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What is UV

Ultraviolet (UV) is part of the spectrum of electromagnetic radiation being emitted by the sun that arrives at the edge of the Earth’s atmosphere. The sun is extremely hot so it emits a large amount of radiation at short wavelengths, below the visible band. However, the atmosphere greatly affect the spectrum of the radiation that reaches the surface.

Spectral range

The solar radiation reaching the earth’s atmosphere consists of approximately:

50%      visible light (VIS)
40%      near infrared radiation (NIR)
10%      ultraviolet radiation (UV)

The UV radiation consists of:

UVC      100 to 280 nm
UVB      280 to 315 nm
UVA      315 to 400 nm

UVC is completely absorbed by Oxygen and Ozone in the atmosphere.
UVB is approximately 90% is absorbed by the atmosphere, primarily by stratospheric Ozone
UVA mostly reaches the surface
UVA radiation at the Earth’s surface is normally 15-20 times greater in intensity than UVB.

The ‘violet’ end of the visible (VIS) part of the spectrum begins at 400 nm.

spectral range of solar radiation

Solar light spectrum

It will be appreciated that reduction of the Ozone concentration will significantly affect the amount of UVB radiation reaching the Earth’s surface. The Brewer Spectrophotometer measures the Total Ozone Column, which largely consists of the stratospheric ‘Ozone layer’.

The World Meteorological Organisation (WMO) and the World Health Organisation (WHO) define the boundary between UVA and UVB as 315 nm and this was universally adopted in 2002/2003. However, some other organisations, particularly in the USA, used an older boundary definition of 320 nm. This makes a significant difference to the amount of UVB measured (over-estimation) and must be taken into account when comparing data from different published information sources and from different UV measuring instruments.

Spectral shift

Measuring UV radiation accurately is difficult as the amount of energy is very small compared to the amount of visible radiation reaching the earth at the same time, particularly for UVB.

Atmospheric conditions have a large influence on the spectrum of solar radiation reaching the Earth’s surface.

Variations of air pressure and temperature within the atmosphere influence absorption and therefore affect the spectrum at sea level and at different heights above sea level. The spectrum of solar radiation received on top of a mountain in a remote region can differ markedly from the spectrum received in an industrial or urban area or near sea level.

Further factors are the variations caused by dynamic changes in the ozone concentration of the atmosphere which increase with rising wavelengths in the range of 280 nm to 315 nm. UV radiation is also influenced by aerosols, clouds, the position of the sun (solar zenith angle) and the amount of atmosphere that the radiation passes through (air-mass). The influence of surfaces causing multiple reflections, such as snow, is also significant.

The graph shows the effect of hazy and cloudy conditions on a typical solar spectrum at sea level. The overall irradiance drops, but the maximum irradiances have been normalised to demonstrate the spectral shift. The UV radiation greatly reduces and the spectral balance shifts to the ‘red’ end of the visible with a higher proportion of near infrared radiation (NIR).

typical solar spectrum at sea level

Spectral shift in diffuse irradiance with sky type

Erythemally active UV irradiance

UV radiation measured with a similar response to a defined typical human skin is termed ‘Erythemally active UV irradiance’ (UVE). There have been a number of response functions used in the past, but the one currently accepted is that proposed by McKinlay and Diffey in 1987 and adopted as the international standard ISO: 17166:1999 / CIE S 007/E-1998. This function covers the range from 250 nm to 400 nm and is used to calculate the Global  Solar UV Index (UVI) for public health information.

The McKinlay and Diffey function is now standardised and is shown in the graph below, together with another function that was sometimes used in the past.

McKinlay and Diffey function

Response functions

Older data may be recorded in MED/hr (Minimum Erythemal Dose per hour). However, this has no universal definition and varies depending upon skin type and condition and it is no longer used. Nowadays, integrated UV irradiance is always measured in W/m2.

As can be seen from the spectral function graphs, UVE includes some UVA radiation and a response towards the UVC band. For this reason a UVB radiometer should not be used to measure UVE and to calculate the Global Solar UV Index.

The Global Solar UV Index (UVI) can be calculated by multiplying the UVE radiation value in W/m2 by 40 m2/W. For example, 0.25 W/m2 of UVE represents a UV Index of 10, and this is the value used for public health information.

As can be seen from the spectral function graph, UVE includes some UVA radiation and a response across the UVB band. For this reason a UVB radiometer should not be used to measure UVE and to calculate the Global Solar UV Index.

Total UV irradiance

‘Total UV’ is the sum of UVA and UVB and is most commonly monitored in meteorology and climatology stations as an extension to the solar radiation monitoring by pyranometers. It is also used for checking the UV output of light sources and solar simulators for renewable energy research.