The glare illusion is commonly used in CG rendering, especially in game engines, to achieve a higher brightness than that of the maximum luminance of a display. In this work, we measure the perceived luminance of the glare illusion in a psychophysical ex- periment. To evoke the illusion, an image is convolved with either a point spread function (PSF) of the eye or a Gaussian kernel. It is found that 1) the Gaussian kernel evokes an illusion of the same or higher strength than that produced by the PSF while being com- putationally much less expensive, 2) the glare illusion can raise the perceived luminance by 20 35%, 3) some convolution kernels can produce undesirable Mach-band effects and thereby reduce the brightness boost of the glare illusion. The reported results have practical implications for glare rendering in computer graphics.