How this calculator works
Directed light carries momentum. For a flat surface, the projected intercepted beam and the normal momentum component each contribute a cosine, giving cos squared for normal pressure. A perfect specular reflector doubles the normal momentum transfer. An absorbing tilted surface also receives tangential momentum; that component is not included here. Isotropic cavity radiation is a different geometry: pressure is one third of energy density. The thermal mode describes a radiation field at equilibrium, not a freely radiating surface.
Formula
P_normal = q L cos^2(theta)/(4 pi d^2 c), with q = 1 for absorption and 2 for reflection. P_thermal = u/3 = 4 sigma T^4/(3c).
Worked example
A normally incident flux of 299792458 W/m2 gives 1 Pa on an absorber and 2 Pa on a perfect reflector. At 60 degrees these normal pressures are one quarter as large.
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Frequently asked questions
Which angle should I enter?
The angle between the incoming ray and the surface normal, from zero to 90 degrees.
Does reflection always double the pressure?
Only for an ideal specular reflector compared with a perfect absorber at the same incidence angle.
Can I use the thermal result for sunlight?
Sunlight is a directed beam. The isotropic thermal expression applies to an equilibrium radiation field.