Fluorescent cooling of objects exposed to sunlight – The ruby example

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 700–2500nm, and also a color specified by a reflectance spectrum in the 400–700nm 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 (~694nm) and near infrared (700–800nm). 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 ~3wt% 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. •Fluorescence can cool objects heated by the sun.•Ruby, Al2O3:Cr, is an important example.•A ruby crystal array emits more than 300Wm−2 in full sun.•Temperature measurements indicate quantum efficiency ≥0.7.