Sun light is defined as the the total electromagnetic radiation emitted by the sun. The world meteorological organisation defines sunlight as direct radiation produced by the sun with a minimum measurement of 120W•m-2 at ground level.

Direct sunlight has a luminous efficacy of about 93 lumens per watt of radiant flux, higher than most artificial lighting, and produces around 100.000 candelas per square metre at the earth surface.

Considered the least harmful for cellular DNA and therefore this is the type used for tanning and psoriasis treatments.

Found in sunlight, we also find this spectrum in incandescent bulbs and "full spectrum" bulbs. UVA is part of the visual spectrum in reptiles by using reflection off individuals of the same species, plants and insects for identification purposes (Honkavaara et al., 2002; Leal et al., 2002; Loew et al., 2002; Sillman et al., 2001; Thorpe et al., 2001). The eyes of red-eared sliders, for example, have cones responding to not only all the colours we recognise, but also higher-wavelength UVA (from 350nm) and low-wavelength infra-red (up to about 750nm); these are almost certainly additional colours in the turtle’s rainbow. “UVA colour” is especially important to many reptile species in recognising conspecifics and even food items (Frances Baines MA VetMB MRCVS).

Mostly absorbed by the atmosphere and along with UVC is partly responsible for the photochemical reactions that produce the ozone layer.

Wavelengths are classified in to spectrums (or spectra) by scientific consensus, with UVB in the USA within 280-320, whilst in Europe it is considered between 280-315nm. The atmosphere blocks wavelengths below 290nm which means that the spectrum on the earths surface is 290-315nm (Europe).

This is the spectrum that allows photosynthesis and production of pre-vitamin D3 (cholecalciferol) and diurnal reptiles are the most likely to suffer a metabolic disorder in its absence. Typical species affected by the lack of UVB in captivity are Green Iguanas, Bearded Dragons and Bosc Monitors.

The conversion rate from pro-vitamin D3 to pre-vitamin D3 is directly related to the quantity of UVB. Although this is true, the active photobiological range, called UV Index should also be considered.

An example is that Panther Chameleons (Furcifer pardalis) kept at 5-15uW/cm2 (Spectronics UV radiometer) or 20-60uW/cm2 (UVP UVB radiometer) for 12h seemed to be optimal (Ferguson et al., 2002). Quantity is species-dependant and will basking behaviour will also vary depending on individual physiological necessities.

Endorfin production is also stimulated by UVB on the human skin creating a wellbeing effect (Holick et al., 2003) which may not be limited just to humans.

Normal household bulbs will not produce this spectrum and therefore we must rely on specialised bulbs.

Wavelengths under 180nm are only found in space with only limited quantities reaching the earths surface as most of this spectrum is filtered through the ozone layer. This spectrum has germicidal effects and therefore may be found in sterilising bulbs.

This is responsible for heating and is also produced by the sun. Three types are established (A, B and C) depending on the wavelength.

That visible to the human eye and also to the reptilian eye, although the latter includes UVA also (Honkavaara et al., 2002; Leal et al., 2002; Loew et al., 2002; Sillman et al., 2001; Thorpe et al., 2001).

The following graph shows solar radiation coming through the atmosphere and through to sea level. Whilst passing through the atmosphere this is partly absorbed by gases with specific absorption bands. Rayleigh dispersion or redistribution of light also ocurrs which is the reason why we sea a blue sky.

Large ranges of UV levels are found in nature and the sun angle plays an important role in this. When the sun is low on the horizon light must pass through a thick layer of the atmosphere, during which, most UV is absorbed. For this reason, the highest reading of UV are at midday when the sun is at its highest, in the tropics or high altitude where the air is thin. In winter, the further we are from the equator, the lower the UVB readings. In January at the equator level we could have a reading of 450uW/cm2, whilst in northern regions a reading may be around 25uW/cm2 at the same time.

Reptile behaviour is species-specific with some species basking in the mornings (ie Green Iguana) and others may choose to bask at higher levels of UVB (ie Bearded Dragons).

Different colours exist within the light spectra and these may be emitted and absorbed depending on the type of bulb used in a captive environment. Different stages of plant growth require different spectra. The initial vegetative stage requires a blue spectrum of light, whereas the later "flowering" stage is usually promoted with red–orange spectra.

REFERENCES

1. Gregory Hallock Smith (2006), Camera lenses: from box camera to digital, SPIE Press, p. 4, ISBN 9780819460936

2. Narinder Kumar (2008), Comprehensive Physics XII, Laxmi Publications, p. 1416, ISBN 9788170085928