The scientific applications of photography and imaging as tools to assist visualization date from the earliest days of the introduction of photographic processes. J. W. Draper's celebrated image of the moon, for example, was taken as early as 1840, and Sir John Herschel had noted the presence and effects of ultraviolet (UV) and infrared (IR) radiation with Henry Talbot's process in 1839. Today, however, by comparison with specialisms such as advertising photography and photojournalism, scientific photography has a low public profile, although its images have roles in many consumer products and health and civic matters.
Scope
Most scientific photographers work for governments or in research institutions, universities, or hospitals. A primary role is the production of images for recording, measurement, and interpretation purposes, principally in the fields of science, technology, and medicine, plus the communication of results in the form of audiovisual material and reports. The specialism is defined as ‘photography and imaging used as scientific tools to provide records that cannot be made in any other way’. This includes cinematography, video, and digital imaging, as these visual media have attributes individually of value, and the most appropriate medium is chosen to suit the recording task. Traditionally, ‘imaging’ refers to recording outside the visible spectrum.
Scientific imaging extends recording beyond the limits of human visual perception, producing permanent records for analysis and evaluation of the subject. Emphasis is on precision and accuracy, as in the micro-electronics industry from techniques such as photofabrication and micro-imaging. Objectivity and absence of ambiguity are necessary in images for clinical, forensic, and public inquiry work. Many scientific images may have little meaning except to specialists and lack general interest except in certain morbid or sensational—e.g. crime-or disaster-related—cases. However, suitable pictures are used in the editorial and advertising sections of popular and learned journals, where their normally non-visible or purely serendipitous nature, or beauty, attracts non-specialist attention. Additionally, compilations of such images are published in book form, on topics such as astronomy, natural history, aerial photography, and human physiology.
In addition to the techniques of studio and location photography, scientific photography uses specialist equipment to record subjects radiating outside the visible spectrum, or that may move too slowly or too fast for changes or events to be readily perceptible, or that may simply be too small or too far away to be examined visually in detail. Recording is possible in situations that are biologically hazardous, giving permanent images to provide accurate dimensional information about the subject without the need for physical contact.
Images may further be optically reduced to micro-images for storage purposes. Storage in digital form allows processing to enhance and emphasize parts of the image to aid interpretation and comprehension. The replacement of film-based recording by digital counterparts has not been a problem in scientific photography; the most appropriate system is chosen for a specific task. The scientific photographer is now involved less with actual recording and more in advising clients as to the most cost-effective method of obtaining results, and in creating the initial experimental design.
Spectral recording
Human visual perception and conventional photography are limited to the spectral band from 400 to 700 nanometres (nm). Information about the nature of a subject is obtained from its ‘spectral signature’ including patterns of emission and reflection outside the visible region, particularly in the ‘near’ UV at 300 to 400 nm and the ‘near’ IR from 700 to 1, 300 nm. Both regions can be recorded with modifications to photographic techniques.
Silver halides are sensitive to all UV wavelengths, but modern optical glasses, the gelatin of emulsions and even the atmosphere may remove much of this actinic radiation. Lenses of fluorite and quartz and special film materials are needed for recording the shortest UV wavelengths. Ultraviolet photography is used, for example, in dermatology (skin) and plant studies. UV fluorescence photography is used chiefly in non-destructive testing, for the detection of labelled molecular groups, and for forensic purposes. This technique employs conventional colour film.
For infrared recording, with extended sensitization, film materials can record to some 1, 000 nm (1 μm). Most optical materials pass infrared but lenses may need a focus shift correction. Apochromatic lenses (i.e. those corrected for the three primary colours) may have the necessary properties. The striking results of infrared photography of landscapes are familiar and used in pictorial photography. The IR reflectance of foliage is indicative of plant types and diseases. The simultaneous recording of reflectance and emission of subjects in visible and IR spectral bands as selected by filters, usually called ‘multi-spectral imaging’, is a useful tool in agronomy, forensic work, and museum conservation.
Earth satellites use multi-spectral imaging to search for water and mineral resources by ‘remote sensing’. Film may be replaced by a suitable solid-state sensor which may be ‘non-imaging’, giving only a single point response, so the subject field has to be ‘scanned’ by rocking or spinning mirrors to build up a full picture.
Thermal imaging (or thermography) records the far IR at 3-5 μm and 8-14 μm, using two atmospheric transmission ‘windows’, and plays an important role in medicine, environmental studies, and military applications. Such ‘thermal cameras’ use the emitted radiation from the subject to provide a ‘heat map’ and any local variations. Digital data can be displayed using designated colours to indicate specific temperatures so giving a ‘false colour’ image.
High energy particles and ionizing radiation such as X-rays and gamma rays have very short wavelengths. Various radiographic techniques use photographic, electronic, or digital imaging to give records of internal details of subjects suited to diagnostic and non-destructive testing purposes.
Optics and illumination
Use is made of various light sources, particularly lasers, and optical equipment such as filters, mirrors, lenses, and prisms, to illuminate subjects in ways that can provide forms of ‘optical fingerprint’.
Back lighting and transillumination, especially using polarized light, can show structure and flaws in simple cases while use of collimated (parallel) light between a pair of parabolic mirrors can show similar dynamic changes in solids, liquids, and gases from changes in refractive index of the subject. Such Schlieren photography can be used in aerodynamic studies and wind tunnel work to show air flows, vortices, and shock waves.
A laser provides an intense, shapable, highly monochromatic beam of light. This is used either to scan subjects, or to illuminate them by a motion-arresting pulse of light or show indirectly very small changes in properties by provision of motion-sensitive interference fringes. Interferometric experiments require a rare skill and patience in alignment and interpretation. The fringe patterns produced as topological ‘maps’ of the subject can be of great visual attraction. The technology of holography provides images from the recording of interference patterns and is a routine technique for suitable subjects.
Special-purpose lenses can give views ranging from extremely wide to very narrow angle or detailed close-ups of a subject. They may have spectral correction for UV and IR use.
Photomacrography and photomicrography
The photography of small details in a subject is called photomacrography, further extended to photomicroscopy. Specialized equipment is needed and, apart from their scientific uses, entrancing and beautiful images can result.
Photomacrography is the recording of an image whose size (magnification) is in the range from lifesize to about 20 times larger. To keep equipment manageable in terms of the considerable distance from lens to film needed (in contrast to the very short lens to subject distance), special ‘macro’ lenses with short focus and suitable aberration correction for a particular magnification or working distance are used. Large camera formats are still popular and camera movements can be used to manipulate the vanishingly small depth of field in the subject to best advantage. Lighting may be provided by ring-flash or fibre optic light guides, but transillumination may be the only way.
Photomicrography uses transillumination for subjects in the form of a prepared glass slide, but alternative reflected illumination is needed for opaque specimens such as in metallurgy and for microcircuit examination. The microscope uses a projection eyepiece to give a real image which can then be recorded using a camera body. No camera lens is needed. Conventional microscopy can give magnifications up to about×1, 000 and details of subjects down to sizes about the wavelength of light (say 500 nm). Thereafter, electronic imaging in the form of scanning and transmission electron microscopes are required, especially for the depth of field given by the former. The ‘confocal’ microscope uses a beam of laser light to scan the subject through the actual optical system. The image is effectively a record of a thin slice of the subject and can be suitably stored, manipulated, enhanced, colourized, or combined with others as need be. A variety of supplementary optical systems are used in microscopy to provide enhanced image contrast or detail.
The study of motion and flow
Many changes in subjects take a long time. The movement of a glacier, the growth of a plant or a person, the change in star patterns, and the onset of corrosion are imperceptible. To aid perception, elapsed time may be contracted by techniques of ‘time-lapse’ photography (or cine or video) whereby images are recorded at suitable intervals for playback at normal frequency. Intervalometer devices, often now built into cameras, aid this task and the typical 72-hour continuous recording capability of time-lapse video recorders is useful in surveillance applications. By contrast, an extended time exposure of a subject can reveal aspects of its intrinsic motion or behaviour hitherto unsuspected, for example the distribution pattern of an aerosol spray.
Other aspects of an aerosol, such as the droplet sizes and shapes, are captured by means of high-speed photography, where a very short exposure duration, usually provided by electronic flash, can freeze a moment in time. Applications such as aerodynamics, fluid flow, and ballistics use ‘microflash’ techniques or pulsed lasers to provide multiple images of phases of the action. Synchronization of the event with the flash can be problematic. The classic image of the corona splash pattern of a falling drop of milk is well known.
For a continuous record of an event, high-speed cine or video is used, where the usual framing rate of 25 pictures per second (pps) is increased to 500 pps (framing camera), 20, 000 pps (rotating prism camera), 20 million pps (spinning mirror or Miller camera), or greater (image tube camera). A normal projection rate of 24 pps then gives a ‘time expansion’ to allow visual comprehension of events of very short duration. A pulsed laser gives suitably high repetition rates for illumination purposes. Much high-speed work is now done using video equipment. The long recording time available is useful, as is the rapid access to the image.
Photogrammetry and 3-D photography
An image contains photometric, positional, and dimensional information about the subject. Knowledge of the location and orientation of the lens axis and the photoplane relative to the subject and/or another camera position, followed by measurement of the image coordinates of chosen points and computer-assisted calculations, give three-dimensional data or contour maps of the subject. A stereo pair of images taken using techniques of three-D photography is a primary requirement to facilitate data extraction. Familiar applications include aerial survey work and map-making, but terrestrial uses of photogrammetry by non-contact methods give precise measurement of subjects and changes otherwise difficult to achieve.
— Sidney Ray
Bibliography
- Darius, J., Beyond Vision: One Hundred Historic Scientific Photographs (1984).
- Krook, H., et al., Lennart Nilsson: Nature Magnified (1984).
- Jussim, E., and Kayafas, G., Stopping Time: The Photographs of Harold Edgerton (1987).
- Ray, S. (ed.), High Speed Photography and Photonics (1997).
- Thomas, A. (ed.), Beauty of Another Order: Photography in Science (1998).
- Ray, S., Scientific Photography and Applied Imaging (1999)
