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High-Speed Still Photography - Magneto-optical Shutter, Electro-optical Shutter (Kerr Cell Shutter), Image Tubes

flash light subject time

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

High-speed photography embraces a variety of techniques that capture short duration events by means of one or more very short exposures. In general this topic covers photographs taken with exposure times shorter than 1/1000 of a second.

Leaf or diaphragm mechanical shutters in still cameras generally have a minimum exposure limit of about 1/500 of second, while modern focal plane shutters have achieved exposure times of 1/8000 of a second. Thus, for shorter exposures, other types of capturing methods, usually electronic, will be used. Short exposures may also be made with a spark, flash gun, or strobe flash combined with an open shutter. In all cases the aim will be the same, to arrest motion (avoid blur) in a picture. An alternative is to move the camera or the imaging detector so that it is synchronous with the subject. In this case static elements in the scene will be blurred.

It should be noted that in film cameras, diaphragm and leaf shutters expose all the detecting area simultaneously. Focal plane shutters build up the image by exposing the detecting area sequentially, via a narrow moving slit.

An empirical formula for the maximum permissible exposure time is:

T = L /500 × V seconds

where L is the largest subject dimension to be recorded and V is the subject speed in the same units as L per second.

For example, if a car was moving at 100kph (62mph), equivalent to 27 m (100 feet) per second, the longest frame dimension would be about 14 m (50 feet).

Thus (using imperial units) T = 50/(500 × 100) or T = 1/1000 of a second.

This would be just on the limit for a normal shutter and no camera motion. Note that both V and L determine the exposure time—increasing V or decreasing L both demand shorter exposures. Many research subjects are both faster and shorter than a car.

A better method for determining exposure time is using the concept of maximum allowable blur based on subject dimensions. A decision as to what this dimension is will be subjective and depends on user-defined criteria. These may include a total absence of noticeable blur at a given degree of magnification of the reproduction, size of the smallest object within the subject of which useful detail needs to be recorded, etc. The formula also includes a factor that is of vital importance, i.e., the direction of subject motion relative to the camera film plane.

T = size of smallest detail within subject

K (velocity of subject) (cos A)

where K is a quality constant, generally a number from 2-4 and A is the angle between the film plane and subject direction.

For example, a projectile traveling at 100 m/s (360 kph), within the imaging field of view where it is desired to perceive detail as small as 1mm will require an exposure of 1/200,000 of a second (5µs). In this example, angle A is zero degrees and the quality constant is 2. A rifle bullet will have a velocity several times greater than this example.

The two main methods of taking high-speed still photographs are the use of a suitable high-speed shutter system or the use of a short duration flash while the shutter is open. High-speed shutters may be magneto-optical, electro-optical, or completely electronic in the form of image converter tubes. High-speed flash systems may use electronic flash discharges, sparks, or single or multiple laser discharges. Other systems available include argon bombs, super-radiant light sources, and X-ray discharges for high-speed radiography.

Magneto-optical Shutter

The orientation of the plane of polarization of light passing through a transparent medium can be rotated by application of a strong magnetic field. This effect, discovered by Faraday, can be used to produce a fast-acting magneto-optical shutter. Dense flint glass is often used as the transparent medium as it reacts well to a strong magnetic field. The glass cell, surrounded by a magnetic coil, is placed between two crossed polarizers that effectively cut off the passage of light. When a strong magnetic field is applied to the glass cell via current in its surrounding coil, the plane of polarization is rotated through 90 degrees to be the same angle as that of the second polarizer, allowing a light beam to pass. The current is
substantial (e.g., 1000 amperes driven by 10,000 volts), and is supplied by discharging a capacitor through the coil using a spark gap switch. The duration of the open time of the shutter can be controlled by varying the discharge and the number of turns in the coil.

Typical exposure times can be as short as 1 µs in duration. The system is placed in front of the camera lens or within the optical system. Magneto-optical shutters have an optical efficiency of around 70 percent and pass light throughout the visual spectrum range. However, the required magnetic field is very high and takes time to activate and remove, which limits the minimum shuttering time.

Electro-optical Shutter (Kerr Cell Shutter)

In 1875, John Kerr discovered that certain media, when placed in an electric field, exhibited birefringence so that light polarized in one plane has a different velocity in the medium to light polarized in a plane at right angles. A typical Kerr cell consists of a glass cell fitted with parallel plate electrodes between which the beam passes and the cell is filled with nitrobenzene. The cell is placed between two polarizers. The assembly is placed in line with the camera system, or within the optical path.

The first polarizer is set such that its polarizing plane is at 45 degrees to the original plane of polarization, i.e., at 90 degrees to each other. If a suitable voltage is applied to the plates (20,000 volts), the phase difference produced by the varying velocities is such, that on recombining at the second polarizer, the resultant plane polarized beam is in angular agreement with this polarizer. When no voltage is present, no phase change occurs and no light passes. Kerr cells can have opening times down to a few thousandths of a microsecond and have good imaging qualities.

The development of image tubes has meant that the use of Kerr cells and electromagnetic shutters has now been reduced
and are now they are only used in special types of research. For everyday use, the need for high voltage and high current supplies is a drawback. Whilst having low image degradation, the Kerr cell has poor optical efficiency with around 50 percent loss due to the polarization function, so operating light levels need to be very high. Another problem is that nitrobenzene is opaque to blue and ultraviolet light, a spectral range much used in high-speed photography.

Image Tubes

Beyond the capabilities of mechanical/film cameras the electronic camera takes over. A very important class of devices among the electronic cameras are those with image tubes. These include image converters, orthicons, image orthicons, and image intensifiers. Some of these can be used as high-speed shutters. Some metals, particularly cesium, have the property that, in a suitable electric field, they emit electrons when irradiated by light (the photoelectric effect). Conversely, some materials can emit light when bombarded by electrons (e.g., the phosphor on a television screen). A combination of such materials in an evacuated tube can act as an image converter.

The Image Converter

In an image converter, the input window is called a photo-cathode and acts as a light detector, while the output screen is the light emitter or phosphor screen. An image is focused onto the photocathode by a lens. The photocathode transforms the light image into emitted electrons, which are directed by suitable electric and magnetic fields, to the light emitter, re-forming a new light image that can then be recorded on film or CCD video cameras.

As an electron “image,” the beam can be accelerated (and thus amplified), shuttered, and deviated electronically similar to an oscilloscope or television tube. At the phosphor screen the accelerated electrons are transformed into photons and
result in an amplified light output. When the accelerating voltage is pulsed, the device can act as a fast shutter. The most widely used systems are now image converters and image intensifiers often combined into a system, either to present the image converter with an amplified light image or to amplify the output image before recording on film or CCD cameras.

The essential difference between image intensifiers and image converters is that intensifiers just produce an intensified light image, whereas converters can act as both intensifiers and manipulators of the image, i.e., a complete camera. Both can act as shutters. Image converters can also convert UV, IR, and X-ray radiation to visible light using appropriate input windows.

Image Intensifiers

In general use are Generation 1, 2, and 3 (G1, G2, and G3) types. G1 proximity-focused types are relatively simple. They have input and output fiber-optic windows on which are deposited a photocathode and phosphor screen, respectively. Applying a short high-voltage pulse (5-15kV) across the tube allows light amplification of around 15 to 100 times and can also acts as a fast shutter.

G2 types incorporate a micro-channel plate (MCP) amplifier between the photocathode and phosphor screen, which allows a greater luminous gain and also shorter shutter times as the necessary shuttering voltage excursion is much smaller. This a smaller and lighter device. Light amplification can be up to 400 times.

G3 types also use an MCP, but in this case the channel surfaces are coated with gallium arsenide (GaAs), which improves response.

Flash Photography Lighting Techniques

Flash photography is closely related to the early developments of photography. W. H. Fox Talbot made the first high-speed photograph using a spark-gap in 1851. At that time and until the beginning of the 20th century, sparks were the only short duration light sources available. In a demonstration at the Royal Photographic Society, he photographed a piece of newsprint glued to a rapidly revolving disk. When the plate was developed the print could be clearly seen and read.

Electronic flash is widely used to supplement available light, but can also be used to arrest motion using an open shutter and short flash exposures. Normal photographic electronic flashes have flash durations of 100-1000 (µs which is too long for most high-speed subjects. However, employing special techniques and components, spark durations can be reduced to microsecond or sub-microsecond duration, while still providing photographically practical intensities, but with more limited camera to subject distances.

As the exposure time shortens, illumination levels must increase. These constraints may bring with them the problems of a material’s reciprocity law failure in high-speed photographic application.

Light sources normally used for high-speed photography include flash tubes, sparks, and argon bombs. Sparks are produced by discharging a high-voltage capacitor between tungsten, steel, or copper electrodes. Spark gaps can take many forms to suit specific needs, e.g., a point source for shadowgraphs or a line source for schlieren photography. Line sources can take different random paths between electrodes, choosing the most easily ionized route. To keep the discharge in the same position each time a preferred ionization path is provided by blowing a fine jet of argon gas across the gap via a hollow electrode. For spark and flash tube sources the afterglow may continue after the main discharge, giving a phantom tail to a moving object. This afterglow is often eliminated using quenching circuits or fusible links.

Another short duration light is obtained by the vaporization of a thin metal wire. A large capacitor bank is discharged through the wire, switched by the breakdown of a spark gap. In vaporizing the wire, the electrical energy is turned into a bright flash of light.

High-speed X-radiology units are available to give sub-microsecond pulses for internal studies of dynamic processes, such as the penetration of a projectile into a target. As the voltage applied to the X-ray tube is increased, the energy of the X-ray beam increases (i.e., the rays becomes “harder”) and penetration distances increase.

Argon bombs consist of a simple tube filled with argon, with a transparent window at one end and an explosive charge at the other. During its travel down the tube, the shock front of the detonated explosive produces a highly luminous output from the argon gas. Exposure time can be varied about the 1 µs period by adjusting the length of the tube and hence the shock travel time through the gas. Because of the need for explosive in these devices, their use is normally restricted to government research establishments.

Currently, high-power laser pulses of nanosecond duration can be used in single or multi-pulse mode. As laser light is controllable to be in a single spectral band, this light source can be used to photograph highly self-luminous events such as explosions. The camera is fitted with a narrow-band filter, which allows only the laser light to be recorded while the self-luminosity is cut out.

Stroboscopic Operation

Using a still camera and a repetitive light source (stroboscope), a moving subject can be photographed in a series of positions in one picture. This is very useful for studying movement sequences of sportsmen or dancers to help them improve their technique. Often the images produced are artistically pleasing.

Alternatively, for rotating subjects, such as a fan, regularly spaced synchronized flashes illuminating the subject in exactly the same position at each revolution can give the illusion that it is stationary, which allows the observation of any changes under rotating conditions to be seen and photographed. Stroboscope flash units are adjustable for repetition rate and normally operate from 100 to 1000 flashes per second.

Lighting Techniques

High-speed, short duration exposures require intense subject lighting. However, self-luminous subjects may give adequate light levels, but extra lighting may be needed to illuminate backgrounds or shadowed areas.

Non-luminous subjects, and those where the self-illumination is not to be recorded, can be lit using either constant intensity sources or flash sources synchronized with camera shutters. In outdoor filming, bright sunlight may be adequate, but of course cannot be relied upon. Tungsten halogen lamps are used for general lighting and low voltage types are very useful where mains voltages are not available. Vortex stabilized argon arcs can produce impressive light intensities. A 300kW version using a 6m parabolic reflector can produce a 6m diameter beam giving a peak irradiance of 400,000 lux at 100m.

Xenon gas discharge lamps, of the short arc, high-intensity point-source types, can provide intense collimated beams where small areas are to be illuminated. A more recent development is the high-intensity photographic illuminator (HIPI), which runs in two modes—normal and boost—that last for 5 seconds. Luminous flux density in its boost mode is 750,000 lux at 75 feet (25m) distance (1 lux equals one lumen per square meter).

Flash Bulbs

These have a useful run time of about 20ms so they can be used for both still and cine illumination. Although they only last for a single flash, the light output in relation to weight, size, portability, and battery operation makes them efficient sources. Output is up to 100,000 lumens per second. When fired simultaneously in large numbers they are briefly equivalent to several kilowatts of light output. Alternatively, ripple firing, where the output of several bulbs is overlapped, can give a longer duration light output. For example, eight ripple-fired bulbs can give about 0.5 seconds of high intensity light. An important consideration for synchronization is an allowance for burn-up time to full brightness (5-35 ms).

Light-emitting Diodes

For short distances, continuous or flash illumination can be provided by light-emitting diodes (LEDs). A typical unit comprising a grouping of 24 diodes, each with a special reflector surround, can provide a nominal output of 28,000 lumens in continuous mode or 28,000 lux at a distance of 58.8cm. Alternatively the LEDs can be pulsed to generate strobe lighting up to 5000Hz. Some types of LEDs can be over-driven by a short pulse to produce very short duration, high-intensity light pulses.

Synchronization

In high-speed photography the precise time at which an exposure is made (to capture the event) is as important as the duration of the exposure (to minimize blur).

To synchronize the exposure to the event, a variety of event sensors (detectors) and associated variable delay devices can be used to initiate a photographic sequence. This sequence may be very simple or very complex depending on the type of event, the camera in use, the lighting requirements, and so on. Event sensors will be selected to suit the event conditions, and can include simple make or break switches, detection of radiation emitted by or reflected from a subject, interruption of an external radiation source by the interposition of the subject, detection of sound emitted by the subject or caused by subject movement, detection of local pressure changes or shock waves from the subject, and detection of changes in electrical or magnetic fields caused by the presence of the subject.

For self-luminous events, radiation detectors can be used and their trigger sensitivity level set such that the exposure is not made until the light level is high enough.

If a common signal source is used, it is essential to be aware of the response time of all the system components, which can include the time for the camera to reach an operation-ready condition, or the burn-up time of flash bulbs, so that appropriate delays can be set for all the individual component start times. This becomes increasingly important as subject velocities increase and exposure times become shorter.

Often in the photography of projectiles an appropriate trigger device will be selected, so that when the projectile reaches the camera field of view it will virtually take its own picture. For example, Ernst Mach (1880s) used a simple wire switch, which made contact when struck by the projectile, and fired the photographic spark.

Supersonic projectiles will produce a shockwave that can be detected and used as a trigger. Moving the acoustic detector along underneath the line of flight, the triggering instant can be fine tuned to vary the instant of the light flash.

Applications

High-speed still cameras are used in the same kind of contexts as high-speed cine cameras. Many of the systems developed for armament research have been applied to more peaceful purposes, e.g., in industry, medicine, complex mechanical processes, and the study of atomic processes for the production of electricity using nuclear power.

Reciprocity law failure

With very long and very short exposures, the normal reciprocal relationship between the effective exposure, the light reaching the detector and exposure time breaks down. This is relevant with silver-based films and is known as reciprocity law failure.

When using film with short exposures and high light intensity, the sensitivity and contrast reaches a minimum with exposure times of about 10 µs. Reciprocity failure at high intensity normally gives a decrease in gamma (image contrast) and a decrease in effective emulsion speed. An increase of development times of about 80 percent for flash photography of 10-100 µs is recommended and for less than 1 µs exposures an energetic developer is suggested.

Reciprocity failure can be minimized by choosing emulsions with the smallest departure of its published reciprocity failure data (see manufacturer’s data). The effects of reciprocity failure are small for modern black and white emulsions, but larger for color film where the color balance may also be affected. Note also that the color output of flash tubes and spark gaps may vary with changes in duration or intensity. In practice, experimental trials using similar emulsions and exposure times to those of the anticipated event trials are necessary to find suitable processing conditions. This will avoid costly failures with the actual event. Digital cameras also may exhibit changes of sensitivities based on duration and hence their sensitivity settings may need to be adjusted.

Stroboscopic Flash

Electronic flash tubes can be fired repeatedly at high frequencies of hundreds to thousands of flashes per second. This method of use has various applications in high-speed photography and motion study. A stroboscopic flash operates like a normal flash discharge circuit but with some modifications to allow a high-repetition rate. Flash tubes were developed from simple spark units by enclosing them in a sealed tube containing a low-pressure gas such as xenon, which enhances the light intensity of the discharge. Following 1931, much of the pioneering work on strobe development was carried out in the United States by Dr. Harold Edgerton. Edgerton was inspired to work on this project as a result of watching bright discharges of mercury arcs in rectifiers.

Theory of the Single Flash Tube

Light is produced when electrical energy stored in a charged capacitor is discharged into a flash tube. During the discharge, the instantaneous power is quite large (it may be several million watts).

However, the practical criterion is the light energy produced. Thus the second most important component of a flash unit after the flash tube itself is the capacitor. The capacitor is the energy store for the unit and it must be able to slowly charge over a period of seconds or minutes and then discharge all its energy in a few microseconds. This cycle places huge electrical and physical stresses on the capacitor and the insulation between its storage plates. Thus flash unit capacitors are constructed specially for the task and have been steadily improved in performance and endurance over many years.

Electronic flash tubes are made in many sizes and shapes to satisfy different requirements, such as straight, U, or spiral shapes. Gas pressures and electrode separation are arranged so that the tube will not strike under normal working voltages. It requires a trigger pulse to be applied to a trigger electrode to start the ionization process and allow the tube to flash.

Most flash tubes are filled with low-pressure xenon gas. High-speed photographic studies of the arc formation reveal an initial thin filament discharge, which quickly grows to fill the tube.

Flash duration can be defined by plotting light output against time on an oscilloscope through the initial delay, the initial rise, the peak, and decay. Provided the motion of the subject is frozen, the total duration of the flash is not critical. A common way to indicate duration is to measure the interval between half-peak value on the rise and fall of the light output curve. However, light intensity below these points may still be capable of forming an image and blurring the subject outline. Thus, this measurement only gives an indication and must be confirmed by actual experiment.

The electronic flash tube has an unusual volt-ampere characteristic. Initially with no discharge, the resistance is infinite. As current starts to flow, the resistance drops to a few ohms during the main discharge. Finally, as current ceases the resistance becomes infinite once more. The circuit is thus similar to the discharge of a capacitor through a resistor, and flash duration can be approximated by the equation:

Flash duration = RC /2 seconds

where C is capacitance in farads, and R is resistance in ohms.

When choosing a flash tube for a specific application, the following factors are important.

The shape—linear, U-shaped, spiral, etc.—of the tube is a factor. Compact tubes are more effective in a reflector to give maximum light control. Each tube has its maximum efficiency for a particular input. Although the tube should be used at or near its maximum efficiency, it may be necessary to operate below this to avoid overheating.

The flash duration is governed by the tube and associated circuitry, so if a particular flash duration is required then choice is limited. Often the most efficient flash tube cannot be used for stroboscopic work, as the high repetition rates and high power required will cause heat storage and conductivity problems.

Stroboscopic Photography

The term stroboscopic photography has come to mean multiple-flash photographs on a single plate or frame. The word stroboscopic is derived from the Greek strobes , act of spinning. The first multi-exposure photographs were made by Eadweard Muybridge, using multiple separate cameras along the line of the subject’s movement, and by Etienne Marey, who used a moving plate and a slotted rotating disk as a shutter. The modern method is to use a succession of electronically produced flashes separated by accurately controlled intervals.

When a flash tube is used intensively as a stroboscopic source several major problems may arise, largely because of the effects of heat build-up. For example, a flash tube may miss occasional flashes, or the gas in the flash tube fails to de-ionize, so the capacitor current continues to leak through the tube and prevents the build up of a full charge, a condition known as “holdover.” A hot flash tube may also short-circuit the triggering spark through surface conduction or may even fail from melting and tube collapse. Quartz tubes are preferred as quartz has a higher melting point than glass.

Another point may be self-flashing of the tube during the charging cycle. Again this is caused by incomplete de-ionization or lower stand-off voltage due to heating of the tube or gas filling. Special circuit elements such as the mercury-pool control tube (MCT) and the hydrogen thyratron can assist in preventing these problems.

Mercury-Pool Control Tube

The MCT is connected in series between the charging capacitor and flash tube and must be capable of handling the peak currents that flow as the tube discharges. The MCT is filled with mercury vapor and includes its own trigger circuit. When fired, it presents the full capacitor voltage across the flash tube and after discharge de-ionizes quickly, shutting down the current path to the flash tube and preventing holdover.

Hydrogen-Thyratron Tube

The hydrogen-thyratron tube (HTT) is similar to a conventional triode valve, and can be triggered by a pulse applied to the grid. It is filled with hydrogen giving it the ability to pass high currents without internal damage. The capacitor and flash tube are connected in series. The HTT circuit is placed in parallel across the capacitor and flash tube. When triggered it completes a current path including capacitor and flash tube, via a common earth. It has a very fast de-ionization time, again effectively preventing holdover current. In current equipment these two systems are often replaced by solid-state components.

Multi-Capacitor Circuits

When a limited number of flashes are required the circuits can be arranged so that each flash is powered by a separate capacitor, feeding either into a common flash tube or into separate tubes. Each circuit will have its individual trigger system. If many flashes are required, such a system can become very bulky. To prevent variations in the light source position a common flash tube is often used. This system allows the variation of flash discharge interval by settings from a master multi-trigger unit. Unless each circuit is very well shielded electrically and visually from its neighbors, there is sometimes the danger of sympathetic triggering of more than one circuit at a time. The above remarks can also be applied when spark sources are used instead of flash tubes, and this may occur when a point light source is necessary.

Applications

Stroboscopic flash has two main applications: analyzing a rapid movement by photographing successive movements on one frame or plate or arresting a rapid cycle or other periodic movement for direct visual observation or photography.

Examples of the first application are multiple photographs of dancers or movements in sport, e.g., detailed study of a golf swing. It is important that the flash repetition rate is adjusted so that successive changes of subject position do not overlap each other too closely, otherwise the picture may become very difficult to analyze. Sequences taken for artistic purposes rather than analysis may be deliberately allowed to merge (see Chromophotography ).

For the second type of application, the flash frequency is matched to the cyclic period of the motion under study. For instance, a machine component rotating at 5000 revolutions per minute will be illuminated by a flash lamp operating at 5000 flashes per minute. If the rates are accurately matched, the flash will illuminate the same phase of the movement each time apparently freezing the motion of the component, which can then be studied visually or photographed. By slight phase-shifting various elements of a large complex motion can be studied in turn.

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