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on reflection

Architect's memo 87: May 2007

Virtually all of the energy the earth receives comes in the form of radiation from our sun. Without this energy pouring down non-stop the earth would freeze into a lifeless icy rock. For all of this dependence, it is only in recent years that, via a NASA sponsored programme and satellite, the type of level and radiation has been studied in detail.

In the vacuum of space, the sun is belting out some fairly lethal stuff in the very short wavelength area, like x-ray and Extreme and Far U.V. These waves are so active that they interact with our atmosphere and never reach the earth's surface. Of what does reach us there is about 5% U.V. light of wavelength between 290-400 nm, 50% of visible light between 400-700 nm and 45% of infra-red (or heat) ranging from 700-2,500 mm. All of these waves interact with the substrates they fall on, either being absorbed, transmitted or reflected. All absorbed energy eventually gets converted to longer wavelength heat.

The 5% of U.V. light punches above its weight because these short waves are very active and cause a lot of damage when they impact on organic materials, such as paint and human skin.

We naturally have a close relationship with visible light as we have, in our eye, a very sophisticated organ that can detect the subtlest nuance in how visible light is absorbed or reflected. The eye detects the presence of different wavelengths and senses them as a 'colour'. If a substrate absorbs all visible light falling onto it the eye sees this surface as black, and white if the surface reflects all light. All primary colours absorb the wavelengths that the eye doesn't see and reflects the wavelengths that the eye then identifies.

The light reflectance value (LRV) of a colour, as measured by, for example, a spectrophotometer, is a measure of how much of the visible spectrum of light is reflected by that colour. The simple corollary of this is that the balance of this value (100 - LRV) is an indicator of the light absorbed by the colour. As stated earlier, any absorbed energy will end up as heat.

LRV is crucially dependant on the saturation of the colour and the amount of white or black that colour contains. It is virtually impossible to have a dark colour with a high LRV or a pastel shade with a low LRV.

The LRV of the same shade can be affected by the surface of the substrate. A glossy paint, even a jet black glossy paint will have some specular reflectance from the surface - rather like a mirror. Flat and textured paints do not have this property and reflect only in a diffuse manner.

LRVs are particularly difficult to determine from semi-transparent finishes such as wood stains. Light travelling through these finishes gets absorbed and/or is reflected by the underlying timber. Hence, a stain applied over Pine will also have a substantially higher LRV than the same stain applied over, say Kwila. This is why there is not a standard LRV provided for wood stain colours.

LRVs can be quite a useful tool to use in colour scheming buildings in specific environments. An, almost lost, thesis by Tim Heath called 'Colours for Structures in the NZ Landscape' measured the LRVs of typical NZ landscapes and recommended sympathetic colours. Some councils will demand LRVs of say less than 25% so that buildings are not too conspicuous in a hilly bush suburb or so that roofs do not cause glare to neighbours looking down on them.

Higher LRVs are often specified for building systems which may be sensitive to heat. This recognises that absorbed light will convert to heat but does not necessarily take into account the 45% of the sun's energy emitted in the infra-red range. This can lead to the situation where two colours can have identical LRVs but dramatically different heat build up depending on whether the pigmentation reflects in the IR range (CoolColour™) or not. Resene CoolColour technology reflects much of the infra-red heat so the colour stays much cooler than its standard colour counterpart even though they both visibly look the same. This is because the total solar reflectance (TSR) of the Resene CoolColour is much higher than the TSR of the standard colour. In these situations it is the heat reflectance of the colour that is important rather than its light reflectance.

LRVs can have a dramatic benefit to mankind. When in the 70s it was believed that the earth was slipping into an ice age, this scribe's solution was to decrease the earth's albedo by painting everything black. Now that the fear is that we are going to overheat, then maybe it's time to paint everything white or in a CoolColour and reflect the heat away.

View the Resene CoolColours brochure. (You will need Acrobat Reader).

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The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.

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