A radiation sensor and a small surface of area A 1 that emits radiation as a blackbody

Question:

A radiation sensor and a small surface of area A₁ that emits radiation as a blackbody are oriented as shown in the figure. Determine the distance L at which the sensor is measuring two-thirds of the radiation rate emitted from surface A₁ corresponding to the position directly under the sensor at L = 0. A L Radiation sensor A 02 H = 50 cm