optical density (original) (raw)

Acronym: OD

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

Alternative term: absorbance

Category: general optics

• optical properties of devices
  • absorbance
  • astigmatism
  • cardinal points
  • group delay
  • group delay dispersion
  • insertion loss
  • optical thickness
  • optical aberrations
  • optical density
  • reflectance
  • retardance
  • return loss
  • transmittance
  • (more topics)

Related: absorbancedecibeloptical attenuatorsneutral density filtersrefractive indexoptical thickness

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Contents

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): \textrm{OD} = \lg \frac{P_\textrm{in}}{P_\textrm{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 attenuation mechanism used. 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.

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 density in the context of light attenuation?

Optical density (OD) quantifies light attenuation. It is the base-10 logarithm of the ratio of input power to output power (($\textrm{OD} = \lg(P_\textrm{in} / P_\textrm{out})$). For example, an OD of 3 means the power is reduced by a factor of 1000.

Why is optical density a useful quantity for filters?

When multiple attenuators or filters are used in series, their individual optical densities simply add up to give the total optical density, which simplifies calculations.

What is the other meaning of optical density?

The term optical density is also used to describe a transparent medium with a high refractive index. For instance, total internal reflection occurs when light travels from an optically denser medium to a less dense one. This usage is unrelated to light attenuation.

Suppliers

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

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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.

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Optogama designs and manufactures motorized laser power attenuators (LPA) for precise, programmable control of laser beam intensity. Covering a broad spectral range from 250 nm to 2000 nm, these devices are compatible with UV, visible, and near-infrared sources.

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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°.

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