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Last updated January 1, 2014

A comparison of the UV-B irradiance of low-intensity, full-spectrum lamps with natural sunlight

James C. Ball, PhD

Abstract
Natural sunlight has a much higher UV-B irradiance than most commercial sources of low-intensity, full-spectrum lamps. The maximum UV-B irradiance near the equator (solar elevation angle < 25 deg.) under clear, sunny skies, is about 250 µW/cm². Commercial full-spectrum light sources, such as the Duro-Test Vita-Lite, have UV-B light irradiances about 100 times lower than natural sunlight. The exposure time required for several commercial lamps to mimic five minutes of UV-B sunlight at the equator are tabulated.

Introduction
The use of low-intensity, full-spectrum lamps (e.g. Vita-Lite) has been reported to be critical for the successful husbandry of lizards, turtles, tortoises, and immature crocodilians as well as other unspecified reptiles (Frye, 1991; Seufer, 1991). The most often cited reason for using full-spectrum lighting is related to the need for the animal to synthesize vitamin D3. The photochemical reaction that occurs in mammalian skin (and presumably reptiles and amphibians) is a ring opening reaction of 7-dehydrocholesterol to form a conjugated double bond system, previtamin D3. 7-Dehydrocholesterol has significant absorption of UV light at the lowest wavelengths of light found in sunlight (290-310 nm). Hence the need for full-spectrum lamps to emit UV-B radiation (UV-B light is defined here to be light between 290-315 NM). Previtamin D3 thermally rearranges, without enzymic catalysis, to form vitamin D3 or cholecalciferol (Holick, 1989; MacLaughlin et al., 1982). Cholecalciferol can be further metabolized to calcitriol (1,25-dihydroxycholecalciferol). Calcitriol acts like a hormone in the regulation of calcium metabolism and is critical for normal growth and development (Frye, 1991; Lehninger, 1976).

There are significant risks to humans exposed to lamps that emit high intensity UV-B radiation because light in this wavelength range can cause sunburns (Armstrong et al., 1985; Berger and Urbach, 1982), cataract formation (Silney, 1983), and has been associated with the development of skin cancer (Freeman et al., 1988; Henriksen et al., 1989). For these reasons manufacturers are reluctant to sell lamp sources that emit significant amounts of UV-B light to the general public. This has led me and others to question how much UV-B radiation is actually emitted by full-spectrum lamps. Gehrmann (1987;1994) has measured the UV-B light output of a variety of commercial lamp sources and has indeed found that one of the most commonly used full-spectrum lamps (e.g. Vita-Lite®; Duro-Test) emits very little UV-B light.

There is very little experimental data on the metabolic or behavioral requirements for natural sunlight or full-spectrum light in maintaining captive reptiles and amphibians. Gehrmann et al. (1991) observed that neither vitamin D3 supplementation nor exposure to UV-B light effected the growth of the heliophilic western fence lizard (Sceloporus occidentalis). Roberts and Gehrmann (1990) compared the growth of bearded lizards (Pogona vitticeps) exposed to either plant lights (Gro-Lux®) or full-spectrum lights (Vita-Lite®). Both groups were also exposed to a black light. There were no significant differences in growth between the two groups suggesting that subtle adaptive physiological and behavioral responses were not dependent on full-spectrum light. The design of this experiment, however, did not allow for a conclusion regarding the effect of UV-B radiation since both groups were exposed to a black light. There is a report suggesting that green iguanas (Iguana iguana) cannot metabolize exogenous sources of vitamin D3 normally (Bernard et. al., 1991). These iguanas initially showed clinical signs of metabolic bone disease, but no new bone fractures were observed after exposure to an experimental UV-B emitting lamp (Sylvania 2096, 15 watt fluorescent lamp), suggesting that iguanas require UV-B light. In addition, anecdotal reports from veterinarians treating green iguanas for metabolic bone disease suggest that full-spectrum lamps are critical for the health of these lizards (Myers, 1994).

The relative intensity of UV-B radiation from different commercial lamps has been systematically studied (Gehrmann, 1987). The purpose of this paper is to compare the UV-B light output of those lamps studied by Gehrmann (1987) with the UV-B light intensity of sunlight from a variety of locations and latitudes.

Methods
The literature was examined for reports on measurements of sunlight as a function of wavelength, time of year, latitude, elevation, and cloud cover. Many of the reports used units of intensity that were not directly comparable (e.g. erythema) to the measurements made by Gehrmann. Hence, only those papers that reported UV-B light in power units (e.g. watts) per area were used. The measurement of the natural irradiance of UV-B light is very difficult because the intensity of light falls off exponentially as a function of wavelength and very little UV-B light strikes the surface of the earth. Some graphical data required manual integration of the intensity of UV-B light over the wavelength range 290-315 NM in order to estimate irradiance. The intensity of UV-B light is heavily biased towards the longer wavelengths of the UV-B range (290-315). That is, the contribution of the light from 290-294 NM is swamped by the intensity of light from 310-315 NM Laboratory measurements of UV light from 290-295 NM are not been readily available.

Results and Discussion
Table 1 shows the intensity of UV-B light at different locations and solar elevation angles (the angle of the sunlight hitting the surface of the earth). The data for locations with solar elevation angles less than 25 deg. are remarkably similar (approx. 250 µW/cm²; range 212-265 µW/cm²), giving credibility to the methods used for estimating UV-B light intensity. It should be pointed out that although there is significant UV-B light at extreme latitudes, such as Finland and Norway, there is no detectable ultraviolet light shorter than 300 NM (Henriksen et al., 1989: Kolari et al., 1986). Since the action spectrum for previtamin D3 has a maximum below 295 NM, natural sunlight at these extreme latitudes will photochemically isomerize very little of this compound.

The data in Table 1 shows that the most commonly used lamps do a poor job of simulating the UV-B light intensity of natural sunlight. The only lamps that come close to the intensity of natural sunlight are sunlamps, which are sometimes used to treat psoriasis. The practical interpretation of Table 1 for the maintenance of reptiles and amphibians will ultimately depend on experiments elucidating the time-integrated exposure of UV-B light necessary for the health of captive reptiles and amphibians. For example, is five minutes of equatorial sunlight sufficient for a particular species to prevent metabolic bone disease or to stimulate breeding? The answer to these questions remains until appropriate experiments are carried out. In the mean time, Table 1 can be used as a relative guide to the use of low-intensity, full-spectrum lamps in the care of reptiles and amphibians.

Acknowledgements
I would like to thank James Harding (Michigan State University) and Dr. Mark Miller (Herpetological Online Network) for their helpful comments on this manuscript. I also thank William Gehrmann (Tarrant County Junior College, Texas) for his comments on an earlier version of this paper, for some key references, and for his detailed comments on this manuscript.

Literature Cited

Armstrong, R.B., G.B Whitman, F.G. Gasparo, E.E. Leach. 1985. Potential hazards in phototherapy with ultraviolet radiation arising from variation in dose required to produce erythema. J. Am. Acad. Dermatol. 13:772-777.

Bernard, J.B., O.T. Oftedal, P.S. Barboza, C.E. Mathias, M.E. Allen, S.B. Citino, D.E. Ullrey, and R.J. Montali. 1991. The response of vitamin D-deficient Green Iguanas (Iguana iguana) to artificial ultraviolet light. Proc. Am. Ass. Zoo Veterinarians 147-150.

Berger, D.S and F. Urbach. 1982. A climatology of sunburning ultraviolet radiation. Photochem. Photobiol. 35: 187-192.

Frederick, J.E. and H.E. Snell. 1988. Ultraviolet radiation levels during the Antarctic Spring. Science 241:438-440.

Freeman, S.E., R.D. Ley, K.D. Ley. 1988. Sunscreen protection against UV-induced pyrimidine dimers in DNA of human skin in situ. Photodermatol. Photoimmunol Photomed. 5:243-247.

Frye, F.L. 1991. Pp. 23,46-56 Reptile Care: An Atlas of Diseases and Treatments. Volume 1. Neptune City: TFH Publications, Inc.

Holick, M.F. 1989. Phylogenetic and evolutionary aspects of vitamin D from phytoplankton to humans. pp. 27-30 In: P.K.T. Pang and M.P. Schreibman, editors, Vertebrate Endocrinology: Fundamentals and Biomedical Applications. Volume 3. Regulation of Calcium and Phosphate. New York: Academic Press, Inc.

Gehrmann, W.H. 1987. Ultraviolet irradiance of various lamps used in animal husbandry. Zoo Biol. 6:117-127.

Gehrmann, W.H., G. W. Ferguson, T.W. Odom, D.T., and W.J. Barcellona. 1991. Early growth and bone mineralization of the iguanid lizard, Sceloporus occidentalis in captivity: Is vitamin D3 supplementation of ultraviolet B irradiation necessary? Zoo Biol. 10:409-416.

Gehrmann, W.H. 1994. Spectral characteristics of lamps commonly used in herpetoculture. Vivarium 5:16-21,29.

Henriksen, K., K. Stamnes, G. Volden, and E.S. Falk. 1989. Ultraviolet radiation at high latitudes and the risk of skin cancer. Photodermatology 6:110-117.

Kolari, P.J., J. Lauharanta, M. Hoikkala. 1986. Midsummer solar UV-radiation in Finland compared to the UV-radiation from phototherapeutic devices measured by different techniques. Photodermatology, 3:340-345.

Lehninger, A.E. 1976. pp. 355-357 Biochemistry. New York: Worth Publishers, Inc.

MacLaughlin, J.A, R.R. Anderson, and M.F. Holick. 1982. Spectral character of sunlight modulates photosynthesis of previtamin D3 and its photoisomers in human skin. Science 216:1001-1003.

McKenzie, R.L., P.V. Johnston, M. Kotkamp, A. Bittar, and J.D. Hamlin. 1992. Solar ultraviolet spectroradiometry in New Zealand: instrumentation and sample results from 1990. Applied Optics 31:6501-6509.

McKenzie, R.L., M. Kotkamp, R. Erb., C.R. Roy, H.P. Gies, and S.J. Toomey. 1993. First southern hemisphere intercomparison of measured solar UV spectra. Geophysical Res. Lett. 20:2223-2226.

Myers, M. 1994. Personal Communication. Hartman Veterinary Hospital, Toledo, OH.

Roberts, D.T. and W.H. Gehrmann. 1990. Light quality and growth in the Bearded Lizard, Amphibolurus: A preliminary study. Bull. Chicago Herp. Soc. 25:101-103.

Seufer, H. 1991. Pp. 44-50 Keeping and Breeding Geckos. Neptune City: TFH Publications, Inc.

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Silney, D.H. 1983. Biohazards of ultraviolet, visible, and infrared radiation. J. Occ. Med. 25:203-206.

Webb, A.R. and M.D. Steven. 1987. Solar ultraviolet-B radiation under cloudless skies. Q.J.R. Meteorol. Soc. 113:393-400.

Table 1. UV-B Irradiance[a] from sunlight and commercial lamps[b]
The data in the left column are: sunlight; location, time, date, weather, latitude, solar elevation angle (SEA), and literature citation.

[a] UV-B light is defined here to be light between 290-315 nm. 
[b] W.H. Gehrmann, Zoo Biology, 6:117-127 (1987). 
[c] n/a - not applicable. 
[d] No longer manufactured; similar sunlamps are offered by National Biological Corp, Twinsburg, OH.

Ball, James. 1995. A Comparison of the UV-B Irradiance of Low-Intensity, Full-Spectrum Lamps With Natural Sunlight. Originally published in the Bulletin of the Chicago Herpetological Society, 30(4):69-71. Reprinted here by request of the author.

James Ball
York Serpentarium
1083 Jewell Road
Milan MI 48160

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