The UV-Index UVI

Francis Massen, LCD ; Nico HARPES, Radiation Protection Office

Since several years many countries (e.g. Australia, USA...) have introduced different strategies to quantify the erythemal UV irradiation in a manner which should be easily understandable by the general public. The starting point for this action was the greater awareness that excessive UVB irradiation significantly increases the risk of contracting skin cancer, and that the population should be informed accordingly.
The definitions used for computing these "UV indices" varied from country to country, and the necessity of a well defined common standard method became more and more apparent. During different meetings, an agreement has been found.
The standard UVI (UV-Index) is now (1996)defined as follows:

1 UVI corresponds to an irradiance of
1/40 Watt/m2 = 25 mWatt/m2
effective UVB radiation

The measured UVB radiation should be the mean of several measures done around 12:00 UTC at an interval of 10 to 30 minutes. Effective UVB radiation means that the CIE action spectrum has been applied to the "raw" values: the intensity (actually: the power) of every infalling radiation wavelength is multiplied by a wavelength dependant coefficient which expresses the particular sensitivity of the human skin for that wavelength. The sum of the resulting "effective power per wavelength" done over the UVB spectrum gives the total effective irradiance..

The minimal erythemal dose (MED) is the radiation dose which gives a detectable skin reddening in skin type II persons.

The maximum "safe" exposition-time can be computed from the UVI as:

max_time = 25/(9*UVI) hours = 167/(UVI) minutes.

Example: If UVI =2, max_time = 25/(9*2) = 1.4 hours = 83 minutes

To compute the UVI from the readings in mMED/h of the UVB_Bio sensor use the following formula:

UVI = reading *25 /9000

The maximum "safe" exposition time can also be computed from the mMED/h readings:

max_time in hours = 1/(mMED/h divided by 1000)

Example: If mMED/h =2000, max_time = 1/2 = 0.5 hours = 30 minutes

Please take a look at the UVI Poster for more details !

You may jump to Safesun's scientific page for more details.

The UVI is published regularly by several Luxembourg newspapers; click here for a scanned picture from the Luxemburger Wort, Friday 31 May 96!

There exist one personal UVI monitor we are aware of: the Sun Watch II from Saitek. This sensor shows the actual UVI and computes continuously the remaining "safe" exposition time. Preliminary checks show a reasonably good accordance to the UVI computed from the UVB Biometer readings. The Sun Watch can be ordered for instance from : Personal Protection Technologies, Postbus 1049, NL-9400 BA Assen, fax (0031) 5920 176 31 (price NLF 89.-) Conrad Electronic GmbH, Klaus-Conrad Str. 1, D-92240 Hirschau, fax (0049) 96 22 30 434 (price DEM 79.95) Another UVI sensor is now (January 1998) available form Safesun Pty. Ltd. in Australia.

See the paper A first calibration check of the Sunwatch II UVI sensor for more details on this sensor and the report A comparison of 3 personal UVI sensors for an extensive paper on 3 personal, portable instruments.

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