Richard, please tell us what fraction of the sky is included in the measurement of UVI. In other words, does the instrument have a range of shutters which only allow a narrow focus of sunlight to reach the plate or cells which measure intensity at varying elevation? Or does The instrument measure
Ultraviolet light coming from a relatively wide angle surrounding the sun?
To measure the UVI, the sensor should accept radiation from all angles, with what’s called a ‘cosine’ angular response function, meaning that light at 60 degrees from the zenith has 0.5 of the weighting of that from directly overhead (and 0.86 for 30 degrees, etc), and zero weighting for light from the horizons (which is in any case zero at UV wavelengths)
Richard, Please clarify. My instrument for measuring Ultra Violet light intensity has a lens like a camera aperture which must be pointed at the Sun to obtain a proper reading. The lens must be rotated to follow the sun as it rises and sits. Does the instrument which measures ultra Violet Light intensity for calculation UVI also have an aperture Which points towards the sun and follows it's movement? Or does it have 180 degree vision fish eye lens which accumulates radiation From across the hemisphere from across the sky?
They usually have a no lens at all. Instead they have diffuser plate made of teflon, or quartz with a ground surface. Both of these have an approximate cosine response which can be improved by shaping. A fisheye lens may have a similar angular response, but you’d have to test it too many sure. If it doesnt have a cosine response, then it’s not really measuring uvi.
Richard, please tell us what fraction of the sky is included in the measurement of UVI. In other words, does the instrument have a range of shutters which only allow a narrow focus of sunlight to reach the plate or cells which measure intensity at varying elevation? Or does The instrument measure
Ultraviolet light coming from a relatively wide angle surrounding the sun?
To measure the UVI, the sensor should accept radiation from all angles, with what’s called a ‘cosine’ angular response function, meaning that light at 60 degrees from the zenith has 0.5 of the weighting of that from directly overhead (and 0.86 for 30 degrees, etc), and zero weighting for light from the horizons (which is in any case zero at UV wavelengths)
Richard, Please clarify. My instrument for measuring Ultra Violet light intensity has a lens like a camera aperture which must be pointed at the Sun to obtain a proper reading. The lens must be rotated to follow the sun as it rises and sits. Does the instrument which measures ultra Violet Light intensity for calculation UVI also have an aperture Which points towards the sun and follows it's movement? Or does it have 180 degree vision fish eye lens which accumulates radiation From across the hemisphere from across the sky?
They usually have a no lens at all. Instead they have diffuser plate made of teflon, or quartz with a ground surface. Both of these have an approximate cosine response which can be improved by shaping. A fisheye lens may have a similar angular response, but you’d have to test it too many sure. If it doesnt have a cosine response, then it’s not really measuring uvi.
I often get asked about this. Thank you for sharing the science :)
Thanks Monique.