameras are in the microsecond range, depending on the recording speed. The exposure time for each individual image is therefore very short (≤ 1 / 15,000 s), which is why stronger and stronger light sources are required as the number of images increases. Since such high image frequencies are mostly used for extremely short processes, powerful flash units or very powerful permanent light sources (several kilowatts of light output) are often used. In general, because of the very short exposure times, high-speed cameras need a lot of light in order to achieve reasonable brightness dynamics and depth of field. For this purpose, the objects to be filmed are illuminated very strongly. It is sometimes the case that the correct lighting of the objects to be filmed causes more effort than the actual filming process and the subsequent image processing. The intense light for high-speed recordings also often means that the objects to be filmed become so hot during the filming process that they can melt or catch fire.
In connection with exposure, it should also be mentioned that black and white (monochrome) functioning high-speed cameras are up to three times more sensitive than color cameras of the same type with the same exposure time. In some cases, color cameras have to use longer exposure times or correspondingly stronger light sources than black and white cameras.
Applications
These cameras are used in the following areas:
- in film (primarily for effect shots, in advertising and for extreme slow motion in sports with up to 4000 fps)
- in basic scientific research, e.g. B. to empirically test theories about turbulence, Particle Image Velocimetry (PIV)
- in the automotive industry, e.g. B. in crash tests
- in defense technology, e.g. B. to analyze deformation of material under fire
- in medicine, e.g. B. to record vocal fold vibrations
- in production lines, e.g. B. in troubleshooting mechanical packaging processes
- in machine and apparatus engineering
- in welding technology, laser welding
- in the laboratory simulation of meteorite , micrometeorite or space debris - impact processes on planets or satellites
Applications in film (similar to television)
In the case of film, high-speed cameras are used to emphasize certain sections of the action in a dramaturgical way. Due to the time expansion, illustrated processes such as B. Explosions bigger and more powerful than the original motif. Accidents look more painful. At the same time, many processes gain aesthetic quality through recording with high-speed cameras.
Because of the high material costs for film material, only digital processes are practically used today for high-speed filming. Since the recordings produced are usually to be shown on HD monitors or in the cinema, it is necessary that the cameras used still deliver an image that is appropriate to the best possible playback medium, even at the highest recording speed. In the cinema z. B. a resolution of 2K or at least Full HD is necessary. Modern high-speed cameras create a resolution of up to 4K, which also leaves enough room for post-processing of the shot material.
High-speed camera recordings of crash tests
In the automotive industry, high-speed cameras are used to analyze crash tests . So-called acceleration-resistant cameras (crash-proof or HighG-proof) are mostly used here, which can also perform their tasks onboard (in the vehicle or in the test setup) due to their robustness against strong impacts and vibrations.
The automotive industry now predominantly uses digital camera systems, but high-speed film cameras are still in use here and there. In the area of crash analysis, high-speed recordings are made at 500 or 1000 images per second, with 1000 images per second being the standard. With a recording speed of 1000 images per second, the interval between two consecutive images (period) is 1 millisecond.
Recording speeds higher than 1000 images per second are rarely required in standard crash tests and are mostly only used for recording airbag deployments or even faster processes. Since the memory of a high-speed digital camera is limited, such a camera can only record for a limited time. If a camera z. B. can save 1500 images in a certain image resolution, a recording with 1000 images per second is finished after 1.5 seconds. If you were to take a picture at 10,000 images per second, the picture would be over after 150 milliseconds. If you want to record and analyze a process over a longer period of time, there are great problems with very fast processes and recording speeds if a camera with the classic sensor → RAM memory principle is used. Modern long-term systems provide a remedy here.
Mechanical resilience of high-speed cameras in crash tests
In crash tests in the automotive industry, high-speed cameras are subject to high mechanical stress requirements. For this purpose, crash- proof high-speed cameras are used, which can withstand a high acceleration of up to 100 g (100 times the gravitational acceleration ) in all axes over a period of up to 25 ms. In addition, these acceleration-resistant cameras must offer a solid connection option to the surrounding structures. Of course, a crash-proof high-speed camera must also have a housing that is robust against impacts. The chemical resistance of the housing and protection against dust and other foreign bodies also play an important role. The lenses to be used must also be able to withstand high loads. Insensitivity to ambient temperature and humidity is also very important. Most high-speed digital cameras have a temperature sensor in the housing that turns the camera off to protect itself if it gets too hot.
Synchronization of several high-speed cameras / 3D recordings
Accident situations are also increasingly being subjected to a 3D analysis. In order to create a 3D high-speed recording, two or more high-speed cameras of the same type (the same type of camera guarantees the same processing speed of the synchronization signals) are aimed at the object to be filmed or the process to be filmed from several perspectives. The process mentioned is recorded synchronously with all cameras. A 3D image is then calculated from the multiple 2D images using graphics processing software on the computer . For the calculation of the 3D recording, the synchronous running of all camera systems involved is essential. Even deviations in synchronicity in the range of a few microseconds can seriously falsify the result of the 3D image.
Modern high-speed cameras , for example in crash analysis, are expected to have high image synchronicity between several cameras in addition to the high recording speed . For a clear analysis of a process, the collision must be recorded from several perspectives. Synchronous recording from different perspectives is therefore essential. Therefore, all modern high-speed cameras from the crash analysis have a variety of synchronization options - for example via an external frequency generator, which supplies all cameras with a highly stable signal at the same time. Another possibility is to use the GPS time signal as a common constant. As a basic requirement, of course, an exact recording speed with minimal deviation in the period between two successive images is essential. A high-speed camera works very precisely and must be regularly calibrated for high synchronicity .
Lenses for high-speed digital cameras in crash tests
High speed cameras require adequate lenses. In crash tests , not only crash-proof high-speed cameras but also crash-proof lenses are used for the so-called onboard recordings (traveling in the test setup or in the vehicle) . As a rule, these are standard lenses from common manufacturers that have been tested by the manufacturers of the high-speed cameras and declared suitable.
Zoom lenses or lenses with an adjustable focal length cannot be used onboard, as these cannot be crash- proof in principle . Compared to lenses with a fixed focal length, zoom lenses are much more complex and have sensitive precision mechanics inside to set the distances between the individual optics segments ( lenses ) of the lens. The said precision mechanics inside a zoom lens cannot usually withstand the high accelerations of a crash test. In addition, zoom lenses are much larger and heavier than fixed focal length lenses, so that the weight and size of the lens can damage its attachment to the camera due to a higher tilting moment or higher lateral force. For stationary use, however, zoom lenses are preferred because they provide a high degree of flexibility when setting the image section.
Very important in terms of lenses is also the light intensity of a lens. When it comes to high-speed cameras in crash tests, the bigger the better. The light intensity indirectly reflects the light transmission of a lens. Since you have to work with short exposure times and high lighting costs with high-speed cameras, lenses with high light transmission are to be preferred. As a rule, lenses with light intensities of 1: 1.2 to 1: 2.8 to 1: 4 are used.
As far as the value of the focal length is concerned, one should orient oneself to the given requirements and the desired image details. The only thing to note is that lenses with a short focal length (≤ 16 mm), i.e. wide-angle lenses , strongly distort the image at the edges and thus make it difficult to analyze the image. Lenses with focal lengths that are too large (≥ 200 mm), i.e. telephoto lenses , can only be used to a limited extent, as the light intensity decreases sharply with increasing distance, making exposure with high-speed cameras more difficult. In crash tests, lenses with focal lengths from 4 mm to approx. 100 mm are used, so that distances of 0.3 m to approx. 15 m to the object to be filmed can be covered without any problems.
Small high speed cameras
The cameras' particularly compact designs open up new areas of application.
| camera | company | Dimensions |
|---|---|---|
| Phantom N5 | Vision Research Inc. | 32 × 32 × 28 mm |
| Micro-G1 | AOS Technologies AG | 30 × 32 × 59 mm |
| Ipcam Race 400 | Genexta | 25 × 20 × 80 mm |
See also
- Chronophotography (high-speed photography)
- Photographic rifle
- MOS (camera technology)
- Spark slow motion camera
Web links
- J. Holzfuss: Analog and digital high-speed cinematography . (PDF; 2.1 MB) In: Technisches Messen Volume 68, Number 11, 2001, pp. 499–506. doi: 10.1524 / teme.2001.68.11.499
- Declaration for laypeople
- Snowboard film "The Art of Flight" as a current example of the use of high-speed cameras in sports films
- Curious Droid: Ultra High Speed Cameras - How do you film a tank shell in flight or a Nuclear bomb test? youtube.com, video (7:13), November 10, 2017 (English) - about mechanical methods
Individual evidence
- ↑ S. Hertegård, H. Larsson H: A Portable High Speed Camera System for Vocal Fold Examinations . In: J Voice . 2014, doi : 10.1016 / j.jvoice.2014.04.002 , PMID 25008381 .
- ↑ https://www.phantomhighspeed.com/products/cameras/mirocnn/n5
- ↑ https://www.aostechnologies.com/high-speed-cameras/self-contained/high-g-rated/micro-g1/
- ↑ https://www.genexta.de/cam_race400.shtml