# What is optical density?

I'm a zoology minor and we are doing protein estimation by colorimetric method. I have stumbled upon a term 'Optical density'. I don't understand the term well. Is it a measure of the extent of light that can pass through a particular object?

I've checked a related question of this community and it doesn't solve my question completely.

You're a little confused probably because there are two usages of the words "optical density".

The first usage is as a synonym for refractive index, as described in the answers to the related question you cite. This is the commoner usage in physics.

The second usage is the total attenuation afforded by a protective screen, neutral density filter, laser goggles or the like. $ODx\; \lambda=y$ or even $ODx\; y$ means that the filter, goggles etc afford a power attenuation factor of $10^x$ at a light wavelength of $y$ or light wavelength range $y$. That is, the power transmitted through the filter is $10^{-x}$ of the incident power when the wavelength is as stated.

For example, laser goggles marked $OD7\;488{\rm nm}$ means that the goggles will reduce incident power at 488nm by a factor of $10^7$. Goggles marked with a lone wavelength rather than a wavelength range are always meant for use with a particular kind of laser. For example, the $OD7\;488{\rm nm}$ goggles are meant for use with an argon ion laser. You cannot rely on them using another source of wavelength 485nm, for example.

For generic use, a wavelength range must be specified. So, for example, one often sees $OD7\;450{\rm nm} - 510{\rm nm}$, meaning, pretty obviously, goggles that will give you seven orders of magnitude of attenuation over the whole range $450{\rm nm} \leq \lambda \leq 510{\rm nm}$.

• To exhaust the topic - optical density can be also called absorbance. It's negative logarithm of transmittance. There is even Wikipedia article on the topic: en.wikipedia.org/wiki/Absorbance – Jarosław Komar Aug 11 '16 at 7:42
• The only field I have seen the "Refractive Index" usage of "Optical Density" has been in corrective lenses in human eyes. Ironically, in such a field the attenuation of light by the glasses is also an important variable (prescriptive sunglasses). – Aron Aug 11 '16 at 8:35
• @Aron The use of the term as a synonym for refractive index is not at all uncommon. – garyp Aug 11 '16 at 11:16
• @JarosławKomar Yes, my impression is that older European texts tend to use this word. I actually find it much more intuitive. In German (which I use quite a bit for my work), one commonly finds "Bechungsindex / Brechzahl" (refractive index) and "optische Dichte" (optical density) equally often in lens and optical system specification, standards and discussion documents ( although as a non-native speaker I find the technical meaning of "brechen" a bit odd, given that "brechen" aside from in optics means to shatter, break, throw up or something equally violent - why German doesn't say .... – WetSavannaAnimal Aug 12 '16 at 8:16
• @WetSavannaAnimalakaRodVance And English term is not entirely devoid of "brutality" for lack of better word. Amongst cognates we have fracture. – Jarosław Komar Aug 12 '16 at 15:10

For all intents and purposes, OD is the negative of the order of magnitude the factor by which the intensity of the light is reduced by the attenuating element with said OD.

In other words: OD = 6, means that the intensity will be reduced by a factor of 10 to the power of -6, a.k.a by a factor of a million.

Optical density is a mathematical way of describing the extent of attenuation of visual lightwaves.

Density is the log of opacity.

Opacity is the reciprocal of transmittance.

So a transmittance of 50% of the light (half of the light) would have an opacity of 2. The log of 2 = .3

So a density of .3 blocks half of the light energy.