Focus on: Biconical Transmittance

Cone diagram

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When a coating is used in a convergent beam (as illustrated in the graphic shown above), TFCalc can compute the biconical transmittance (also called the cone-angle average in TFCalc). This computation is especially important if the performance of the coating varies dramatically with the incident angle, such as a bandpass filter or a beamsplitter. Note: because spectrophotometers (which use convergent beams) are used to measure the performance of coatings, this computation may be important even for coatings that are used at only one incident angle. That is, the spectrophotometer measurement may indicate that the coating is defective when it is actually correct. For example, consider a wideband, dielectric, immersed 45-degree beamsplitter, whose reflectance is shown below.

Plot of BS performance

Note that both polarizations are within 50% ± 0.5%. When it is measured with an f/8 convergent beam, the performance degrades to 50% ± 8%:

Plot of BS performance using f/8 cone

There is also the capability, added in TFCalc 3.4, of specifying the radiation distribution inside the cone. Previously, it was assumed to be Lambertian. Now it can be any distribution that the designer desires.

In TFCalc 3.5, the reflectance and/or transmittance of a cone of light can be optimized. See the information about cone-angle targets.

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