Optical Density
Author: the photonics expert Dr. Rüdiger Paschotta
Acronym: OD
Definition: a logarithmic measure of the power attenuation, or the refractive index
Alternative term: absorbance
DOI: 10.61835/6zs Cite the article: BibTex plain textHTML 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
Encyclopedia articles:
Suppliers
The RP Photonics Buyer's Guide contains 79 suppliers for optical attenuators. Among them:
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.
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.
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.
Questions and Comments from Users
Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.
Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.
By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.
Share this with your network:
Follow our specific LinkedIn pages for more insights and updates: