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Fluorescence Photography - Ultraviolet Fluorescence, Infrared Fluorescence, X-ray Fluorescence, Cathodoluminesence

filter radiation visible light

DAVID MALIN
Anglo-Australian Observatory, RMIT University

Fluorescence is electromagnetic radiation emitted by gaseous, liquid, or solid materials as a result of an irradiation with light, X-rays, or electron beams. In fluorescence, all or part of the absorbed energy is re-emitted at the same or longer wavelengths. If the emission of energy continues after the excitation source is removed, the phenomenon is called phosphorescence. The emission can be in the visible or ultraviolet (UV) region of the spectrum if a molecular electronic transition is involved, or in the infrared region (IR) if it is a vibrational transition. The transitions to this state are brief, so the emission occurs within nanoseconds. Other mechanisms are involved with fluorescence outcomes induced by electron beams and X-rays.

Ultraviolet Fluorescence

In most applications of interest to photographers, the exciting radiation is UV energy, often from a “blacklight,” a mercury vapor tube coated with Woods glass (Kodak Wratten 18A or equivalent) to filter out the visible emission lines. Some flash units also strongly emit in the UV region, especially if their UV filters are removed and a Woods glass filter is used. UV energy is dangerous to the eyes and skin of humans and its effects are not immediately observed, so care is essential when working with continuous UV sources.

There are two principal types of UV fluorescence: primary (autofluorescence), in which fluorescence is intrinsic, and secondary in which fluorescence occurs because of the treatment of the sample with a fluorescing substance (a fluorochrome). Primary fluorescence is most often seen in mineral specimens, and a common example of secondary fluorescence is clothing that has been washed in “whiter-than-white” detergents. The selective dyeing of specimens with fluorochromes is important in fluorescence microscopy. For completeness, it should be mentioned that many of the colorful clouds of glowing gas (emission nebulae) seen in astronomical photographs are fluorescent hydrogen and other elements excited by UV radiation from hot stars. The earth’s atmosphere filters out much of the UV radiation from the sun.

Apart from the astronomical applications, to photograph these effects the camera is usually set up in a darkened room or enclosure containing the UV source. Two filters are normally required. One is a blocking filter that restricts the subject illumination to the required wavelengths, and is often Woods glass. A visually colorless, UV-absorbing barrier filter is used on the camera lens. This prevents UV radiation from reaching the film or detector and obscuring the fluorescence effect. The filter also prevents components of the lens itself from fluorescing. Since the light to be recorded is visible light, both film and digital cameras can be used, however, exposure times can be long and are usually determined by trial and error. Also, Woods glass filters have a “red leak” that does not affect film but may be visible to charge-coupled devices (CCDs), so an additional cyan filter may be required on a digital camera.

UV fluorescence photography is used in forensic, geological, medical, and ophthalmologic photography.

Infrared Fluorescence

In this situation, the fluorescence is the emitted radiation (rather than the exciting radiation). Certain materials emit IR radiation when visible or UV radiation is absorbed. This property is sometimes called luminescence but is actually a special form of fluorescence. Again, two filters are required. A pale blue-green, heat-absorbing filter is placed over the light source as a blocking filter and prevents any IR emitted by the source from reaching the subject. A second, barrier filter such as a Kodak Wratten 88A that only allows IR to pass is placed over the camera lens. This is visually opaque and its function is to block all visible light. In most cases exposures are very long, though the excellent IR sensitivity of some digital cameras can be usefully exploited. Some astronomical objects also emit IR fluorescence from interstellar hydrocarbons.

The exciter filter can be omitted if the incident radiation can be limited to visible wavelengths by using a source that emits only wavelengths shorter than IR. Wavelength-tunable lasers provide such a source, and they have the added advantage of easily selecting from a wide range of wavelengths for exciting fluorescence in the infrared or other areas of the spectrum. Applications include document photography for forensic work and examining works of art. Chlorophyll also has a natural IR fluorescence and the technique is also used
with fluorochromes to differentiate cell types in medical imaging, or with nanocrystal materials (quantum dots) for use in biology, drug development, and diagnostic procedures.

X-ray Fluorescence

The most common example of X-ray fluorescence is the intensifying screen used to record radiographs. X-ray film is placed in close contact with two sheets coated with the fluorescing material. The effect of the excited fluorescence on the silver halide particles is greater than from direct absorption of X-ray energy alone, thus reducing the required X-ray dose.

Most elements fluoresce under sufficiently powerful X-rays and images can be made of this X-ray fluorescence. Uses include archaeology, semiconductor research, crystallography, and many applications where non-destructive chemical analysis and the spatial distribution of trace elements is required. These techniques require rather elaborate sources and detectors.

Cathodoluminesence

Some compounds emit visible light when bombarded by an electron beam. The most familiar example is the face of a CRT TV screen. However, in a scanning electron microscope (SEM), the visible-light luminescence induced by the beam can be collected with a photomultiplier and used to make images in the same way as normal SEM images, revealing differences in chemistry and composition on small scales. The same electron beam can generate X-rays that can also be imaged in the same way.

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