Fluorescent cooling of objects exposed to sunlight The ruby example
Abstract Particularly in hot climates, various pigments are used to formulate desired non white colors that stay cooler in the sun than alternatives. These cool pigments provide a high near infrared (NIR) reflectance in the solar infrared range of 700 2500 nm, and also a color specified by a reflectance spectrum in the 400 700 nm visible range. Still cooler materials can be formulated by also utilizing the phenomenon of fluorescence (photoluminescence) Ruby, Al2O3:Cr, is a prime example, with efficient emission in the deep red ~694 nm) and near infrared (700 800 nm) A layer of synthetic ruby crystals on a white surface having an attractive red color can remain cooler in the sun than conventional red materials. Ruby particles can also be used as a red/pink pigment. Increasing the Cr:Al ratio produces a stronger (darker) pigment but doping above ~3 wt% Cr2O3 causes concentration quenching of the fluorescence. The system quantum efficiency for lightly doped ruby pigmented coatings over white is high, 0.83+ 0.10.