The UV Index is an international standard measurement
of the strength of the ultraviolet (UV) radiation from the sun is at a specific
place on a particular day. The scale is principally used as part of a weather
forecast, aimed at the general public, to inform people of the potential UV
exposure they can expect on a given day. This enables the public to protect
themselves against excessive exposure to UV, which can cause sunburn, eye
damage (e.g. cataracts), skin ageing and skin cancer .
The index is an open-ended scale, with higher values
representing higher UV exposures and therefore greater risk of skin damage due
to the UV exposure. An index as high as 8 is rare in the UK, but indices higher
than 11 are quite common in the southern hemisphere where the ozone layer is
depleted. Values as high as 17 have apparently been recorded in Carnarvon,
In 1992, three scientists from Environment Canada
developed the UV index and Canada became the first country to broadcast
forecasts of the predicted daily UV levels for the next day. UV indices started
to be used by other countries, but using different methods of calculation.
Until recently, the methods of calculating and reporting a UV index varied from
country to country.
The World Health Organization eventually standardised
the UV Index method and now the international UV Index specifies a standard
calculation method and standard graphics for reporting forecasts for worldwide
The erythema curve indicates the UV exposure required
to induce erythema of human skin - a redness of the skin resulting from
inflammation, in this case, as caused by sunburn.
Note that the erythema curve includes the human skin
response to UV-A (wavelengths between 315nm and 400nm) and UV-B (wavelengths
between 280nm and 315nm). UV-C is absorbed by the ozone layer and does not
reach the earth's surface. However, in the Southern Hemisphere there are holes
in the ozone layer and UV-C must be considered here.
Taking a couple of readings from the Erythema Curve, it
shows that, for example, at a wavelength of 295nm the skin is a thousand times
more sensitive than to UV at 340nm.
The Sun Radiation at Sea Level curve  shows typical sunny day values of sun radation in
the USA (UV index = 3.66). By multiplying the sun radiation curve by the
Erythema Curve we produce the Relative Danger curve. This shows that most
dangerous wavelength from the sun is 310nm. This wavelength is not the highest
output from the sun nor is it the wavelength that is the most dangerous to the
human skin, it is however the most dangerous when both effects are taken into
Two of our UV sensors have filters which match the erythema action
curve of human skin.
Eryca - based on the
EryF* - based on the
The filter window material in each sensor corrects the sensor
response so that it matches the erythema curve and therefore the output current
will be directly proportional to the UV Index. Sensitivity curves are given
Calculation of UV Index
Using the sample sun radiation figures above, together
with the sensitivity plots we can calculate the output current produced by the
Sensor Output (A/nm) = Sun radiation (W/m^2/nm) x
Sensor Sensitivity (A/W) x Sensor Size (m^2)
To find the actual sensor output, it is a simple matter
of integrating the above curves.
For the Eryf*, we get a figure of 15nA. For the Eryca,
we get a figure of 83nA.
To calculate the actual UV index value for the typical
sample data above, we multiply the sun radiance figure by the Erythema curve
(which gives us the Relative Danger curve above) and then integrate. This gives
us an Erythema corrected irradiance figure of 0.0916 W/m2.
The UV Index itself is an irradiance scale calculated
by multiplying the Erythema corrected irradiance in watts per m2 by
40. This gives us a UV index figure for the
sample of 0.0916 x 40 = 3.66.
We can use this figure to find the conversion figure
from sensor output to UV index.
UV Index measured by Eryf* = Current Output x 3.66
/ 15x10-9 = Current Output x 2.4x108
UV Index measured by Eryca = Current Output x 3.66
/ 83x10-9 = Current Output x 4.4x107
The above calculation assumes that sunlight being measured matches
the profile in the example data above. In practise this is unlikely to be the
case due to differences in water vapour in the air amongst other effects. It is
therefore strongly recommended that the above calculations are not relied upon
and that comparison with a calibrated uv index meter is used.
EryF* has a ±5% variation in Smax
from unit to unit. Long exposure to high UV radiation does not affect its
EryF and Eryca have an actual output up to 50% higher
than the given figure for Smax. These sensors are not as reliable as
EryF* but there is no measurable degradation over lifetime either.
For sun UV detection we recommend using the
ERYCA because it is stable enough and, once calibrated,
it provides stunning precision performance, far superior to that of SiC or
New products, including a hydrid component combining a
chip and an amplifier which will be calibrated during manfacture, are currently
under development. Please contact us us for