Question

If you exposed an image to a UV lamp, a tungsten lamp, or an IR lamp—would you expect the images to look the same or different? Explain. A complete answer will include the quantitative comparison of photon energies.

Answer 1

The electromagnetic spectrum is comprised of a series of waves arranged in order of wavelength. The various types of radiation forming this spectrum differ widely, and only a very minute part of this spectrum has any relevance to photography. Usually photography is confined to the visible part of the spectrum, those waves that the eyes see as "light'. This part of the spectrum is comprised of wavelengths from approximately 400 - 700 nanometers (nm), which can be seen by the eye as a change of color. The shorter wavelengths are blue, the longer ones red. At either end of the visible spectrum lie two "invisible" spectra: the ultraviolet which extends from x-rays to the blue end of the visible spectrum, and the infrared which extends beyond the red and into heat (Figure 1). One important function of photography is to extend the range of spectral visualization of the human eye and record these "invisible" spectra. Infrared and ultraviolet photography therefore acts as investigative tools that are capable of discovering new facts about the subject. In some fields of investigation, extensive work has been reported on the use of invisible radiation photography. Other areas of application, however, remain unexplored and await the attention of the research-oriented photographer.

At either end of the visible spectrum lie two "invisible" spectra: the ultraviolet which extends from x-rays to the blue end of the visible spectrum, and the infrared which extends beyond the red and into heat. Both ultraviolet and infrared photography offer a visible interpretation of an invisible state - no one has ever seen what the subject looks like under these radiations because the retina is insensitive to them. There is, therefore, no "correct" density to print to. It is also sometimes very difficult to interpret the infrared or ultraviolet record. It is for these reasons that one should always include a control photograph taken with visible light to provide an exact comparison of the subject. It is also worth pointing out that clinicians and scientists will often have an incomplete understanding of the value of infrared and ultraviolet techniques. The competent photographer, however, will always be alert to the possible application of these techniques, and may indeed need to correct misunderstandings about their use. There are two distinct techniques of ultraviolet photography: the reflected or direct method, and the ultraviolet fluorescence method. Reflected ultraviolet photography requires the subject to be lit with ultraviolet radiation, and filtration used so that only ultraviolet radiation is allowed to reach the film. The ultraviolet fluorescence technique requires that only ultraviolet radiation is allowed to fall on the subject, and the camera (if there is any fluorescence) records the emitted visible light.

The ultraviolet spectrum extends from approximately 10 to 400nm, overlapping x-rays at the shorter wavelengths and running into the violet end of the visible spectrum (Figure 2). The ultraviolet spectrum is further divided into near UV (320-380nm), middle UV (200- 320nm) and vacuum UV (10-200nm) by physicists, or into UVA, UVB, UVC OR UVD, by biologists. The UVA extends from 320nm to 400nm and is known as the glass transmission region, the UVB extends from 280nm to 320nm and is known as the erythemal or sunburn region, while the UVC extends from 185nm to 280nm and is known as the bacterial region. The UVB region is best known for the erythemal effects - which stimulate melanin production as a means of protection - sun tanning. The suntan was once popular as a symptom of a healthy lifestyle (Figure 3) but the dangerous effect of ultraviolet in triggering malignant melanoma of the skin is now recognized. The photographer's interest lies in the near ultraviolet, or UVA, region although the researcher should understand the effects of the other regions of ultraviolet. For example, there are two significant biological effects of ultraviolet radiation: germicidal and erythemal. Short wavelength ultraviolet is very effective as an antibacterial agent. Some caution needs to be exercised with "continuous" sources of ultraviolet, as there is a real risk of burning yourself and/or your patient, or of getting conjunctivitis. This is because the eye cannot "see" below 400nm, but the peak of erythemal activity occurs at 297nm (Figure 4); also there is no heat emission from most sources of ultraviolet to warn the photographer of harmful exposure.

A quantitative comparison of multiple?photon absorption for a number of polyatomic molecules has been performed. A basis for this comparison has been developed that takes into account the different experimental conditions and molecular parameters, and provides suitable normalization of the fluence and absorption parameters. The normalization, and consequently the generalized interaction process, can be specified in terms of the spectroscopic absorptioncross section of the molecule ?0(?) and effective fraction of molecules that interact with the radiation field ?f?. The results of the study indicate that in terms of the normalized absorption variables, multiple?photon absorption is a general phenomenon that is qualitatively the same for all molecules; quantitative differences can be related to differences in ?0 and ?f?. The absorption of all polyatomic molecules can be described in terms of the number of photonsabsorbed per molecule ? (?) at a fluence ?. The data indicate that in spectral regions where ?0(?) ?0.1 (max), ? (?) can be represented by the function ??where ?=1 for ?0?/?f?<1 and ??2/3 for ??/?f??1. Quantitative values of ?f? and the functional dependence of ? (?) on the exerimental parameters such as gas pressure, optical bandwidth, and optical pulse duration are derived and shown to be in good agreement with experimental data for SF6. These results are also compared with the anharmonic oscillator model for multiple?photon excitation of polyatomic molecules.