[7.5.3.9] \boldsymbol{\tau}(\boldsymbol{\theta}_{\mathrm{i}}, \boldsymbol{\phi}_{\mathrm{i}}; \boldsymbol{\theta}_{\mathrm{t}}, \boldsymbol{\phi}_{\mathrm{t}})

The ratio of incident flux collected over an element of solid angle surrounding the given direction to essentially collimated incident flux.

Note: The directions of incidence and collection and the size of the solid angle “elements” must be specified.

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