The share of light that passes through a material, measured wavelength by wavelength.
Transmitted radiant flux divided by incident flux, from 0 (nothing passes) to 1 (everything passes). It is the sum of regular transmittance, for light that keeps its direction, and diffuse transmittance, for light scattered on the way through. Reflectance, transmittance and absorptance together account for all the incident light.
High transmittance does not mean transparent. Opal glass can pass most of the light that reaches it and still hide everything behind it, because the light is scattered.
A deep red wine transmits a good deal of red light and almost no green or blue through a centimetre of liquid.
It is how the colour of drinks, oils, glass, films and filters is measured, and it changes with thickness, so a figure needs its path length.
The CIE defines transmittance as transmitted radiant flux divided by incident flux, a ratio between 0 and 1 with no unit, usually quoted per wavelength. It splits into regular transmittance, for light that keeps its direction, and diffuse transmittance, for light that is scattered on the way through. An instrument measures the beam with and without the sample in place. Clear liquids and filters are read straight through in a cell of stated length. Hazy films and diffusers need an integrating sphere to collect the scattered part.
| Path length | Transmittance | Absorbance |
|---|---|---|
| 0.5 cm | 70.7% | 0.151 |
| 1 cm | 50.0% | 0.301 |
| 2 cm | 25.0% | 0.602 |
| 5 cm | 3.1% | 1.505 |
Doubling the depth from 1 cm to 2 cm squares the transmittance, 50.0% to 25.0%, and at 5 cm only 3.1% emerges. Absorbance rises in a straight line over the same steps. This is why a drink looks far darker in a deep glass than in a shallow one, and why any transmittance figure is meaningless without its path length.
How this was calculated: For a clear, non-scattering liquid the transmittance over a path is the one-centimetre transmittance raised to the power of the path length in centimetres. The 50% starting figure is chosen for illustration.
A coloured liquid is approved in a small sample bottle and sold in a large one.
The longer path removes more light at every wavelength, and removes most where the liquid already absorbs, so the product looks darker and its hue shifts. Colour for liquids is specified at the path length the customer will see.
Tinted glass is chosen from a thin sample for a thick structural pane.
The green of ordinary float glass, barely visible in a thin sheet, becomes pronounced as thickness grows. Low-iron glass is the remedy where a colourless edge or a true view of colours behind the glass matters.