Short wavelength cut-offs and maximum spectral irradiance wavelengths
comparisons of the spectra for erythema and pre-vitamin D3 synthesis
A.V. Parisi, D.J. Turnbull and J. Turner
Faculty of Sciences, University of Southern Queensland, Toowoomba, 4350. Australia.
Ph: 61 (0)7 46312226, Fax: 61 (0)7 46312721. Email: [email protected]
Materials and Methods
The horizontal plane spectral pre-vitamin D3 effective
solar UV irradiances on a horizontal plane (UVD3)
were calculated by weighting the UV spectrum with
the action spectrum for the synthesis of pre-vitamin
D3 (Webb, 1993). The UV spectra were also weighted
with the erythemal action spectrum (CIE, 1987) to
calculate the spectral erythemal UV (UVEry).
The UV spectrum on a horizontal plane was
measured at five minute intervals throughout the day
between 05:00 and 17:00 Australian Eastern Standard
Results
A sample solar UV spectrum at a cloud free time and
the associated spectral UVEry and UVD3 are provided
in Figure 1 for a solar zenith angle (SZA) of 13.8o on
12 February 2003 at 12:00 AEST (Australian Eastern
Standard Time). At this SZA, the peak in the spectral
UVD3 shifts to longer wavelengths compared to that
for the spectral UVEry.
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10000
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The risk of skin cancer and sun-related eye disorders
increases with exposure to solar ultraviolet (UV)
radiation. On the positive side, solar UVB (280-320
nm) exposures are required for the initiation of the
synthesis of pre-vitamin D3 (Webb, 1993). The
relationship between the burden of disease and UV
exposure displays a minimum in the burden versus
exposure curve (Lucas and Ponsonby, 2002). In order
to optimize the UV exposure of humans by
minimizing the UV exposures to a population, while
maintaining vitamin D sufficiency, a complete
understanding of the solar UV environment is
necessary. This requires an understanding of the
complex interactions between the factors influencing
the solar UV environment. Previous research has
investigated the short wavelength cut-offs (λc) of the
spectral irradiances for the erythemal UV (Kollias et
al., 2003) and for pre-vitamin D3 synthesis (Parisi et
al., 2007). The aim of this research is to compare the
λc and the wavelengths of the maximum spectral
irradiances (λMax) of the spectral biologically
effective UV for pre-vitamin D3 synthesis and for
erythemal UV.
At the start of each spectral scan, the fraction of cloud
cover was measured with a Total Sky Imager (TSI)
(model TSI-440, Yankee Environmental Systems,
MA, USA) with approximately 160o field of view of
the sky. The TSI provides the fraction of cloud cover
at each five minute interval.
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Introduction
Time (EST) with a UV spectroradiometer (model
DTM300, Bentham Instruments, Reading, UK) at a
Southern Hemisphere site at Toowoomba, Australia
(27.6 °S, 151.9 °E, 693 m above sea level) over the
interval 1 January 2003 to 30 June 2003. This
spectral UV data was recorded from 280 to 400 nm in
0.5 nm increments and each spectral scan took
approximately two minutes as described elsewhere
(Parisi and Downs, 2004). The λc for UVD3 and UVEry
were defined as the wavelengths at which the
respective spectral irradiances were 0.1% of the
maximum biologically effective irradiances for each
scan.
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100
Spectral UV (mWm
-2nm -1)
Spectral UV (mW m nm )
Abstract This paper has compared the short wavelength
cut-offs and the wavelengths of the maximum irradiances
for the biologically effective spectral UV for erythema
(UVEry) and pre-vitamin D3 synthesis (UVD3) over a six
month period for the range of solar zenith angles from 4.7o
to 80o. When averaged over the six month period, the short
wavelength cut-off for the UVD3 is higher by 0.9 nm than
that for UVEry. The wavelength of the maximum spectral
irradiance is higher for the UVD3 compared to the UVEry
for SZA less than approximately 60o. For higher SZA,
there is a change with the wavelength of the maximum
spectral irradiance for UVD3 being reduced compared to
that for the UVEry.
1010
11
0.1
0.1
0.01
0.01
0.001
0.001
280 300
300 320
320 340
340 360
360 380
380 400
400
280
Wavelength(nm)
(nm)
Wavelength
Figure 1. A typical UV spectrum with the corresponding
spectral UVEry (thin line) and UVD3 (thick line).
The variation of the λc throughout a typical summer’s
day on 8 January 2003 is shown in Figure 2. On this
day, the cut-off wavelengths range from 290 at noon
to 297 nm at 06:00 and 18:00 EST. For the UVD3, the
λc are shifted to longer wavelengths by an average of
1 nm.
Cut-off (nm)
300
298
296
294
292
wavelengths of the UVD3 and the UVEry spectral
irradiances over a six month period for the range of
solar zenith angles from 4.7o to 80o. The λc for the
UVD3 is higher by 0.9 nm than that for UVEry when
averaged over the six month period. This is due to the
influence of the shapes of the action spectra as the
action spectrum for the synthesis of pre-vitamin D3 is
shifted to longer wavelengths between 300 and 315
nm compared to the action spectrum for erythema.
The λMax wavelength is higher for the UVD3
compared to the UVEry for SZA less than
approximately 60o. For higher SZA, the ratio of the
λMax for UVD3 to that for UVEry is predominantly one
or less.
290
8.4
8.6
8.8
Day
Figure 2. Short wavelength cut-offs during the day on 8
January for the UVD3 (◊) and UVEry (•).
UVD3/UVEry cut-off wavelength
Averaged over the six month period, the difference
between the λc for UVD3 and UVEry was 0.8 nm. The
ratio of the λc for the UVD3 to those for UVEry for the
range of SZA encountered over the six month period
are provided in Figure 3.
1.06
UVD3/UVEry maximum
wavelength
8.2
1.04
1.02
1.00
0.98
0.96
0
20
40
60
80
SZA (degrees)
1.06
Figure 4. Ratio of the wavelengths of the maximum irradiances
for UVD3 and the corresponding UVEry.
1.04
1.02
Acknowledgments
The UV spectroradiometer was funded
by the Australian Research Council. The authors acknowledge
USQ technical staff and Nathan Downs for assistance in setting
up the spectroradiometer and Dr Jeff Sabburg for the TSI data.
This research was supported by a Project Grant awarded by the
Queensland Cancer Fund.
1.00
0.98
0.96
0
20
40
60
80
SZA (degrees)
Figure 3. Ratio of the cut-off wavelengths for UVD3 and the
corresponding UVEry for the range of SZA.
The ratio of the λMax wavelengths for the UVD3 and
the corresponding UVEry over the six month period
are provided in Figure 4 for the range of SZA. The
ratio is higher than one for SZA of less than
approximately 60o. As the SZA increases beyond 40o,
the ratio starts reducing and for SZA of higher than
60o, the ratio is predominantly one or less.
Discussion
This paper has compared the λc and the λMax
References
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