Transmittance

Author: the photonics expert Dr. Rüdiger Paschotta (RP)

Definition: the ratio of transmitted optical power to the incident optical power at some partial transparent object

Category: article belongs to category general optics general optics

optical properties of devices

Units: (dimensionless quantity)

Formula symbol: ($T$)

Page views in 12 months: 821

DOI: 10.61835/vr5 Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn

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What is a Transmittance?

The term transmittance is defined as the ratio of transmitted optical power to the incident optical power for some object, for example an optical system. For transmission through flat unstructured surfaces, it is the same as the transmissivity. However, the transmittance is a more general term and can be specified in a wider range of situations:

Some objects can cause scattering of light. One may then specify the hemispherical transmittance, which is based on the total transmitted radiant flux. Also, there is the directional transmittance, defined as the ratio of transmitted and incident radiance; it is a function of observation angle.
There are extended objects, where light can penetrate, is internally scattered and thus partially transmitted and partially reflected. The transmittance simply quantifies the amount of light getting through the object, regardless of the path.
When light is incident on a transparent plate with parallel surfaces, for example, Fresnel reflections occur on both surfaces. The transmitted power can be affected by interference effects, making the transmittance strongly wavelength-dependent.

The results of transmission measurements with spectrophotometers, for example, are usually called transmittance rather than transmissivity, since one is often dealing with extended samples.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is optical transmittance?

Transmittance is the ratio of the transmitted optical power to the incident optical power for an object, for example, an optical system.

What is the difference between transmittance and transmissivity?

Transmissivity is the transmittance specifically for a single flat, unstructured surface. Transmittance is a more general term for a whole object, which may have multiple surfaces, cause scattering, or exhibit interference effects.

How is transmittance defined for scattering objects?

For objects causing scattering, one can specify the hemispherical transmittance, which is based on the total transmitted radiant flux. Alternatively, the directional transmittance is the ratio of transmitted to incident radiance, depending on the observation angle.

Why can the transmittance of an optical element be wavelength-dependent?

For an element like a transparent plate with parallel surfaces, interference effects between reflections from the front and back surfaces can make the transmittance strongly dependent on the wavelength of light.

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