Measurement of the reflectance function

Since Pt is zero outside the limits of integration, the limits may be taken as oo, yielding a Fourier transform relationship. The Fourier transform may be inverted to yield 

by measuring V(u) and inverse Fourier transforming it, the reflectance function may be deduced. Four practical constraints are immediately apparent from the theoretical formation. 

Equation (8.4) is valid only for 1 t cos d0 outside this range Pt(t) in the denominator is zero. No information about the reflectance function can be obtained outside the aperture angle of the lens. 

The complete Fourier transform requires measurement of V(u) over an infinite range. Even though V(m) may be small outside the range that can be measured, the truncation will introduce errors. 

The inversion procedure is most straightforward when attenuation in the coupling fluid is ignored. This may present problems in high-frequency applications. 

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