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UV Sensor Application Note: UV Index Measurement

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.

erythema curve

 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.

About public non-scientific instruments for sun UV radiation measurements

Research-grade UV-Index measurements

 Governmental institutes professionally obtain the UV-Index while using research-grade spectrometers with a cosine field of view. (The requirement of a cosine field of view is part of the UV-Index standard). These institutes publish actual values and daily forecasts that are e.g. distributed by newspapers, radio information or web services.

Non-scientific personal UV-Index measurements

 Compared to this research-grade method of obtaining the UV-Index, personal radiometers provide the benefit of more realistic and appropriate values measured at the place where the person is exposed to the sun. However, this method suffers from a reduced measurement precision. In 2010, Carrea et. al (Photochem. Photobiol. Sci. 2010, 9, 459 – 463) tested a selection of commercially available personal UV-Index radiometers while comparing them with values obtained by a professional spectrometer. Most of these radiometers suffered from a much to low precision. Only one instrument showed an acceptable error of +-5%.

sglux UV-Index sensors with an error of +-3% only

 sglux UV-Index sensors are not scientific instruments, but considering an error of +-3% only these sensors can be a very good alternative to research-grade UV spectrometers. sglux achieves this precision using SiC UV photodiode chips with extreme visible blindness of more than ten orders of magnitude and sophisticated filter technology. Nowadays a lot of weather stations and other instruments base on the sglux UV-Index sensors. The below table shows the sglux UV-Index sensor product line. It ranges from OEM components working inside the customer’s sensor unit until ready-to-use PTB traceable calibrated water proof sensors.

 All sglux UV-Index sensor solutions are stand-alone instruments that do not need further meteorological information gathered from remote servers. No data connection, no computer and no smartphone is needed to benefit from the high precision of the sglux sensors.

 The products listed below are available with or without a PTB traceable UV-Index calibration:

TOCON_E_M

TOCON_E_OEM

amplified TO5 metal housed high precision UV-Index sensor working inside the customer’ sensor unit and using customer’s window (e.g. a dome) as cosine correcting component, 0….5V voltage output.

 

TOCON_E2

TOCON_E2

amplified TO5 metal housed high precision UV-Index sensor with included diffusor for cosine correction. Designed to directly look to the sun through a hole of the customer’s sensor unit. Easy sealing against water with an O-ring is possible, 0….5V voltage output.

 

UV_Cosine_UV-Index

UV_Cosine_UV-Index

ready-to-use high precision UV-Index sensor, wafer proof, cosine corrected with voltage or current output.

 

DWG_E_SMD

TOCON_E_SMD

current development project aiming at a miniature SMD housed high precision UV-Index sensor. Available early 2015

 

 

 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.

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