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Optical Density

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

Acronym: OD

Definition: a logarithmic measure of the power attenuation, or the refractive index

Alternative term: absorbance

Category: article belongs to category general optics general optics

DOI: 10.61835/6zs Cite the article: BibTex plain text HTML Link to this page! LinkedIn

The term optical density is used with two different meanings:

Optical Density as the Degree of Attenuation

Optical density may mean the absolute value of the logarithm with base 10 of the power transmission factor of an optical attenuator (e.g. as used for a laser safety glass):

$${\rm OD} = 10 \: \lg \frac{P_{\rm in}}{P_{\rm out}}$$

For example, an optical density of 3 means that the optical power is attenuated by the factor 10³ = 1000. That would correspond to an attenuation by 30 decibels.

If several attenuators are used in series, their optical densities can simply be added. That is the key advantage of using such a logarithmic quantity.

To avoid ambiguity, it is better to use the term absorbance, as far as absorption is the used attenuation mechanism. However, attenuation may also be achieved with reflection or scattering.

Optical densities depend on the optical wavelength, although that dependence may be weak, e.g. in neutral density filters.

Optical Density and Refractive Index

A transparent medium is sometimes said to have a high optical density (or to be relatively dense) if it has a high refractive index. For example, one may say that total internal reflection is possible only if the beam comes from the optically more dense medium. That use of the term has nothing to do with attenuation.

More to Learn

absorbance decibel optical attenuators neutral density filters refractive index

Suppliers

The RP Photonics Buyer's Guide contains 78 suppliers for optical attenuators. Among them:

Vortex Optical Coatings

We design and manufacture custom attenuator coating across a range of 300 nm – 6000 nm for a variety of applications. The main uses of attenuators are protecting sensitive detectors from overload, attaining accurate laser power measurements and being used in filter wheels to balance the system for particularly strong signals.

LASEROPTIK

LASEROPTIK can provide coatings for variable optical attenuators. One can change the transmission by tilting an edge filter, with highest transmission at normal incidence and typical operation for angles of incidence between 0° and 45°.

Gentec Electro-Optics

Gentec Electro-Optics offers optical attenuators for measurements on high-power laser beams.

EKSMA OPTICS

EKSMA Optics offers variable attenuators for linearly polarized laser beams with high performance optics. They are available for Nd host laser applications in the 1040–1070 nm range and for femtosecond applications with broadband Ti:sapphire and Yb:KGW/KYW laser pulses. Manual and motorized versions available, highly customizable and designed for use in laboratory settings. Compact motorized laser power attenuators provide stable and reliable performance even in high power lasers for industrial applications.

Optogama

Optogama designs and manufactures motorized laser power attenuators (LPA) for laser power control. LPA are available for use in the UV, visible, and NIR spectral ranges from 250 nm to 2000 nm. These devices feature a large clear aperture dedicated for considerable size beam applications. All optical elements of these laser power attenuators are made for high LIDT and provide a stable and reliable performance even when using them with high power lasers in industrial applications.

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