Rotating mirror high speed camera application
S. Kirner, from the Universität der Bundeswehr in Munich used the stand-alone image intensifier module, Quantum Leap, to construct a rotating mirror high speed camera. The rotating mirror camera takes image sequences with a corresponding frame rate of 1 million frames per second.
Image sequence of the Triplex-plasma source taken with a corresponding frame rate of 1 million frames per second. The triplex-plasma source is a multi cathode plasma source which ensures the independent formation of three electric arcs. Figure reprinted with permission of the Universität der Bundeswehr, Munich.
Rotating mirror intensified CCD camera principle
For the high speed camera application S. Kirner used a rotating mirror to move a image section over the input of the image intensifier module. The image intensifier opens the ultra high speed shutter for multiple times and pictures the image section multiple times site by site on one single image. The result is a ultra high speed image sequence taken with a corresponding frame rate of 1 million frames per second. The number of image sections on one exposure and the corresponding frame rate can be adjusted by the rotation speed of the mirror and the timing of the image intensifier module.
Comparison to conventional high speed video cameras
Multiple technical approaches are on the market to facilitate ultra high speed imaging. The most common technique are CMOS sensors. High speed CMOS cameras offer high resolution images and framing rates up to thousands of frames per second. However, these cameras can increase the frame rate only at the expense of significantly reduced image resolution. Faster frame rates require smaller image sizes since the maximum number of read-out pixels per second is constant at CMOS cameras. Another approach for capturing images at ultra high speed are In Situ storage CCD chip (IS-CCD). The IS-CCD chips are modified interline transfer CCD sensors with an additional register for each pixel. The design enables a quick transfer of the recoded, individual pixel charge into the attached register. After the recording the register is read-out image by image. This technique enables frame rates up to 1 million fps at full CCD resolution with the drawback that the maximum number of frames is limited by the size of the register.
Image sequence of the F4-plasma source (by Sulzer Metco) taken with a corresponding frame rate of 1 million frames per second. The F4-plasma source is using a single cathode which has the drawback of fluctuating plasma formation. Figure reprinted with permission of the Universität der Bundeswehr, Munich.
Rotating mirror camera using a image intensifier module
To overcome this limitations the researcher from the Universität der Bundeswehr in Munich used a different approach. They mounted a rotating mirror in front of a image intensifier module which provides an ultra high speed shutter of up to 3MHz gate repetition rate. The rotating mirror moves an image section along the input window of a image intensifier module. The ultra high speed shutter opens at multiple positions of the moving image section. The result is a ultra high speed image sequence. The figure above shows a image sequence of 10 image sections taken of a F4-plasma source with a corresponding frame rate of 1 million frames per second.
Limitation of rotation mirror high speed cameras
The limitation of the rotation mirror camera is not the CCD sensor. Since the frame rate is only determine how fast the image section is moved over the input window of the image intensifier module and the shutter repetition rate. These two constrains permit frame rates up to 3 million frames per second. Therefore, the rotation mirror ICCD camera provide highest frame rates with comparable simple and cost-effective design.
a) b)Animation of a ultra high speed image sequence of the Triplex-plasma source (a) and F4-plasma source (b). The image sequences are taken with a frame rate of 1 million frames per second. The Triplex-plasma source is more stable due to a multi cathode design. In comparison the F4-plasma source has a single cathode with the drawback of a highly fluctuating plasma formation. Figures reprinted with permission of the Universität der Bundeswehr, Munich.
Investigation on high speed fluctuation of plasma sources
The researcher from the Universität der Bundeswehr in Munich used the rotating mirror camera to investigate the free plasma torches of the Triplex- and F4-plasma generators both made by Sulzer Metco. As it can be seen in the animated figure above the F4-plasma generator is fluctuating much more. With the high speed rotating mirror camera the researcher could determine a parameter dependent plasma fluctuating frequency of the F4-plasma generator. Whereas, the Triplex-plasma generator provides an almost stable plasma free torch. The reason for the difference torch characteristics is the design of the plasma generators. The F4-plasma generator has a single cathode which forms a single electric arc for the plasma torch. This electric arc is unstable depending on the adjusted parameter. The Triplex-plasma generator has three cathodes which separately and independently form an electric arc. All three electric arcs in combination ensure a stable plasma torch. This examples shows that the rotating mirror ICCD camera enables ultra high speed imaging and provides researcher and scientists with a cost-effective and compact tool for ultra high speed analysis.
Title: HOBAS: Ein innovatives Hochgeschwindigkeits-Bildaufnahmesystem -
Fertigstellung des Prototypen und erste messtechnische Anwendungen
Author: Stefan Kirner
Institute: Universität der Bundeswehr, Munich, Germany
Technical University of Munich, Germany