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B1. A point light source of intensity I is placed at a large distance z (z >> r) from a spherical Lambertian scatterer, as shown in Figure B1. r is the radius of the scatterer. (a) Assuming 100% of the light illuminated on the spherical surface will be scattered, please derive the radiant exitance of the scattered radiation at different positions of the spherical Lambertian scatterer. (7 marks) (b) A photodetector with a responsivity of R and a sensitive area of A is placed between the light source and the scatterer, pointing at the scatterer. The distance between the scatterer and the photodetector is x (x >> r). Ignoring the effect of the shadow cast by the photodetector, calculate the output signal of the photodetector. (8 marks) X Light source photodetector Figure B1 Scatterer