Coaxial lighting is a technique whereby light is transmitted to the subject along the axis of the lens, from the direction of the camera (Fig. 7-6). This method of lighting is accomplished by placing an optical flat or beam-splitter at a 45 ° angle between the camera and the subject. The beam-splitter should be housed in a box that has an opening above and below the splitter and is lined with black velvet or flocked black paper ( Fig. 7-7). The box
eliminates all light except for that from the intended light source, and absorbs all unnecessary light transmitted or reflected by the splitter. A light is directed at the beam-splitter that reflects it downward along the lens axis where it reflects off the subject and back up through the beam-splitter to the lens. The use of this technique results in a loss of 75 percent of the light, as each time the light strikes the splitter, 50 percent of it is either transmitted or reflected uselessly.
Best results are achieved with this technique when the subject is relatively flat and reflective such as a polished sur face. Coaxial lighting reveals in minute detail the microtopography of the sub-ject's surface, otherwise difficult to record due to its very low relief. This sort of
i mage can be a great aid in the analysis of Figure 7-8Growth pattems on a quartz crystal face illuminated by coaxial lighting
BEAM-SPLITTER
ANGLED SURFACE
SPOTLIGHT WITH PARALLEL BEAM
PERPENDICULAR SURFACE
SUBJECT
Figure 7-9 Principle of coaxial lighting. Surfaces perpendicular to the light path reflect more light back to the camera, appearing brighter than surfaces inclined to the light path. (After Blaker, 1989)
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SPOTLIGHT
SPOTLIGHT MIRROR
Figure 7-10 Near-axial lighting methods (a) Lamp axis Glose to that of lens. (b) Mirror redirects the li ght Glose to lens axis.
the surface detail of low relief fossils and growth features on crystal faces ( Fig. 7-8).
The technique is also useful in illuminat-ing objects in deep cavities.
Figure 7-9 illustrates how coaxial lighting works. A flat surface parallel to the film plane will reflect more light and therefore appear brighter than a surface inclined to the film plane.
The beam-splitter must have optically flat, parallel surfaces to minimize distor-tion. A certain amount of linear distortion is unavoidable due to the offset of the light beam by refraction. This problem can be minimized by using the thinnest beam-splitter possible. Beam-beam-splitters are avail-able commercially through scientific supply houses such as Edmund
Scientific. You can make your own, of somewhat lesser quality, by using micro-scope cover slides, glass for mounting 35-mm slides, or a glass photographic plate with the emulsion removed (by soaking in bleach).
For best results, the light suurce should provide a beam of light that is as parallel sided as possible. Hine (1971) dis cusses this point in some detail. For our purposes, a good focusing microscope light will do.
There are several ways to achieve near axial lighting, all of which avoid the loss of light that occurs with the use of a beam-splitter. These techniques however, do not achieve quite the same results as coaxial lighting (see Fig. 7-10).
LENS SHIELD
RINGLIGHT
MATTE-SURFACE MIRROR WITH RECTANGULAR HOLE
SPOTLIGHT
Figure 7-10 (c) Ringlight surrounds the lens to provide near-axial lighting from all around the lens.
( d) Matte surface mirror (matte side of aluminum foil) with a rectangular hole can be closer to lens axis because of the small hole size.
The closest approximation is achieved by the use of a mirror with a hole in it. It is positioned and illuminated just as with the beam-splitter. Instead of a mirror, a sheet of cardboard can be coat-ed with aluminum foil, matte side out, and pierced in the same manner. The matte side of the foil diffuses the light, making it more even and eliminating the possibility of "hot spots" from the use of the shiny side.
Another option is the use of two mir-rors placed at a 45 ° angle to the light source, just as in the use of the beam splitter. The camera looks through the gap between the mirrors. The mirrors must be perfectly parallel, in the same plane, and their adjoining edges must be
aligned with the lamp axis. Achieving proper alignment with this technique can be difficult.
Ringlights are also very useful for near axial lighting. As mentioned in Chapter 6, many flash manufacturers make them, but you have the problem of not seeing what you are getting until the film is processed. For black-and-white work, ringlights are available with neon or fluorescent tubes.
Less useful, but another possibility, is the placement of small mirrors near the lens, aimed at the subject, and illuminated by a focusable light source. And finally, you can place your light source or sources close to the lens and aim it at the subject.
Background choice is more limited 61
DEPTH OF FIELD
MOTORIZED STAGE PLANE OF LIGHT
MOTORIZED SI.GE
DEPTH OF FIELD
Figure7-11a Scanning light photography principles and depth-of-field limits.
Figure 7-11bScanning light photography setup and projector mounting. (Drawings by Michael junius, from Sharp and Kazilek, 1 990)
with coaxial and near-axial lighting. Any background will be uniformly lighted and little can be done to make it more artistic.
For paleontological photography, this is not a problem usually, as backgrounds typically are plain black or white for sci-entific publication. With near-axial light-ing uslight-ing either a rlight-inglight or pierced mirror, a white background can be creat-ed using aluminum foil. Use it matte side up and be careful not to wrinkle the foil.
When using a spotlight or mirror nearly on axis, a shadow will be created if the subject is resting directly on a white back-ground. The specimen may be placed on a sheet of glass suspended above a sepa-rately lighted background material.
You may find that with axially lighted small subjects, the background is magni-fied enough to reveal distracting texture.
If this is the case, raise the subject above the background on a small pedestal.