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A review of the methods of high-speed photography
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1957 Rep. Prog. Phys. 20 379
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A REVIEW OF THE METHODS OF HIGH-SPEED PHOTOGRAPHY
BY J. S. COURTNEY-PRATT Research Laboratory for the Physics and Chemistry of Surfaces,
Department of Physical Chemistry, University of Cambridge
C O N T E N T S
4 1. Introduction.. .............. ....................................................... 4 2. Streak records .......................................................................... 4 3. Single exposures .......................................................................
4 5. Series of separate pictures ............................................................ 6. Series of pictures by image dissection .............................................
4. Multiple exposure without displacement of the frames. .......................
4 7. Conclusion. .............................................................................. References .....................................................................................
PAGE 380 383 386 3 96 397 41 1 428 429
Abstract. The variety, range and precision of methods available for photographic recording of fast phenomena have been increasing rapidly. The capabilities of the newer techniques are considered, classifying them by the kind of record obtained. Streak records with drum cameras can give a time resolution of sec ; rotating mirror cameras at present approach lo-' sec and may eventually achieve t x sec ; deflecting image converters may go much further. Single flashes of light, bright enough for silhouette recording, can be as short as sec for near objects, or 10-B sec for a field of view a metre square. Kerr cells can operate with an exposure of lo-' sec, and image converter tubes Several pictures can easily be taken at short intervals super- imposed on the one plate. Frames may be separated by intermittent film movement at rates less than 300 pictures per second (p.p.s.) ; or for speeds up to lo4 p.p.s. by using continuously moving film and short exposures, or by using some form of image movement compensation where exposures are a significant proportion of the interframe interval. With smaller pictures of lower resolution, higher speeds are possible. For higher speeds still one uses effectively separate cameras exposed in succession by mechanical means (such as a rotating slotted disc) up to 105p.p.s., by optical means (such as the use of a rotating mirror) up to lo7 p.p,s., or by electronic means (such as phased shutters or phased spark sources) up to lo7 p.p.s. Comparably high speeds with less elaborate equipment are made possible by image dissection. Simple dissection plates with clear lines or holes in an opaque ground allow recording rates of lo6 to lo6 p.p.s., but with low throughput of light. Using a rotating mirror camera to traverse the image elements rates of lo* p.p.s. have been achieved for brilliant objects. Dissection by means of lenticular plates allows in many cases a con- siderably greater throughput of light. Simple cameras are now available that can take 300 pictures at 250 000 p.p.s. Lenticular plate image dissection has been applied to cine- micrography so that similar series of 300 pictures can easily be taken at lo6 p.p.s. at magni- fications up to 2000x. Alternatively, the use of fibre light guides for dissection allows long series of pictures of low resolution at lo6 p.p.s. The combination of image dissection and deflecting image converter provides means for taking a series of 50 pictures at rates of the order of loo p.p.s.
Using one or another of these techniques, reasonable photographs may be taken of most macroscopic phenomena that are not too remote, though synchronization problems are sometimes of overriding significance. The difficulties are greater when the interest is in fine spatial detail. These problems are accentuated in cine-micrography where the image moves many times faster than the event. In such cases one must take advantage of the most advanced techniques, and there is urgent need for the continued development of better methods.
sec, and for reflected-light recording
380 J . S. Courtney-Pratt
3 1 . I N T R O D U C T I O N W I D E field of photography is customarily included in the term
It may be taken to include the recording A of single-shot pictures with exposure times of less than one-thousandth of a second, and the taking of series of pictures at rates above 200 frames per second, and the taking of other records such as streak records which allow a time resolution shorter than a millisecond. The different branches of the subject might be classified by their historical or geographical development, or by their speed range, and so on ; but one of the most convenient classifications is by the kind of record that is made, and the method by which the recording is made.t
Many of the phenomena that need to be studied by high-speed photographic methods can be adequately ' frozen ' by the use of a single short flash of light. That is, a single picture is sufficient. Many more require speeds of a few thousand frames per second. Compact and efficient cameras for this sort of job are commercially available. Such applications as these constitute the bulk of the work that is undertaken, to the extent even that they may be called the classical methods of high-speed photography. However, they are, just because of this, mentioned only briefly below. The emphasis of the presentation is on the newer techniques, their advantages, their limitations and their inherent possi- bilities. There is little discussion of the applications of the techniques, except in so far as is evident from the illustrations. A number of other volumes and reviews have been written presenting different aspects of the subject (Bourne 1948, Chesterman 195 1, Collins 1957, Courtney-Pratt 1953 a, Fayolle and Naslin 1948, 1949, Jones 1952, Naslin and Vivie 1956, Schultze 1955, Society of Motion Picture and Television Engineers 1949-54, Spencer 195 1, 1954).
Before I begin to describe the various systems, I wish to draw attention to an important point. One can easily take pictures with a standard 35-mm cine- camera at 32 frames per second. These pictures are each 16 x 24 mm, and the quality of the pictures can be high. We may measure the quality as the number of black and white line pairs that can just be resolved across the frame in each dimension. One may expect to resolve in good ' still ' cameras about 1000 line pairs across the frame each way. In commercial cinemas the resolution under ordinary projection conditions is rarely better than 350 line pairs from top to bottom of the picture on the screen. We still think of these as good pictures. However, if we drop by a further factor of two-thirds in resolution, the pictures are noticeably worse.
When we wish to increase the frame speed we are tempted to subdivide the picture area or reduce the film size, as shown in figure 1, as this would allow us to take more pictures per second without increasing the accelerations and the stresses in the film and/or the moving parts. If this sub-division is directly at the expense of the number of lines resolved across the frame, then little really has been gained. In any assessment of a technique of high-speed photography it is essential to consider both the framing rate and the resolution achieved.
' High-speed Photography '.
t A brief survey of some of the methods of high-speed photography appears as part of an article I wrote some years ago, and I would like to thank the Editors of theJournaZ of Photographic Science for permission to make use again of matter that was published there.
A Review of the Methods of High-speed Photography 381 Figure 2 shows six pictures taken by Professor Schardin (Schardin 1953).
They are all of the same scene, but the resolution in each is different. The pictures have been arbitrarily divided up into blocks of constant density. The total number of picture elements is indicated on each picture. Where the number is only 500, i.e. about 20 linesx 25 lines, the picture quality is very low, and might well be taken as the lowest limit of useful picture quality. I t would be just possible with a picture such as this to tell with an accuracy to 4 or 5% the position of large and obvious items in the field of view. On the other hand, where the number of picture elements is 20000, i.e. about 125 x 160, the quality, subjectively judged, is quite good. We have usually taken, as a guide, that we should aim at a minimum resolution of 200 x 200 for high-speed pictures, though if severely pressed a figure of 70 x 70 is still acceptable, as may be seen by reference to the fifth of Schardins pictures, which has only 5000 picture elements.
Full size 24 x 2 4 mms
iVote: 2% i iqures indicaLe picture sequence or&r
Figure 1. A diagram illustrating the principle of frame division. It is possible to achieve a larger number of small frames per second without increasing the accelerations and stresses in a camera mechanism. The advantages are however slight if this is achieved at the expense of proportionate loss of picture quality. (Reproduced by courtesy of W. Deryck Chesterman.)
Still higher picture quality is almost always worth aiming for, particularly where one is able to make the subsequent analysis or measurement directly on the primary negative. I have added this proviso, as only too frequently the quality of a really high resolution picture is degraded by inadequate playback, measuring, printing or copying equipment.
In the sections that follow there will be frequent reference to spatial resolution and to time resolution. The choice of a suitable means for studying any event depends primarily on these two resolutions ; but there are other limitations, and the most serious of these are available light level at the photographic emulsion,
382 r. S. Courtney-Pratt
A Review of the Methods of High-speed Photography 383
synchronization difficulties, length of sequence required, repeatability of the event, convenience or otherwise of operation, labour in reading the recording, and cost.
No one of the following methods, nor even any single combination of them, can provide a universal answer. It is often the best plan to make a qualitative overall survey by some simple means, and to study the particular feature of greatest interest by some more specialized but limited technique.
$ 2 . STREAK RECORDS For streak records, the attention is confined to events that occur along some
chosen line. The image of this line is focused on the sensitive surface of photo- graphic film which is moved during the event. The result is thus a record of the development of the phenomenon along the chosen line as a function of time. A simple diagram is shown in figure 3. Continuous film cameras have writing
I i I
L e n * I
Figure 3. A diagram of a simple drum camera. From measurements of the streak record, velocities of movement of the object can be determined relative to the speed of movement of the film. (Reproduced by courtesy of the ' Photographic yournal '.)
speeds of not more than a few tens of metres per second, and the minimum time between two events that may just be resolved is sec (Chesterman 1951, Chesterman and Myers 1951, Hercock 1947, Jones 1952). I n a recent camera a short loop of film is accelerated and, as its inertia is so much less, time delays and film wastage are much reduced (Prudence 1957). With drum cameras the writing speed can be 100 or 200 m/sec, and the time resolution low6 sec or a little less (Chesterman 195 1, Courtney-Pratt 1952, Courtney-Pratt and Thackeray
384 J . S. Courtney-Pratt
1956 b, Eyles 1941, Dr. Frank Frungel G.m.b.H. (C.I,.)t, Hercock 1947, Jones 1928, Jones 1952, Payman, Shepherd and Woodhead 1937, Weibull 1947).
Instead of a fixed image and moving film, the beam of light may be reflected from a rotating mirror or prism so that the line image moves over film that is held stationary. A writing speed of a thousand metres per second is easily possible with a lens aperture of f/S, and higher speeds have been attained at the expense of smaller light-gathering power (Adams 1950, Armament Research Establishment 1952 b, Beckman and Whitley Inc. (C.L.), Brixner 1957, Cairns 1944, Chesterman 195 1, Croney 1948, Herzberg and Walker 1948, Jones 1952, Payman et al. 1937). In fact the ratio of writing speed to numerical aperture is a convenient figure of merit and a value of 150 mjsec has rarely been exceeded, for any drum camera, or a value of 1000 m/sec for any mirror camera.
Film of explosive,
Slit Armour plate glass
mage converter tube
mage converter tube
Ordinary plate camera
A diagram of image converter tube apparatus used to obtain streak records of explosive (Reproduced by courtesy qf ' Reseavch '.)
Figure 4. reactions.
Professor Schardin at the recent Congress on High-speed Photography (Schardin 1957 b) considered the ultimate limits to which one might press a rotating mirror streak camera. The minimum width of an image of the slit is determined by diffraction and the aperture of the system. Using, for the mirror, materials of the highest specific strength available, the peripheral speed of the mirror can hardly be greater than 500 metres/sec. From this it follows that the time resolution cannot be better than 2 x sec, whether one uses a high numerical aperture and correspondingly low writing speed, or any other combina- tion. Mirror streak cameras built so far, fall considerably short of this ultimate limit.
If the image of the chosen line is focused on the photosensitive surface of an image converter tube a corresponding line image would appear on the fluorescent screen. The electron
It is possible to avoid all mechanically moving parts.
t References to commercial literature are indicated by (C.L.).
A Review of the Methods of High-speed Photography 385 stream can be deflected by a changing transverse electric or magnetic field to provide a fast time-base. The record on the fluorescent screen of the whole event can be photographed by an ordinary stationary camer...