There are two distinctly different techniques for taking photographs using ultraviolet radiation, and since they are often confused with each other, both will be described.
In the first technique, called reflected-ultraviolet photography, the photograph is made by invisible ultraviolet radiation reflected from an object. This method is similar to conventional photography in which you photograph light reflected from the subject. To take pictures by reflected ultraviolet, most conventional films can be used, but the camera lens must be covered with a filter, such as the Wratten ISA, that transmits the invisible ultraviolet and allows no visible light to reach the film. This is true ultraviolet photography; it is used principally to show details otherwise invisible in scientific and technical photography. Reflected-ultraviolet photography has almost no application for motion picture purposes; if you have questions about reflected ultraviolet photography information is given in the book "Ultraviolet and Fluorescence Photography," available from Eastman Kodak Co.
The second technique is known as fluorescence, or black-light, photography. In motion-picture photography, it is used principally for its visual effects. Certain objects, when subjected to invisible ultraviolet light, will give off visible radiation called fluorescence, which can be photographed with conventional film. Some objects fluoresce particularly well and are described as being fluorescent. They can be obtained in various forms such as inks, paints, crayons, papers, cloth, and some rocks. Some plastic items, bright-colored articles of clothing, and cosmetics are also typical objects that may fluoresce. For objects that don't fluoresce, fluorescent paints (oil or water base), chalks or crayons can be added. These materials are sold by art supply stores, craft shops, department stores, and hardware stores. Many of these items can also be obtained from Wildfire, Inc., 10853 Venice
Blvd., Los Angeles, California, 90034, which manufactures them specially for the motion-picture industry.
Fluorescence may range from violet to red, depending on the material and the film used. In addition to the fluorescence, the object reflects ultraviolet light, which is stronger photographically. Most film has considerable sensitivity to ultraviolet, which would overexpose and wash out the image from the weaker visible fluorescence. Therefore, to photograph only the fluorescence, you must use a filter over the camera lens (such as the Wratten 2B, 2E or 3, or equivalent) to absorb the ultraviolet.
The wavelengths of ultraviolet light range from about 10 to 400 nanometers. Of the generally useful range of ultraviolet radiation, the most common is the long-wavelength 320 to 400nm range. Less common is the short to medium-wavelength range of 200 to 320nm. In fluorescence photography you can use long-, medium-, or shortwave radiation to excite the visible fluorescence depending on the material. Some materials will fluoresce in one type of ultraviolet radiation and not in another.
Certain precautions are necessary when you use ultraviolet radiation. Warning: You must use a source of short- or medium-wave ultraviolet with caution because its rays cause sunburn and severe, painful injuries to eyes not protected by ultraviolet-absorbing goggles. Read the manufacturer's instructions before using ultraviolet lamps.
Eye protection is generally not necessary when you use long-wave ultraviolet because this radiation is considered harmless. However, it's best not to look directly at the radiation source for any length of time, because the fluids in your eyes will fluoresce and cause some discomfort. Wearing glass eyeglasses will minimize the discomfort from long-wave sources.
There are many sources of ultraviolet radiation, but not all of them are suitable for fluorescence photography. The best ultraviolet sources for the fluorescence technique are mercury-vapor lamps or ultraviolet fluorescent tubes. If an object fluoresces under a continuous ultraviolet source, you can see the fluorescence while you're photographing it.
Since the brightness of the fluorescence is relatively low, the ultraviolet source must be positioned as close as practical to the subject. The objective is to produce the maximum fluorescence while providing even illumination over the area to be photographed.
Fluorescent tubes designed especially to emit longwave ultraviolet are often called black-light tubes because they look black or dark blue before they're lighted. The glass of the tubes contains filter material which is opaque to most visible light but freely transmits long wavelength ultraviolet. These tubes, identified by the letters BLB, are sold by electrical supply stores, hardware stores and department stores. They are available in lengths up to 4 feet and can be used in standard fluorescent fixtures to illuminate large areas. Aluminum-foil reflectors are available to reflect and control the light.
Mercury-vapor lamps are particularly suitable for illuminating small areas with high ultraviolet brightness. When these lamps are designed for ultraviolet work they usually include special filters which transmit ultraviolet and absorb most of the visible light. Mercury vapor ultraviolet lamps are available in two types, long-wave and short-wave. Some lamps include both wavelengths in the same unit so that they can be used either separately or together. If you use a light source that does not have a built-in ultraviolet filter, you must put such a filter over the light source. The filter for the radiation source is called the exciter filter.
You can use a Kodak Wratten Ultraviolet Filter, No. 18A, or Corning Glass No. 5840 (Filter No. CS7-60) or No. 9863 (Filter No. CS7-54) for this purpose. The Kodak Filter, No. 18A, is available in 2-and 3-inch glass squares from photo dealers. The dealer may have to order the filter for you. The Corning Glass is available in larger sizes from Corning Glass Works, Optical Photo Products Department, Corning, New York 14830. The filter you use must be large enough to completely cover the front of the lamp. The scene is photographed on a dark set with only the ultraviolet source illuminating the subject. In order for the film to record only the fluorescence, use a Kodak Wratten gelatin filter, No. 2A or 2B, or an equivalent filter, over the camera lens to absorb the ultraviolet. When used for this purpose, the filters are called barrier filters. Since the fluorescence image is visible no focusing corrections are necessary. Focus the camera the same as for a conventional subject.
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