The goal of solar ultraviolet (UV) simulation is to reproduce the natural solar UV spectrum. However, this spectrum changes continuously depending on parameters, such as latitude, season, time, cloudiness, etc. From spectra recorded worldwide throughout the year, a realistic ("standard") solar UV spectrum at Earth level was defined by the Deutsches Institut f|r Normung e.V. (DIN) to represent a "worst" case situation. Exposure of human skin to such a spectrum is likely to result in intense biological effects. Simulated solar UV spectra should match the standard spectrum as closely as possible. Here, we present a method to assess the match between a laboratory spectrum and the standard spectrum. Representative UV sources such as xenon arcs, metal halide lamps and fluorescent tubes, along with various filters, have been measured. Differences between the relative irradiance of UV candidate spectra and the standard are calculated for each wavelength. These differences are squared and summed. The lower the sum, the better the match of the source spectrum to the standard sun. This method may be used with or without biological weighting by an action spectrum. We have selected the erythema action spectrum to assess and rank candidate sources. Our analysis shows that filtered ultraviolet B fluorescent tubes are the worst way of simulating solar radiation, with and without weighting by the erythema action spectrum. UV spectra from solaria equipped with combinations of ultraviolet A and ultraviolet B fluorescent tubes are also far from satisfactory. In general, metal halide lamps rank slightly better than the fluorescent UVB tubes. The choice of UV filter plays a significant role in the compliance of candidate UV source. In conclusion, the suggested method allows the determination of the most appropriate UV source to simulate real solar exposure for any targeted biological damage.

1. Diapositive 1
International Congress on Photobiology
San Francisco, 1-6 July 2000
1-
Choosing a solar UV simulator
with an appropriate spectrum
François J. Christiaens and A. Fourtanier
L´ORÉAL Research, Clichy-France
Research, Clichy-
Good afternoon,
I appreciate being able to speak to this group.
We would like to talk to you on how solar radiation and solar
simulators used in photobiological experiments compare with
another. We also want to show you a method to help choose
the best solar UV simulator.

2. Diapositive 2
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Sunlight Variability
0.16
Spectral irradiance (mW.cm-2.nm-1) Zenithal sun (DIN 67501)
0.14 Morning / afternoon
Early morning / late afternoon
0.12 Sunrise / sunset
0.1
0.08
0.06
0.04
0.02
0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
This slide shows a series of solar UV spectra. The most
important feature is that there is not a single ubiquitous solar
UV spectrum. Anyone outdoors will be exposed to many
different spectra, depending on sun altitude above horizon and
local weather conditions.
The sun has its zenith (i.e located just above your head) at
precise locations and given dates. The zenithal sun spectrum
(dark blue curve), defined as the “worst” case spectrum, is
given by the DIN 67501 standard.
Exposure of human skin to such a spectrum is likely to result in
intense biological effects. This spectrum will be referred as the
standard UV sun spectrum.

3. Diapositive 3
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Ultraviolet Sources
* Fluorescent tubes
0.025
Standard sun
0.02 UVB tube
Relative spectral irradiance
1B, 6A
UVA-340
0.015
0.01
0.005
0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
We are going to show some typical UV sources which are
commonly used to simulate the solar UV spectrum in
photobiological applications.
On this slide, the spectra of fluorescent tubes are plotted.
Because sources are operated at a wide range of irradiances, a
method had to be developed for comparing each other: To
normalize the curves, measured spectral irradiances have been
divided by their total spectral irradiance. In other words, the
reciprocity law is assumed to be valid.
UVB tubes (blue curve) are typically TL12/20 when
manufactured by Philips Company or FS20 by Westinghouse.
Here, they have been filtered with Kodacel film to remove any
radiation below 290 nm. They are widely used, for example by
Dr. Kripke, Ley, Cooper etc. for phototherapy.

4. A combination of 1 UVB fluorescent tube with 6 UVA
fluorescent tubes (green curve) has been used and is described
in the literature (Reeve, Halliday). These tubes come from the
Westinghouse or Philips companies; in both case the spectrum
is the same.
The UVA -340 tubes come from Qpanel Co (pink curve). They
have been used by Roberts and Beasley.

5. Diapositive 4
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Ultraviolet Sources
* Metal halide lamps
0.025
Standard sun
0.02
Relative spectral irradiance
Metal halide 1
Metal halide 2
0.015
0.01
0.005
0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
Metal halide lamps come from Dermalight or Atlas.
They emit complex spectra with intense peaks, particularly that
of mercury, and high UVA energy.
They have been used by Moyal, Lowe…, mainly for UVA
applications.

6. Diapositive 5
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Ultraviolet Sources
* Xenon arcs
0.025
Standard sun
Short arc xenon
0.02
Relative spectral irradiance
Long arc xenon, WG320
Long arc xenon
0.015
0.01
0.005
0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
Short arc xenon lamps are used in Oriel and Solar Light solar
UV simulators. These sources are widely used, for example by
Dr. Kripke, Ullrich, Roberts, Sayre, Moyal, Marrot, Bernerd,
Fourtanier, Duval, Young, Guéniche…
Long arc xenon come from Atlas and are used for photostability
testing. They are also used in pharmaceutical testing.
These lamps can be filtered with a Schott WG320 to improve
the shape of the UV spectrum. This modified spectrum has
been used in photocarcinogenesis studies.

7. Diapositive 6
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Least Square Method
Comparison at each
0.025
Relative spectral irradiance (A. U.)
wavelength
Standard sun
0.02 Sum of the squared
Short arc xenon
differences
0.015
0.01
0.005
The lower the sum, the
0 closer the simulator
290 300 310 320 330 340 350 360 370 380 390 400 spectrum to the reference
spectrum
Wavelength (nm)
After normalization of the spectra, defined with an increment
step of 1 nm, the spectrum of each candidate source is
compared to the standard sun spectrum at each wavelength.
The difference calculated at each wavelength is squared, so
that a lack at a given wavelength does not compensate for a
excess at another wavelength.
Here the spectrum of a short arc xenon lamp is being compared
to the standard sun spectrum.
Then all the squared differences are summed over the UV
waveband (290-400nm). The lower the sum, the smaller the
difference between the two spectra, the closer the simulator
spectrum to the reference spectrum.

8. Diapositive 7
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Results: Ranking
1 800
1 600
1 400
1 200
1 000
800
600
400
200
-
UVB tube 1 B, 6 A UVA-340 Metal Metal Short arc Long arc Long arc
halide 1 halide 2 xenon xenon + xenon
WG320
In the table the squared spectral differences have been
reported. For example, for correctly filtered short arc xenon
lamp, the sum may be as low as 6E-4.
Then the inverse of the sum was calculated. So the higher the
inverse, the better the match. Candidate sources have been
ranked according to their inverse value.
About long arc xenon lamps: The commercially available
spectrum is plotted with a plain rectangle. The empty rectangle
stands for a long arc xenon lamp filtered with a WG320.
Correctly filtered xenon arcs sources prove to be the best match
of the standard sun spectrum.
We can also notice see that, although filtered, UVB fluorescent
tubes provide the worst solar simulation.

9. Diapositive 8
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Considering a biological action spectrum
1.6
Standard sun
Spectral irradiance (mW.cm-2.nm-1)
Relative efficacy (arbitrary units) 1.4
Erythemal action spectrum
1.2 Standard erythemal sun (x 100)
1
0.8
0.6
0.4
0.2
0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
Now, let ’s consider that we irradiate biological systems. Most of
photobiological effects show a high dependence on the UVB
waveband. A representative, commonly used and internationally
recognized action spectrum is the erythema action spectrum,
sponsored by the Commission Internationale de l’Eclairage
(pink curve).
The “new” reference spectrum is now the efficacy spectrum of
the standard sun, i.e. the standard sun spectrum multiplied by
the erythema action spectrum (blue curve with yellow marks).
Spectra of candidate sources are multiplied by the erythema
action spectrum and they are compared to the new reference
spectrum.

10. Diapositive 9
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Efficacy spectra
* Fluorescent tubes
2.5E-4
2.0E-4
Relative spectral irradiance
Standard erythemal sun
UVB tube
1B, 6A
1.5E-4
UVA340 tube
1.0E-4
5.0E-5
0.0E+0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
Here are represented the efficacy spectra of the fluorescent
tubes. All the efficacy spectra show a strong peak in the UVB
waveband.
The efficacy spectrum of UVB fluorescent tubes do not follow
the standard sun efficacy spectrum.
When 6 UVA fluorescent tubes are combined with one UVB
tube, the resulting efficacy spectrum is almost superimposed to
the UVB alone efficacy spectrum.
The UVA-340 tube efficacy spectrum shows the best match, in
the fluorescent tubes family.

11. Diapositive 10
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Efficacy spectra
* Metal halide lamps
3.5E-4
3.0E-4
Standard erythemal sun
Relative spectral irradiance
2.5E-4 Metal halide 1
Metal halide 2
2.0E-4
1.5E-4
1.0E-4
5.0E-5
0.0E+0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
On this slide, efficacy spectra of metal halide lamps show big
discrepancies with the standard sun efficacy spectrum, in the
UV range.

12. Diapositive 11
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Efficacy spectra
* Xenon arcs
2.5E-4
2.0E-4
Relative spectral irradiance
Standard erythemal sun
Short arc xenon
1.5E-4 Long arc xenon, WG320
Long arc xenon
1.0E-4
5.0E-5
0.0E+0
290 300 310 320 330 340 350 360 370 380 390 400
Wavelength (nm)
Last but not least, xenon arcs efficacy spectra are very close to
the standard sun efficacy spectrum. Commercially available
long arc xenon, plotted in violet, may be re-filtered so that its
efficacy spectrum becomes much closer to the reference
spectrum.

13. Diapositive 12
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Results: Ranking of efficacy spectra
1.6E+08
1.4E+08
1.2E+08
1.0E+08
8.0E+07
6.0E+07
4.0E+07
2.0E+07
0.0E+00
UVB tube 1B, 6A UVA340 Metal Metal Short arc Long arc Long arc
tube halide 1 halide 2 xenon xenon + xenon
WG320
The least square method, described above, is applied again.
Again, correctly filtered the xenon arc sources provide the best
solar UV simulators, by far.
Ranking is different for UVA-340 fluorescent tube and for long-
arc xenon lamps. However, correctly filtered xenon lamps still
provide the best matching spectra.

14. Diapositive 13
Standard sun Sources Physical spectra match Biological spectra match Conclusion
Summary
Xenon based solar UV simulators have a spectrum close to the
standard UV sun
This ranking is valid whether the action spectrum is known or not
not
Special attention must be paid to the short wavelength filtration
filtration
These are the thoughts that I wish to leave you with.
Often, the spectrum of a UV source is said to be close to the
sun peremptorily. We have presented a method to rank the
different laboratory sources according to the closeness of their
spectrum to the solar spectrum. The results show that xenon
based solar UV simulators reproduce the solar UV spectrum
very closely.
Thus, from the physical point of view as well from the erythemal
point of view, xenon based solar UV simulators are the best
choice.
Furthermore, it is clearly possibly to filter solar simulators to
closely match sunlight and its erythemal risk.
A WG-320 of convenient thickness removes irrelevant UVB and
UVC rays.

15. Thank you for your attention.

16. Diapositive 14
Consequences for SPF when solar simulator spectra
deviate from the spectrum of the sun
SPF 15 sunscreen absorption
36
Monochromatic Protection
31
26
21
Factor
16 36
11 31
6
26
1
290 300 310 320 330 340 350 360 370 380 390 400 21
SPF
Wavelength (nm) 16
11
6
1
UVB 1 B et 6 UVA-340 Metal Metal Short Long Long
tube A halide 1 halide 2 arc arc arc
(Atlas) (Hönle) xenon xenon + xenon
WG320
This slide shows a direct result on what happens on Sun
Protection Factor when it is assessed using sources whose
spectra deviate from the spectrum of the sun.
Here (upper chart) we assume that the sunscreen absorbs
mainly in the UVB waveband, to provide some protection
against erythema. It is not indirectly linked to existing absorbers.
The lower chart shows the gaps between SPFs that would be
measured with different solar UV simulators. The SPF baseline
is set at 15. We can notice that SPF is strongly overestimated
when assessed with fluorescent tubes.
These results are in full agreement with those found in vivo by
Uhlmann et al. in 1996 (Int.J.Cosm.Sci. 18, 13-24) and by Noda,
Kawada et al. in 1992 (J.Dermatol. 19, 465-469).