The light transmittance factor is the proportion of visible daylight that passes through a glazing unit. It is written as a number between 0 and 1 or as a percentage; in European product data it appears as τv (tau-v) or LT. The Dutch term is lichttoetredingsfactor, abbreviated LTA.
An LT of 0.78 therefore means: 78 per cent of the visible light falling on the pane comes through. The remaining 22 per cent is reflected or absorbed. The value is determined by a European calculation method (EN 410 for the optical properties of building glass), so that figures from different manufacturers are comparable.
Typical values
Guide values for clear glazing without solar control coating:
- Single glazing: around 0.90
- Standard double glazing: roughly 0.80
- Low-E double glazing: roughly 0.75 to 0.80
- Triple glazing: roughly 0.70
- Solar control glass: widely variable, from around 0.70 to well below 0.40
The more panes, coatings and interlayers, the lower the LT. Laminated safety glass and grey, bronze or tinted glass also cost transmittance.
LT, g-value and U-value
Three figures belong together when choosing glass:
- LT — how much light comes through.
- g-value (solar factor) — how much solar energy comes through.
- U-value — how much heat is lost through the pane.
These three are not tied to one another. Glass exists with a high LT and a low g-value: plenty of light, little solar heat. Exactly what you want in an office or a south-facing living room where overheating is the problem.
The ratio LT divided by g-value is called selectivity. Above roughly 1.5 the glass counts as selective solar control glazing: it holds back the heat without darkening the room. So with solar control glass, always ask for both figures, not just “the solar control”.
Where you meet the LT value
- In daylight calculations. In new build and conversion, the daylight provision of a habitable room is calculated from the glazed area, corrected by a factor for the glass. Choose darker glass and you need more area for the same requirement.
- In shading and overheating decisions. The choice between solar control glass and external shading turns on whether you are willing to give up light.
- In listed buildings and historic glass. Old drawn glass has a different transmittance and far more distortion than modern float glass.
Pitfalls
- The LT of the glass is not the daylight in the room. Frame profiles, deep reveals, a canopy, a balcony above, neighbouring buildings and dirt all take something off. In a borderline case, have a daylight calculation done rather than relying on LT alone.
- Compare whole make-ups, not coatings. Two units with the same coating but a different glass thickness or cavity fill can have noticeably different LT values.
- Beware of “equivalent”. If a quotation offers the glazing as “equivalent”, ask for LT, g-value and U-value in writing. “Equivalent” often refers to the U-value only.
- Dark glass feels different. An LT of 0.45 looks unremarkable on paper, but the room reads noticeably greyer and outdoor colours shift. Ask for a sample held to the light before you decide.
Related terms
- Light transmission
- G-value
- Daylight factor
- Building glass
- Thermal resistance (R-value)
- Continuous rooflight
- Clerestory
Want to know more about building terms? Take a look at our knowledge base on fredsdiyplans.com.