What is reflection, absorption and transmission?
Reflection, absorption and transmission are the three things that can happen to light or other radiation striking a material: it is bounced back, taken in or let through. Together they always add up to the whole: ρ + α + τ = 1, or 100%. The more that is reflected, the less is left for absorption and transmission.
The three terms
- Reflection (ρ, rho). Radiation is bounced back. On a smooth surface the angle of reflection equals the angle of incidence (specular reflection). A rough surface scatters light in all directions (diffuse reflection). Reflection is mainly a property of the surface.
- Absorption (α, alpha). Radiation is taken in and converted into heat. This is why a dark surface in the sun gets hotter than a light one.
- Transmission (τ, tau). Radiation passes through. The thicker the material, the more is absorbed on the way and the less gets through.
An opaque material transmits nothing, so ρ + α = 1. Whatever is not reflected is absorbed.
Application in buildings
- Glazing. Light transmittance is the transmission of visible light. The g-value covers the total solar energy transmitted. Solar control glass lets less through, usually thanks to a more reflective coating.
- Colour and daylight. Light reflectance value (LRV) expresses on a scale from 0 (black) to 100 (white) how much visible light a surface reflects. The test method is BS 8493. In the UK, Approved Document M uses LRV differences to define visual contrast for people with impaired vision. Dark walls and floors soak up light, so a room needs more daylight or artificial light to feel as bright.
- Radiant heat. The same principle applies. Aluminium reflects a lot, absorbs little and therefore emits little. This is the basis of reflective foil.
- Sound. Sound is also reflected, absorbed or transmitted. That is the foundation of room acoustics.
Practical tips
- Always compare glazing on both light transmittance and g-value. Plenty of daylight with little solar heat calls for a different glass than maximum solar gain.
- In dark rooms, choose light finishes with a high LRV, especially for the ceiling.
- Allow for heat build-up: dark cladding and roofing absorb more solar radiation.
Related terms
- Light transmittance factor
- g-value
- Daylight factor
- Reflective foil
- Acoustics in construction
- Frosted glass
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