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Fig. 1.1 Two area elements \"'E R -""" perpendicular to a my.r path. 11111 that; 1.1 11113an of radiative tnrfer by {Ml at cm}, then according to [12} the radiation falling on dial; in time cit after passing through art-I11 is I\": {1.42 d: {\"12 (It). From considerations of symmetry... this should also be equal to I\" (41 {11' dl du, where am is the solid angle subtended by till; at JAI. Equating these two expressions and noting that {1.42 (IA. d. = F1 dz = we get I\" = In} [2.10] 11