Photography provides two-dimensional records of three-dimensional scenes; the whole information about spatial objects can be obtained by a totally different imaging process - holography. Modern imaging techniques often no longer use photosensitive layers to store images, but make use of the possibilities of electronic data processing. To process images by computer they have to be converted into a computer-compatible format, i.e. digitalized; in photographic models, scanners are used. To convert light signals into electrical signals, either photomultiplier tubes, pholodiode-arrays, or charge-coupled-devices (CCD) are used. All three systems are photon counting devices, meaning the output signal is proportional to the number of photons received on the surface. With CCD-cameras, a two-dimensional resolution is achieved. They have to be connected with the computer system by analogue to digital converters.
Neither fast nor very slow motion processes can be registered in detail by direct observation. However, various photographic techniques can evaluate such processes.
Time-lapse photography allows analysis is of slow changes of an object for example the melting process by subsequent observation or measurement. With νA as recording frequency and as projection frequency in frames per second then νA<νP in the case of time-lapse photography. When taking such pictures cinematographic cameras are normally used (super-8, 16-mm-, 35-mm-film). The camera is adjusted to single image recording and triggered by a control device at selected time intervals.
With slow motion or time-stretch photography very fast running processes can be analysed by evaluating single images of different phases of motion (Here νA>νP). For investigation of boiling phenomena for example, high speed cinematography is a helpful tool. All cinematographic cameras - except image converting cameras—use moving parts for separating the images. The specific camera construction depends on the recording frequency. Cameras with continuous film transport make it possible to record up to 104 frames per second. During exposure the image has to be driven like the film in order to compensate for the effect of film transport and avoid distortion; this problem is solved by optical components like the rotating prism. Figure 1 shows photographs made by such a camera. With a drum camera recording frequencies up to 105 are possible and with rotating mirror cameras even 107 frames per second. With ultra high speed video cameras frequencies up to 2·107 are possible.
For high speed photography exposure times may be as short as nanoseconds (10−9 s). With such short exposure times, achieved by illumination with sparks or flashes a sharp image of a specific phase of very fast running processes can be recorded. In the case of fast moving objects, flash and motion have to be synchronized and only limited sequences can be recorded.
This technique depends upon small light sources or reflectors capable of moving with the object being photographed. For example reflecting particles can be added to a streaming fluid and their paths recorded on film by long time exposure. With this technique movement sequences show up very clearly.
Photography with radiation outside the visible range often provides information which is not accessible within it. Transformation of the spectral range to be registered can be carried out by electronic image converters. Often a signal amplifier is also used.
Investigation of heated objects with temperatures between 250°C and 525°C is possible by recording their own radiation (heat radiation) on film sensitive to the infrared range. For recording radiation from objects with lower or higher temperatures, for example thermographic cameras, image converters, are needed.
Evaluation of the natural spontaneous emission of photons from particles in gaseous systems is one of the oldest optical techniques for the determination of concentrations and temperatures. A major field of application of this technique is combustion. Low spectral signal intensities lead to limited time and spatial resolution and advanced detector technology is needed. As an example, Figure 2 shows a system for self fluorescence investigations of high speed flames. The main components are the CCD video cameras with controller and image recording and processing units. The image intensifier of the camera provides the required increase in sensitivity, gateability and spectral shift to maximum sensitivity from the visible/red region to the UV. With shutter speeds reduced to 10 s the dynamic structure of the flame can be investigated.
Interference techniques work with superposition of at least two light waves. The waves have optical paths which differ in length with the effect that superposition causes an interference pattern containing information about the process under investigation. The interference techniques have the advantage that photographic recording shows immediately the modifications of the refractive index field caused by diffusion, temperature gradients or flow in a two-dimensional field. (See also Interferometry)
In normal photography, the chosen point of view decides which perspective of the three-dimensional object is shown on the photograph. The basic idea of holography is to store the totality (holos) of a wavefront influenced by an object by adding a reference wave generated by the same coherent light source. The interference pattern is recorded on a film. Illuminating the chemically treated photo plate with the reference wave the object is reconstructed three-dimensionally (see Figure 3). (See also Holography and Holographic Interferometry)
Figure 2. Schematic of setup of cameras for self fluorescence investigations of high speed flames, observed from two directions.
The speckle-method is an interferometric method which has found wide application since the laser has been developed. The granulation which could be seen when illuminating a diffuse reflecting surface by laser light is called "speckle". The single points of such a raw surface acts as coherent light sources of waves with different phases. These waves interfere and form a statistically irregular interference pattern. The fundamental characteristic of the speckles useful for this technique is that the speckle pattern follows a surface translation perpendicular to the optical axis. By analysing the movement of single parts the surface deformation can be determined. The conditions before and after the deformation are stored on a film by double exposure technique (specklegram).
Schlieren and shadowgraph methods depend upon the fact that solid, liquid or gaseous objects influence a transmitted light beam by changing its direction according to retractive index differences relative to the homogeneous surrounding. The simplest schlieren method is the shadowgraph technique. (See entries on Interferometry and Shadowgraph Technique)
Gonzalez, R. C. and Wintz, P. (1977) Digital Image Processing, Addison-Wesley Publ. Co. Reading, MA.
Kingslake, R. (1965-1983) Applied Optics and Optical Engineering, Academic Press, New York.
Saxe, R.-F. (1966) High-Speed Photography, The Focal Press, London.
Walls, H. J. and Attridge, G. G. (1977) Basic Photo Science, 2nd edition, Focal Press, London.
- Gonzalez, R. C. and Wintz, P. (1977) Digital Image Processing, Addison-Wesley Publ. Co. Reading, MA.
- Kingslake, R. (1965-1983) Applied Optics and Optical Engineering, Academic Press, New York.
- Saxe, R.-F. (1966) High-Speed Photography, The Focal Press, London.
- Walls, H. J. and Attridge, G. G. (1977) Basic Photo Science, 2nd edition, Focal Press, London.
Heat & Mass Transfer, and Fluids Engineering