Key points are not available for this paper at this time.
The rayleigh, originally defined as a unit to express the total column light emission rate 10(10) photons sec(-1) (m(2)column) (-1) can equivalently be defined as a unit for apparent photon radiance ((1/4)pi 10(10) photons sec(-1) m(-2) sr(-1)). The selection of the appropriate definition will depend upon the physical situation and the interests of the user. The applicability of the unit for expressing the quantitative measurement of all extended light sources, including optically thick media, is both handy and valid.
Baker et al. (Sun,) studied this question.