Figure 5.

The notion of ‘brightness’ or ‘brilliance’. (a) A fireplace does not have high brightness since its emission, coming from a large area and being spread over a broad angular range, cannot bring much radiation into the zone of utilization. (b) A flashlight is more effective, i.e. it has high brightness. (c) We define the brightness (Margaritondo, 1988 ▸, 2002 ▸; Winick, 1995 ▸; Willmott, 2011 ▸; Mobilio et al., 2015 ▸; Bordovitsyn, 1999 ▸) by combining [equation (8)] the emitted flux F with the geometric parameters Ω (solid angle of emission) and Σ (source area).