A beam hitting a surface at an angle delivers only its normal component, so the effective area and pressure are scaled by cos(theta), the projection onto the surface normal. The arithmetic is exact only for the stated irradiance, area, and incidence angle in the ideal absorbing-surface model. Real beams add finite bandwidth, alignment error, nonideal reflectivity, surface roughness, detector calibration, and measurement/source uncertainty.
An oblique absorber case uses an exact normal projection factor. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Oblique incidence uses a normal projection
The only allowed oblique factor in this book is an exact one-half.
I⊥=6⋅1/2=3W/m2
Projected irradiance sets projected pressure
The absorber pressure uses the projected normal irradiance.