def incident_luminosity(luminosityA, aR, bR, d):
    crossSectionalAreaB = np.pi * bR**2
    surfaceAreaD = 4 * np.pi * d**2
    fractionIncident = crossSectionalAreaB / surfaceAreaD
    incidentLuminosityB = luminosityA * fractionIncident
    
    return incidentLuminosityB