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Open Access Publications from the University of California

Fluorescent cooling of objects exposed to sunlight - The ruby example

  • Author(s): Berdahl, P
  • Chen, SS
  • Destaillats, H
  • Kirchstetter, TW
  • Levinson, RM
  • Zalich, MA
  • et al.

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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.

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