I know there are chemical pigments/dyes which produce an immediate reversible colour change in UV or direct sunlight. Is there a way to postpone the colour change so that it does not happen immediately but rather over a period of 30 minutes for example? Would it have to do with the concentration of dye or is it the chemicals themselves which have different colour change times.

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    $\begingroup$ Older photochromic glasses (the ones that went grey in sunlight) were pretty slow in changing, particularly when cold. I have no idea what changed over the years to speed them up. $\endgroup$ – Jon Custer Jan 10 at 14:17
  • $\begingroup$ @jonCuster they probably found better chemicals to use. $\endgroup$ – Leja Jan 17 at 10:42

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