Lawrence Berkeley National Laboratory
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.
Published Web Locationhttps://doi.org/10.1016/j.solmat.2016.05.058
© 2016 Elsevier B.V. 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.