Expressions for the reflectance of finite, perfect diffuse, and perfect specular samples measured in an integrating sphere wlth finite ports and a uniform, perfectly diffuse reflecting wall, are derived by solving a set of linear equations. The derivation is easy compared with the long computations necessary when using an integral equation approach.
Expressions are given both for the comparison and the substitution method of measurement, with curved or flat sample and standard. For finite, flat, diffuse, and specular samples, computed examples indicate the range of sample sizes within which previous approximate expressions may be used, without introducing significant errors.
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Sample reflectance r2 in the perfect diffuse case, r1 = reflectance of sphere wall, r2 = reflectance of sample (s). r3 = reflectance of standard (st), L1 = luminous emittance of sphere wall, and F = form factor [see Eqs. (3.3), (4.1)].
Flat sample (small-area approx.), curved standard with r3 = r1
Flat sample and standard (small area approx.)
Flat sample and standard of finite area
Table II
The reflectance r2 of diffuse samples computed from the expressions of Table I, with L1s/L1st=0.5, r1 = r3 = 0.97, A4 = A5 = 0.01A, and values of A1, A2, and A2c as shown.
Sample reflectance r2 in the perfect diffuse case, r1 = reflectance of sphere wall, r2 = reflectance of sample (s). r3 = reflectance of standard (st), L1 = luminous emittance of sphere wall, and F = form factor [see Eqs. (3.3), (4.1)].
Flat sample (small-area approx.), curved standard with r3 = r1
Flat sample and standard (small area approx.)
Flat sample and standard of finite area
Table II
The reflectance r2 of diffuse samples computed from the expressions of Table I, with L1s/L1st=0.5, r1 = r3 = 0.97, A4 = A5 = 0.01A, and values of A1, A2, and A2c as shown.