The aperture of an optical system is proportional to the minimum effective lens diameter within the system, and indicates the light gathering ability of the optical system. Cordin cameras generally have apertures that are determined by the relay lenses. The internal aperture of a rotating mirror camera also determines the relationship between framing rate and exposure time, with wider apertures creating longer exposure times at a given framing rate.
Rotating mirror cameras can have one of two types of trigger access; continuous or synchronous. In a continuous access camera, light from the subject reaches an image capture device for every position of the mirror. This means the camera can capture an image independent of mirror position, and therefore the camera can be slaved to the event. It also means that images from some time before the trigger is received can be captured.
In any captured image, dynamic range indicates what meaningful information is captured above the noise floor and below the saturation threshold. Information outside of this range is either too dark to be above the noise or too bright not to cause saturation. In digital imaging, this term is often reported in bits. All too often, the resolution of the analog-to-digital converter is specified and quoted as the dynamic range of the system. A complete imaging system cannot have a dynamic range higher than the resolution of the A/D converter, but in practice, it is often lower.
Rotating mirror assemblies are either electric drive or gas turbine drive. Electric drive assemblies use a high speed motor and a sophisticated gearing mechanism to drive the mirrors as fast as practically possible. They are more convenient to operate than gas turbine drives, but achieve only one fifth the speed.
The time for which light signal is collected for a given image. In a sequence of images, or framing record, the exposure time refers to that of each individual image. For rotating mirror framing cameras, exposure time is a function of framing rate, and is the same for all images in a sequence. For gated intensified cameras, both the exposure time and the interframe time are fully programmable. This means the exposure time for images in a sequence can vary, as can the time between images.
Rotating mirror cameras have a fixed relationship between exposure time, internal aperture width, and framing rate. Very short exposure times are often desirable to minimize image blur from a moving subject. To get shorter exposure times at a given framing rate, the traditional method is to reduce the aperture width. But this also reduces the light gathering ability of the camera. Continuous access rotating mirror cameras can run in an Extended Record mode, whereby the aperture widths are kept the same, the mirror is spun twice as fast, and then every other channel in the record is read out. This creates an image record at a given framing rate but with half the normal exposure time.
Framing Rate (Frames per Second)
The speed at which a camera can capture images of the subject is the framing rate of the camera, expressed in frames per second. For rotating mirror cameras, the rate is constant throughout the record, and is a function of the rotational speed of the mirror (revolutions per second) and the angular density of the image capture devices (channels per revolution). In a gated intensified camera, the time between frames is randomly programmable, and need not be equal. Minimum interframe time is therefore a related, but more meaningful term for gated intensified cameras.
A camera that captures a sequence of two dimensional images is called a framing camera. Most motion picture and video cameras in existence fit into this category. Cordin makes framing cameras based on rotating mirror and image intensifier technology.
Gas Turbine Drive
Rotating mirror assemblies are either electric drive or gas turbine drive. Gas turbine assemblies are driven either with dry compressed air or helium. Helium drive allows for mirror speeds approximately five times faster than air or electric drive.
Gated Intensified Camera
One of the basic methods for capturing a sequence of images at very high speeds is to use MCP intensifiers, which not only amplify the image, but can also gate, or switch on and off, at very high speeds. These cameras take an image from an objective lens and distribute it using beam splitters and relay lenses to the photocathodes of multiple MCPs. The image is presented constantly to each MCP, but each MCP will gate the image at different times. These gated images are captured by CCDs, forming a high speed image sequence.
The maximum speeds of rotating mirror cameras are achieved by filling the camera interior with helium. This helium fill reduces the gas fluid friction on the rotating mirror, due to helium’s lower mass.
Image Converter Tube
Ultra high speed streak cameras are based on image converter tube technology. These tubes rely on a photon-electron-photon conversion. Initially they were used for both framing cameras and streak cameras, but the MCP approach has superseded the image converter tube for framing applications. In the streak application, a thin line of image is formed on the photocathode of the image converter tube, where it is converted to electrons. This line array of electrons is electrostatically accelerated toward a phosphor screen at the back of the tube, and also deflected to the top of that screen. During image capture, the polarity of these deflection plates is reversed, causing the line array to “streak” across the back phosphor screen. This allows for capture of transients in the picosecond time domain.
The interframe time of an image sequence is simply the time between frames. In rotating mirror cameras, the interframe time is equal between all frames of a record, and is the reciprocal of the framing rate (which is constant). In gated intensified cameras, the interframe time is randomly programmable, and therefore can vary between frames as the user so desires. In rotating mirror cameras, there is also a fixed relationship between the interframe time and the exposure time.
Line Pair per Millimeter (lp/mm)
The traditional method for measuring the spatial resolution, or sharpness, of an imaging system is in line pair per millimeter. This refers to how fine a set of alternating black and white lines can be resolved on the final recording medium, which was traditionally photographic film. A resolution of 40 lp/mm means that black lines 12.5 microns wide, alternating with white lines of the same thickness can be seen on the final image. Finer patterns of lines would appear grey. Test patterns for this, like the USAF 1951 resolution chart, have both vertical and horizontal patterns, as the resolution of an optical system is not always the same in both directions.
Micro-Channel Plate (MCP) Intensifier
The gated, intensified framing cameras are based on a micro-channel plate (MCP) image intensifier device. Like an image converter tube, this relies on the photo-electric effect of a photocathode to convert photons from the incident image to an array of electrons. These electrons are accelerated through a charged, conductive plate with many tightly spaced, small diameter holes, or micro-channels, which creates a cascading effect of electrons, thereby amplifying the signal. Electrons exiting the micro channels fall onto a phosphor screen, recreating an optical image. These devices are also the key component in night vision systems.
The optical phenomenon that rotating mirror camera technology is based on was first discovered by Cearcy Miller in the 1930s, and is named after him. This is the effect of relaying an image through a rotating mirror to a final image point in such a way that the motion of the mirror is cancelled out, thereby resulting in an array of images centered about the mirror, without blur from the optical system.
When an image is converted from photons to electrons, and then electrically manipulated, the conversion from electrons back to photons is achieved with a phosphor screen. It relies on the natural electroluminescence of the element phosphorous for this. In this process, wavelength (color) information from the incident photons is not preserved, meaning all imaging devices using a photocathode and phosphor are inherently monochrome.
When an image is converted from photons to electrons to be electrically manipulated, this initial conversion is achieved with a photocathode. The photocathode can be made of a variety of materials, all of which rely on the photoelectric effect. Different materials have different efficiencies across the wavelengths of the visible spectrum, and so photocathodes are generally specified to select a region of wavelengths for peak sensitivity.
In a continuous access rotating mirror camera, CCDs are continuously being exposed to images in sequence as the mirror rotates. Whether or not these images are captured is determined by the electronic triggering of the camera. Because of the way this triggering is managed, images on a subset of the CCDs can be saved from before when the trigger is received. This pre-trigger enables these cameras to have what is essentially a negative response time.
A lens forms an image of an object. It can also form an image of another image – the lens does not distinguish if the light entering it is from an actual object or an areal image. When a lens is used in this way, it is referred to as a relay lens, as it “relays” the image from one point to another. The magnification of the image may or may not change, depending on the design of the optical system. Most Cordin cameras use some kind of relay lens system.
The amount of detail from a subject captured in an image is referred to as the resolution, or “sharpness” of the image. When photographic film was the preferred method of image capture, resolution was specified in line pair per millimeter. With CCDs and CMOS imaging chips now the preferred method of image capture, resolution tends to be specified in number of pixels or megapixels (Mpix). While it is possible for the number of pixels to be the determining factor of ultimate resolution, it is more common that the resolution of the imaging system is determined by the optical system at a level lower than what the pixels could ideally resolve.
All Cordin cameras and most support equipment are triggered with an electronic signal. The response time of a system is the time between the arrival of the electronic trigger signal and the initiation of image capture is the response time of the system.
Rotating Mirror Camera
One of the best methods of high speed image capture is a rotating mirror system. This is the approach based on the Miller principal whereby light from the subject is relayed through a rotating mirror to an array of image capture devices. The speed of the camera is determined by the speed of the mirror and the angular density of the image capture devices. Since acquisition speed is not dependent on image read-out, the images can be read out at optimum speeds, preserving image quality and dynamic range.
A camera that records a thin line of image continuously is called a streak camera. The record of a streak camera is a graph of one dimension of spatial information across time. The image of the subject is reduced to a line by use of a mechanical slit, which consists of two parallel plates that block all of the light except that from the space between them. Streak records are not intuitively readable the way framing records are, but they have the advantage of continuous recording.
There is a method by which a streak camera can be synchronized with the speed of a projectile to capture a very high resolution, very sharp image of the projectile in flight. As the projectile passes in front of the slit of the streak camera, the streak camera captures an image. This method is technically identical to the imaging systems used to determine the finish of a horse or track and field race.
Rotating mirror cameras can be one of two configurations; continuous access or synchronous. In a synchronous camera, light from the subject is reaching an image capture device for only certain angular positions of the mirror. This means the camera can only capture an image for some fraction of the 360° rotation of the mirror, and therefore the camera system must be the master of the timing. The action of the subject must then be slaved to the camera system.
Since a streak camera image is a record of one dimension of spatial information over time, the resolution in the time axis, or temporal resolution, is a very meaningful specification. It indicates the minimum time interval that can be resolved by the camera. In a rotating mirror streak camera, the temporal resolution is a function of the optical resolution of the camera and the mirror speed. In an image converter streak camera, it is a function of the spatial resolution and switching speed of the image converter tube.
The trigger access of a rotating mirror camera determines if the subject needs to be synchronized with the phase of the rotating mirror or not. If it does, the camera is of synchronous access type, and the camera must be the master of the timing system – which is to say it must determine the timing of events in the subject. If the subject need not be synchronized with the phase of the camera, it is of continuous access type, and the camera can either be the master of the timing of events, or be a slave, where it receives a trigger and must respond. Trigger access is only meaningful in rotating mirror cameras.