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pigmented colorants: dependence on media and time - Cornell ...

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Figure A.2: The path of light between two homogeneous layers.<br />

Adapted from [Kub54]<br />

Summing the porti<strong>on</strong>s reflected by the layered specimen, we have<br />

T1T1 ′R2<br />

Rtotal = R1 + T1T1 ′R2(1 + R1 ′R2 + R 2 1 ′R2 2 + ···)=R1 +<br />

1 − R1 ′R2<br />

207<br />

(A.23)<br />

For any homogeneous layer, R1 = R1 ′, or the material reflects light equally<br />

when illuminated from both sides. Also, it can be shown that T1 = T1 ′,orthe<br />

transmittance has the same value, if we illuminate a homogeneous layer from <strong>on</strong>e<br />

side or the other. Therefore, the formulas simplify to:<br />

Ttotal = T1T2<br />

1 − R1R2<br />

Rtotal = R1 + T 2 1 R2<br />

1 − R1R2<br />

(A.24)<br />

(A.25)<br />

The compositing equati<strong>on</strong>s can be used to calculate the reflectance of spec-<br />

imens that c<strong>on</strong>tain more than two layers (which is often the case in painting).

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