A spectral optical transmission function tau(lambda) for sun protection insulating glasses is proposed in order to reduce the solar energy load of a building's interior and thus reducing overheating. tau(lambda) is based on standard functions such as spectral distribution of solar radiation S-lambda(lambda), spectral photopic luminous efficiency V(lambda), standard illuminant D-65(lambda), and the CIE color-matching functions (x) over bar (lambda),(y) over bar (lambda),(z) over bar (lambda). In the framework of the present approach an optimized spectral transmittance tau(min)(lambda) with a minimal normalized solar energy load (i.e., solar direct transmittance normalized to light transmittance, tau(e)/tau(v)) has been obtained on the condition of color neutrality of the transmitted light. A comparison with the performance of actual commercial sun protection glasses on the market shows that the present model for an optimized spectral transmittance could reduce the solar energy load by roughly one third for equal light transmittance tau(v). (c) 2007 Elsevier Ltd. All rights reserved.