Performance metrics & physics
Photopic Response Curve
The photopic response curve is the human eye's sensitivity across visible wavelengths, peaking near 555 nm (green); VLT measurements weight transmission by this curve.
Visible Light Transmission is not a simple "how much visible light gets through" measurement. It is weighted by the photopic response curve — the eye's sensitivity at each wavelength.
A film that transmits 75% at 555 nm (peak sensitivity) but only 50% at 700 nm (lower sensitivity) might still measure 70% VLT because the photopic weighting emphasises green over red.
This is why VLT is the relevant spec for LTA compliance — it reflects what the driver actually sees, not raw photon counts.