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AQ-00073-000, Rev. 7 24

where S' represents the contribution of the specular component of the test sample to

the apparent hemispherical reflectance factor of the sample. This quantity is analo-

gous to the value M' given in Eq. 7 and its relation to the actual specular component

of hemispherical reflectance (S) is described in Eq. 10:

5. On this analysis, the best estimate for the 8°/hemispherical reflectance factor of the

test sample (R

), is given as follows:

Transmittance Measurements

ASTM E 179-91a describes three categories of transmittance measurement: regular transmit-

tance, diffuse transmittance, and total transmittance. These three terms relate to each other in the

same fashion as regular or specular, diffuse and total reflectance. Regular transmittance is the

ratio of the undiffused transmitted flux to the incident flux. Diffuse transmittance is the ratio of

transmitted flux measured at all forward angles except the regular transmittance angle, to the

incident flux. Total transmittance is the ratio of flux transmittance at all forward angles to the

incident flux. An illustrative description from ASTM E 179-91a is shown in Figure 10.

SEX

S' R

SIN

SEX

–

()ρ

⁄

and

Eq. 9

SIN

SEX

–

()ρ

Eq. 10

SD+

SIN

–

()

SEX

()ρ

Eq. 11

Regular Transmittance

Impinging Light Beam

Retroreflection

Specular Reflection

Diffuse

Reflectance

Diffuse

Transmittance

Figure 10. Components of reflectance and transmittance phenomena.

Brand	Labsphere
Model	RSA-PE-20
Category	Laboratory Equipment
Language	English

Labsphere RSA-PE-20 User Manual

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