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High-Speed Cinematography - Rotating Prism Compensation, Rotating Drum Systems, Rotating Mirror Cameras, Capping Shutters, Multiple cameras, Optical Delay Cameras

film image framing time

PETER W. W. FULLER
Consultant in Instrumentation and Imaging Science

High-speed cinematography involves motion picture recording at rates exceeding about 50 pictures, or frames per second (fps). It is used to photograph movement and events too fast to be perceived by the unaided eye or by lower framing rate methods. Such recording requires camera and lighting systems specially built for the task.

High-speed cine is also capable of expanding time. If an event is photographed at a high framing rate, with standard film cameras (see Group 1 and 2 below), it can later be replayed at a lower framing rate, apparently slowing the rate at which events occur and allowing the unaided eye to follow the motion. For cameras in Group 3 and 4, the same basic principles can be used, but playback and analysis is more complex. Originally, film was the only available recording medium, but now much of the recording is achieved using electronic systems. Present techniques enable framing rates up to about 100,000,000 fps to be achieved.

The various regions of high-speed cinematography have been defined in many ways and have been extended as the technology advanced. One classification system is as follows: high speed is 50 to 500 fps using intermittent film motion and mechanical shuttering; very high speed is 500 to 100,000 fps using continuously moving film, image compensation, and digital video systems; ultra high speed is 100,000 to 10 million fps using a stationary film with moving image systems and image converter cameras; and super high speed is in excess of 10 million fps where film has been largely superseded by purely electronic imaging and recording.

The boundaries between groups are not precise as cameras employ combinations of techniques. It is unfortunate that the superlatives were initially used too readily before the abilities of the last two groups became possible.

High-speed cameras of Group 1 are specially developed versions of normal cine cameras. Framing rate is limited by the physical strength of the film. The stresses imposed by starting and stopping the film as it is moved intermittently through the gate can exceed the breaking point of the film base, with tears usually occurring at the sprocket holes.

The fastest framing rates are 300 fps for 35mm film and 1000 fps for 16mm film. On the higher quality cameras in this range pin registration is used. During the exposure phase, the film is precisely located by pins to ensure accurate alignment and stability. These cameras are limited to about 500 fps.

In Group 2 cameras, film stress is relieved by moving it smoothly and continuously through the camera. However, to avoid blur, some form of image compensation is required to ensure the image is focused on the film and moving at the same speed during the exposure period.

In cameras of Groups 3 and 4 the film is stationary or completely supported on a drum and the image is moved along the film. As the required framing rates rise, electronic cameras increase in importance and take over from film.

Rotating Prism Compensation

These cameras were designed by F. J. Tuttle in the 1930s. A rotating glass prism of regular form (a parallel sided block, cube, or octagon) is placed between the objective lens and the moving film in this type of camera. Prism rotation, shutter opening, and film transport are synchronized by gearing. By refraction or reflection, the incoming image is moved in the same direction and at the same speed as the film. Thus during exposure there is no relative motion between image and film. Suitable masks can be used to limit exposure time to the period when compensation is possible. Smaller apertures can be used to produce shorter exposure times.

Film strength limits its rate of transport speed through the camera and maximum full frame rates are limited to 20,000 fps for 8mm film and 10,000 fps with 16mm film. Increasing the number of facets on the prism can increase frame rates to 20,000 for half-height frames on 16mm and 40,000 for
quarter-height frames. This limits possible applications but can still be used on, for example, projectiles in flight, where the reduced height is acceptable.

Rotating Drum Systems

Some improvement in speed, at the expense of total recording time, can be obtained by attaching a length of film to a rotating drum so that the film is supported and the limit is set by the strength of the drum. Drum cameras can include a simple type where the film is on the outside of the drum and the image is focused by means of a lens and a slit at right angles to film motion. This forms a simple streak camera. This can be changed to a framing camera by removing the slit and illuminating the event with a short duration multiple pulse laser or strobe light. In both cases access to the film must be limited to one complete resolution to prevent overwriting.

In more complex camera types, the film is placed on the inside of a rotating drum and image compensation provided by a spinning multifaceted mirror with the image conveyed to the film by a stationary mirror. Again, image access must be limited to prevent overwriting. Framing rates may reach 100,000fps. The films produced do not correlate film frames with sprocket holes, so for projection purposes considerable manipulation of the record is required. For the highest speed drum cameras it may be necessary to evacuate the drum chamber to overcome air drag.

Rotating Mirror Cameras

Development of these very high framing rate cameras was necessary to satisfy the need to photograph atomic bomb research toward the end of World War II. They are designated as working on the “Miller” principle, named after the inventor.

In these cameras the film is stationary and fixed inside a circular housing and can cover an arc of 90, 180, or 360 degrees. The subject is imaged near the surface of a rotating mirror and is then reflected to be re-imaged onto the film by an arc of lenses. As the mirror rotates the incoming image passes sequentially across the lenses and on to the film at a very high rate. Exposure time can be varied by changing aperture stops. Mirrors can have one or more faces, allowing another to take over when the first loses the image, and recording continues by an alternative optical path.

These cameras do not produce photographic records directly that are available for projection as with cine films. To avoid the reprinting required and as the number of frames is usually small, analysis is done frame by frame.

Because of the nature of these recording systems and the high level of synchronization required, drum and rotating mirror cameras are often allowed to control the event and its lighting. Using multiple programmed switching, a sequence can be set off by the camera so that by the time it is ready to record, lighting is on, access shutters open, the event has reached the required part of its sequence, and the recording is made. When all film has been used the capping shutter is shut.

Capping Shutters

Capping shutters are used in two main applications: when photographing self-luminous subjects with high-speed cameras, where the luminosity can continue after the required exposure has been completed, and again when using rotating drum or rotating mirror cameras. In the latter case, they are used to open the shutter at the instant when the mirror or drum is in the correct position to start recording and to close the shutter after all the film has been exposed, to prevent over-writing.

Such capping shutters are required to operate in 10 µs or shorter increments, either in opening or closing mode. These timing requirements are much beyond the capability of normal shutters, so special systems must be used. Cutoff systems include the spattering of lead wire onto a glass window by passing a high current through the wire or by explosively spraying an opaque mixture (e.g., lamp black plus a greasy binder) onto a window, explosively shattering a glass window sandwiched between two sheets of Plexiglas such that it becomes opaque by multiple scattering, or explosively collapsing tubes through which the beam is passed.

Cut-on systems are mostly operated by electronically or explosively removing a barrier in the beam. Most capping systems involve a discharge circuit requiring thousands of volts and large capacitors.

Multiple cameras

A series of pictures taken in sequence by a number of single-shot cameras can be considered as a form of cine photography (see the section Stroboscopic Photography in High-Speed Still Photography ). Advantages are better image quality per frame, flexibility in frame interval, and a reduction of image movement.

The basic principle of these systems is illustrated by reference to the Cranz-Schardin type of shadowgraph camera. For non-self-luminous events, an arrangement of short duration light sources is grouped together and correlated with a similar grouping of cameras or camera lenses. The event takes place between light sources and cameras. All light source beams are passed in turn through a common condensing lens or via a concave mirror, allowing them to illuminate the event. The beams are then focused by their associated lenses onto individual cameras, or onto a common film. Framing rates up to 100,000 fps can be achieved. However, the number of frames is relatively small, 10-20. The pictures obtained tend to be qualitative rather than quantitative, and analysis is difficult because each picture is taken at a slightly different angle to the others.

Kerr cell and Faraday shutters have been used in these systems (see High-Speed Still Photography ), but their use has mostly been superseded by electronic camera systems having their own high-speed shutters.

Optical Delay Cameras

A convenient quantity to remember is that light travels approximately 1 foot (about 30cm) in 1 ns. Thus if the incoming image is sent over a number of increasingly long paths to the camera, a series of photographs can be obtained with very short intervals between frames. The increasing path length for successive beams is achieved by the use of beam splitters and mirrors. A disadvantage of the systems using beam splitters is that the image intensity gets progressively weaker due to losses in the beam splitters. Alternatively, the image can be presented successively to each path mirror by reflecting the incoming image from a mirror rotating at high speed.

Electronic Cameras

Over the last decade the abilities and usage of electronic cameras has increased rapidly. In the lower framing rate regions up to around 50,000 fps, charge-coupled device (CCD) video cameras are now used, often in preference to film cameras. There are several advantages, including almost immediate access to the record, and in some cameras the ability to change framing rate during an event. Image storage is on magnetic tape, disk, or solid-state memory, and processing and analysis is done using a computer. There is also the ability to check lighting levels and image quality under event conditions before committing to actual tests. These advantages allow operation without a photographically trained operator as well as giving general overall savings in time, money and manpower.

For higher framing rates the image converter camera has become widely used. The essentials of such a camera is shown in Figure 42. The shuttering and deflection properties of image tubes can produce a short sequence of pictures on one tube with shorter exposures and higher deflection rates than other techniques. They can even intensify the available light, which makes it possible to record events previously not sufficiently bright to record on ordinary systems.

An advanced production camera, the Imacon 468, has eight combined camera systems, which record images in sequence with an exposure range of 10 ns to 1ms, independently variable in 10-ns steps, with framing rates from 100 to 100,00000,000 fps. This is made possible by using a complex form of pyramid beam splitter that relays the image simultaneously to all eight cameras, which are arranged in a circle around it. The images are recorded by intensified CCD cameras, which are exposed in sequence. During the interval between each successive exposure for an individual camera (cyclic interval period), there is time for the image to be stored and the camera made ready for its next picture.

Timing

Moving-film cameras require timing marks to be recorded along the film to assist analysis. In many cases the film never achieves a steady velocity, so marks recorded at known time intervals along the film edge are needed to indicate film velocity (more precisely, the time resolution), at any point on the film. These timing marks can be produced by a variety of techniques and are regulated by an independent stable reference source. Earlier systems used a tuning fork interrupting a light beam. Current systems are mostly electronic and use adjustable crystal oscillators to drive argon, neon, or xenon lamps or light-emitting diodes (LEDs). Their light is imaged onto the film edge via narrow slits. At fixed intervals a longer mark will indicate the passing of a complete period.

Some sophisticated systems can project coded signals onto the film, which can give information such as time since start.

At framing rates below 1000 fps, an analog or digital clock can be imaged in the field of view and is visible during projection. In rotating mirror cameras, the mirror can be used to sweep a beam of light alongside the film, where it can, via suitable masks, record timing marks. Simultaneously, the beam is received by a photo-electric detector that can send out electrical pulses suitable for triggering events, light sources, and making measurements of camera sweep speed.

Illumination

Up to 50,000 fps a variety of light sources can be used, which include tungsten lamps, photofloods, flash bulbs, and electronic flash tubes. There are also several systems in which devices such as xenon arc lamps can be overdriven for a short period to produce a high-intensity source. In very high-speed cameras, metal vapor lasers can also be synchronized with mid shutter-open time to illuminate the event with a short duration pulse, allowing much shorter exposure times than that of the built-in shutter. For recording at the higher framing rates more illumination is required. Some events are sufficiently self-luminous to provide adequate light, otherwise high-power flash tubes, lasers, or argon flash bombs can be used, synchronized to the event and camera.

Streak Cameras

In this special type of high-speed camera, a very narrow segment of the subject is selected for analysis and continuous successive exposures of this segment are made so that all the visual changes during the recording period are observed. Thus the final photograph has exchanged one of its two space dimensions for a dimension in time. In its simplest form, the subject is imaged by the objective lens onto a narrow slit, thus selecting the portion to be analyzed. The slit image is then re-imaged onto the film. In this system the film motion must be at right angles to the slit orientation, or if the image is swept across stationary film, the slit must be at right angles to the axis of sweep. In electronic systems the same principles must be applied. The movement of the event is along the axis of the slit.

The main point of streak cameras is that the record has no inter-frame loss and is continuous. It is thus very useful for
fine resolution of very fast-changing subjects. The drawback is that the pictures produced show the changing positions of the boundaries between dissimilar media, i.e., a solid medium and its surrounding gas or flame fronts in media. As the record is one spatial dimension versus time, it is not a normal two-dimensional image and it can only be interpreted with background understanding of the manner in which the event is investigated.

The writing speed of streak cameras may be quoted in two ways, either as the relative speed of the motion of the slit image and film, or by the time it takes for the slit image to be moved by an amount equal to its own width. The first is quoted in millimeters per microsecond and the second in terms of time (seconds or fractions of seconds). This is the time resolution of the camera.

For cameras that are not continuously viewing, the event must be synchronized to the period during which the event can be recorded. Some cameras are designed to record both streak and framing records. These are very useful as the framing record can be used to help interpret the streak record.

For higher writing speeds, drum, rotating mirror, or image converters are used. In image converters, a slit-image of the subject is imaged on the photocathode. A streak image is produced by electronic deflection within the tube.

Synchroballistic Cameras

A technique that fits between streak and framing categories is the synchroballistic system. Here, the basic system is as in streak photography but the subject motion is parallel to the film or sweep motion. It is widely used in ballistic experiments to photograph projectiles with high image resolution. The set up is arranged using appropriate positioning of the system elements and chosen lenses, so that the slit is at right angles to film motion and the slit image on the film is moving in the same direction as the film and at the same speed. Each portion of the projectile will be photographed in turn and the film will record a still picture of the whole projectile. This is a streak image of the changes in a given slit in space, but it is also equivalent to a very short exposure with a focal plane shutter. During the recording period, all objects traveling along the same path will be recorded. If the velocities of the swept image and film are not exactly equal, the images will be elongated or compressed accordingly. The system is a high-speed version of the photo-finish camera used to record the relative positions of competitors at the instant the first reaches the winning post.

Streak versus Framing Cameras

The cameras are not comparable as they are complementary. Framing cameras record two spatial dimensions at programmed intervals in time, thus it is possible for a change to occur between two frames that are not evident in either frame. On the other hand, a streak camera records one spatial dimension continuously in time, such that no change within the time period is missed. The streak camera is ideal for studying events of uniform growth or decay, e.g., an expanding sphere, where the rate of dimensional change is to be measured; however, the images can be easily misinterpreted. Streak cameras are also known as velocity recording cameras. A combination streak and framing camera provides the maximum amount of information and minimizes interpretation errors.

Applications

For application of up to about 20,000 fps, the subjects for high-speed cinematography cover a vast number of fields in both industry and research. The technique has made a huge difference in solving problems in industrial machines, e.g., in cigarette manufacture, where a small error in the setting of a minor part of a large machine can cause huge losses and long down time. These problems were originally solved by very long and tedious trial-and-error adjustments. High-speed photography allows the mechanism movements to be shown in slow motion, and the faults to be readily located. In safety research, typical applications are the impact of birds on aircraft windscreens and their ingestion into jet engines.

In research areas such as the development of internal combustion engines, high-speed cinematography allows the observation of burning processes and the behavior of items such as fuel injection mechanisms. For higher speed cameras the accent is on research in regions where subject movements are very fast indeed, e.g., armament research, missiles and explosives (both conventional and nuclear), supersonic aircraft research and space exploration, plasma studies for atomic energy reactors, and crack propagation in materials such as glass.

High-Speed Still Photography - Magneto-optical Shutter, Electro-optical Shutter (Kerr Cell Shutter), Image Tubes [next] [back] High Midnight

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