What's the difference between transmittance and transmissivity?

Transmittance is the measured ratio of light at normal incidence, whereas transmissivity is the ratio of the total light that passes through the glass.

The Radiance glazing parameters require transmissivity and glazing manufacturers often quote transmittance. There is a button on the project construction dialog in ApCDB which pops-up a small dialog box allowing the user to calculate transmissivity from transmittance.

Alternatively, the "Derive from visible light normal transmittance (Tvis)" option can be ticked and then a value can be entered in the visible light normal transmittance box to get transmissivities values.

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