Light transmission is the extent to which a material lets visible light through. It is expressed as the τv value (tau-v): the proportion of incident visible light that emerges on the other side, usually given as a percentage or as a number between 0 and 1.
The distinction from “transparent” matters. Milky white polycarbonate can transmit 80 per cent of the light without your being able to see through it at all; light transmission says something about the quantity of light, not about the image.
Where does it matter?
- Window glass. The more panes and coatings, the less light comes through.
- Plastic sheet such as polycarbonate and acrylic, in multiwall or solid form, clear or opal.
- Dome rooflights. A clear plastic dome transmits of the order of 90 per cent of the light, an opal white version roughly 80 per cent. In return the opal version gives diffuse light with no hard shadows and no view in.
- Net curtains, drapes and insect screens. These take light away too — at a window that only just gives enough light, a fine mesh screen can make the difference.
- Greenhouse construction. Here light transmission is an economic factor: more light means in principle more crop growth, up to the point where temperature and scorching become limiting.
Approximate values for glass
As guide values for clear glass without solar control coating:
- Single clear glass: around 90 per cent.
- Standard double glazing: roughly 80 per cent.
- Low-E double glazing: roughly 75 to 80 per cent; the low-emissivity coating costs a few per cent of light.
- Triple glazing: roughly 70 per cent, since a further pane and coating are added.
- Solar control glass: widely variable, from around 70 per cent down to less than half of that.
Always ask the supplier for the figure for the specific make-up; two nominally similar units can differ by several percentage points.
Less light is sometimes exactly what you want
Light transmission can be reduced deliberately: by etching or sandblasting the glass, by a printed or interlayer pattern, by solar control film, by choosing an opal plastic, or by incorporating solar cells in the glazing unit. That last option generates energy and damps the light at the same time — a solution mainly seen in canopies and atria.
Practical tips
- Light transmission is not heat transmission. There is glass that passes plenty of light and little solar heat, and vice versa. Always judge both figures.
- Calculate on the glass area, not the structural opening. Frames and profiles easily take up 20 to 30 per cent of the opening; slim profiles really do deliver more light.
- Dirt counts. Glass left uncleaned for a year, or a weathered plastic dome, transmits noticeably less light. On rooflights with no maintenance access that is a permanent loss.
- Plastic yellows. Polycarbonate and acrylic without good UV protection lose transmission over the years. Ask about the UV protective layer and the warranty on it.
- More glass is not always more light where you want it. A high window lights deeper into a room than a wide window low down — see the article on the clerestory.
Related terms
- Light transmittance factor
- Daylight factor
- G-value
- Building glass
- Dome rooflight
- Continuous rooflight
- Clerestory
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.