Activation In sunlight

UV Irradiance W/m²

Reference Irradiance W/m²

Activation Time Constant s

Time to reach 63% of full colour at the reference irradiance.

Maximum Colour Change ΔE

Sunlight Exposure s

Reversal Indoors

Reversal Half-Life at Reference s

Fabric Temperature °C

Reference Temperature °C

Fading Rate per 10 °C ×

Time Indoors s

Visible Colour Threshold ΔE

Colour Developed

— ΔE

Colour change reached at the end of the exposure

Switching Behaviour

Activation Rate: — per s
Saturation Reached: — %
Reversal Half-Life at Temperature: — s
Colour Remaining Indoors: — ΔE
Time to Clear: — s

Activation and reversal are modelled as independent, which they are not — both run simultaneously in sunlight, so the true steady-state colour outdoors is lower than the activation curve alone suggests, and increasingly so as temperature rises. Treat the colour developed here as an upper bound and the temperature sensitivity as the real design constraint. Photochromic dyes also fatigue: the achievable colour change falls with accumulated switching cycles and UV dose, so maximum colour change is a property of a new fabric rather than a fixed one, and any lifetime claim needs an accelerated cycling trial.

Photochromic Dye UV Activation & Reversal Time Modeler — free, with the formula and a worked example, at Textile School.