UV Index

Introduction

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 ^[1].

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, Western Australia^[1].

History

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 use.

Erythema Curve

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 ^[2] 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 account.

Two of our UV sensors have filters which match the erythema action curve of human skin.

Eryca - based on the TW30SX

EryF* - based on the SG01S

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 below.

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 sensors.

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^[3]. 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

Notes:

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 S_max from unit to unit. Long exposure to high UV radiation does not affect its sensitivity.

EryF and Eryca have an actual output up to 50% higher than the given figure for S_max. 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 AlGaN sensors.

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 more details.

References

1. UV Index courtesy of Wikipedia.

2. Reference Solar Spectral Irradiance - American Society for Testing and Materials

3. Definition of UV Index from Environment Canada Website.