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First background: I'm working from Planck's law and Stephan's law. The temperature of my target will be between 1250 and 1750K. From Stephan's law I've worked out this will give me a total radiated power of 5.67e-8 * T in W per sq meter of surface area. or 0.138 - .532W/M^2. Now I've built a spread sheet using Planck's law to get spectral distribution. I confirmed that the total under the curves sums to match Stephan's law, so I feel good about that so far. Next I multiplied by the sensitivity curve for my photodiode in A/W vs. wavelength. That gives my a photodiode current if every bit of of radiant energy from a given area of the target landed on the PD, which of course it won't.

Here's where I'm getting confused . . .

My detector will mask off all but a 10mm^2 area of the target and the photodiode active area itself is 0.785mm^2 and will sit 400mm from the target surface, which is flat. How do I work out what portion of the total radiant energy will land on my detector? Does the surface of a black body radiate equally in all directions? Do I assume the total power from the exposed section spreads evenly over a hemisphere of radius 400mm and take the area of my detector as a fraction of the area of that hemisphere? That's the best I can come up with, but I'm not sure.

Any thoughts?