Sun, sky and UVb lamp comparison for Vitamin D synthesis
Note. This new plot (2021-02-17) uses a revised spectrometer UV flux calibration that reduces the difference between the sun and the sun+sky fluxes in the critical region around 310nm. It also reduces the irradiance of UVb lamp at this wavelength. The new calibration, based on my deuterium lamp and the NIST measurements of similar devices, should result in a significant increase in accuracy.
This plot illustrates the comparison between direct winter sunlight (solar altitude 22°), skylight, and an Hg fluorescent UVb lamp for vitamin D synthesis in the skin. It was made as part of my efforts to understand the conditions necessary to synthesise the precursor chain that results in calcitriol, which is the hormone that circulates in the body to produce the multiple beneficial of the vitamin.
This is based on just a single measurement and I consider it to be just a starting point for further investigations.
The black line spectrum on the left with a peak close to 300nm shows the action spectrum (inverse dose in cm^2/J, peak at 10.5) for the conversion of 7-dehydrocholesterol to previtamin D3 in human skin when illuminated by UV-b radiation (Reproduced in "Sunlight and Vitamin D, A global perspective for health" Matthias Wacker and Michael F. Holick (2013) from Holick M F. Vitamin D status: measurement, interpretation, and clinical application. Ann Epidemiol 2009; 19:73-8; PMID:18329892; dx.doi.org/10.1016/j.annepidem.2007.12.001.)
The blue and the red-dashed lines show measurements of the solar irradiance measured on a plane perpendicular to solar direction from a sky with some high level thin cirrus from Bath, UK on 2021-02-12. The blue-line spectrum extending into the UV is from a Maya2000Pro spectrometer using a cosine corrector with a relative but not absolute flux calibration. The red-dashed line is an intensity calibration from a visible light Sekonic C-7000 spectro-radiometer extending only as far as 380nm.
Parts of the horizon were obscured for these observations and so the irradiance measurements should be considered as upper limits.
The thin red line is a predicted solar radiance, using an extinction model with Rayleigh and aerosol scattering and ozone absorption elements, for an altitude of 22°. This is for direct sunlight and does not include any sky radiation. The blue shading then shows the contribution of the sky to the available effective UVb radiation.
The violet and green-dashed lines are measurements made with the same devices of a 13W UVB 10.0 mercury fluorescent lamp (Desert Terrarium Compact Fluorescent Lamp) measured from the side at a distance of 10cm.
These spectra illustrate the very marginal availability of direct solar UVb radiation capable of synthesising vitamin D in winter. The only effective radiation at this time comes from the sky, principally from light that is Rayleigh scattered above the ozone layer and follows a path to the ground from close to the local zenith that minimises the ozone absorption.
The calibrated UVb lamp spectrum shows that it is straightforward to provide more vitamin D effective UV radiation than that from winter Sun + skylight by using appropriate simple, relatively cheap, mercury discharge lamps that are sold to illuminate pet tortoises!
Sun, sky and UVb lamp comparison for Vitamin D synthesis
Note. This new plot (2021-02-17) uses a revised spectrometer UV flux calibration that reduces the difference between the sun and the sun+sky fluxes in the critical region around 310nm. It also reduces the irradiance of UVb lamp at this wavelength. The new calibration, based on my deuterium lamp and the NIST measurements of similar devices, should result in a significant increase in accuracy.
This plot illustrates the comparison between direct winter sunlight (solar altitude 22°), skylight, and an Hg fluorescent UVb lamp for vitamin D synthesis in the skin. It was made as part of my efforts to understand the conditions necessary to synthesise the precursor chain that results in calcitriol, which is the hormone that circulates in the body to produce the multiple beneficial of the vitamin.
This is based on just a single measurement and I consider it to be just a starting point for further investigations.
The black line spectrum on the left with a peak close to 300nm shows the action spectrum (inverse dose in cm^2/J, peak at 10.5) for the conversion of 7-dehydrocholesterol to previtamin D3 in human skin when illuminated by UV-b radiation (Reproduced in "Sunlight and Vitamin D, A global perspective for health" Matthias Wacker and Michael F. Holick (2013) from Holick M F. Vitamin D status: measurement, interpretation, and clinical application. Ann Epidemiol 2009; 19:73-8; PMID:18329892; dx.doi.org/10.1016/j.annepidem.2007.12.001.)
The blue and the red-dashed lines show measurements of the solar irradiance measured on a plane perpendicular to solar direction from a sky with some high level thin cirrus from Bath, UK on 2021-02-12. The blue-line spectrum extending into the UV is from a Maya2000Pro spectrometer using a cosine corrector with a relative but not absolute flux calibration. The red-dashed line is an intensity calibration from a visible light Sekonic C-7000 spectro-radiometer extending only as far as 380nm.
Parts of the horizon were obscured for these observations and so the irradiance measurements should be considered as upper limits.
The thin red line is a predicted solar radiance, using an extinction model with Rayleigh and aerosol scattering and ozone absorption elements, for an altitude of 22°. This is for direct sunlight and does not include any sky radiation. The blue shading then shows the contribution of the sky to the available effective UVb radiation.
The violet and green-dashed lines are measurements made with the same devices of a 13W UVB 10.0 mercury fluorescent lamp (Desert Terrarium Compact Fluorescent Lamp) measured from the side at a distance of 10cm.
These spectra illustrate the very marginal availability of direct solar UVb radiation capable of synthesising vitamin D in winter. The only effective radiation at this time comes from the sky, principally from light that is Rayleigh scattered above the ozone layer and follows a path to the ground from close to the local zenith that minimises the ozone absorption.
The calibrated UVb lamp spectrum shows that it is straightforward to provide more vitamin D effective UV radiation than that from winter Sun + skylight by using appropriate simple, relatively cheap, mercury discharge lamps that are sold to illuminate pet tortoises!