# Intensity distribution of a Lambertian LED as a function of angle

I have a practical spherical geometry problem that I'm having trouble cracking. I'm illuminating a planar surface with an LED that has a Lambertian intensity distribution, i.e. the intensity drops off as the cosine of the angle from the perpendicular (Intensity = $I_0 * cos($\theta\$)). So if an LED at point ( 0,0,1) is pointed normal to a planar surface at the origin (0,0,0), the intensity distribution in the x-y plane is easy to determine - maximum at (0,0,0) dropping off with intensity with the cosine of the angle. What I'd like to figure out is the intensity distribution across the x-y plane when the LED is pointed at the origin, but at any arbitrary position on the surface of a half-sphere/dome with radius normalized to 1, pointed at the origin. In other words, I'm looking for an equation that will describe the intensity at any point in the x-y plane (z=0) for any LED position/angle on that sphere, not just the normal direction. I think I can figure it out for the direction for any point along the projection of the xyz radius vector onto the xy plane, but am having problems generalizing it for any arbitrary x-y point. I have the LED position coordinates in xyz format, but can convert them to spherical coordinates if necessary. I've already figured out how to compensate for the change in intensity due to inverse-square drop-off; I'm only looking for the intensity variation from the Lambertian intensity distribution. Any help would be appreciated.

• Would you be willing to say what you mean by "intensity"? What are the units of intensity here? – John Hughes Feb 20 '15 at 0:43
• Intensity from the LED would be luminosity, lumens, lux, whatever units you want. – Dr. P Feb 20 '15 at 3:04