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 of 0 is theoretically possible, but unlikely to actually occur. 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 graph, it shows that, for example, at a wavelength of 295nm the skin is a thousand times more sensitive than to UV at 340nm.

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

EryF - based on the TW30SX

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.

Calculation of UV Index

The UV Index itself is an irradiance scale calculated by multiplying the Erythema corrected irradiance in watts per m² by 40^[2].

Sample Calculation

IMPORTANT - see comment below regarding this calculation.

An EryF* sensor gives an output current of 1.485 x 10^-8 amps when exposed to sunlight on a certain day.

From the EryF* datasheet:

Sensitivity, S_max of EryF* = 0.11 Amps/Watt

Active area of EryF* = 0.054 mm² = 5.4 x 10^-8m²

So in this sample calculation, the UV Index is calculated as 10.

IMPORTANT - The above calculation does not take into account the missmatch between the UV sensor filter characteristics and the Erythema curve which can introduce an error of up to x5 in the UV index calculation figure. We will update this web page once we have more information on this.

Notes:

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.

Please contact us with any questions or queries.

References

1. UV Index courtesy of Wikipedia.

2. Definition of UV Index from Environment Canada Website.