optical density

Bruce Roe BROE at AARDVARK.UCS.UOKNOR.EDU
Sun Mar 31 08:40:00 EST 1991

```Ed Stokes wrote:
=> Message-Id: <9103302101.AA11433 at genbank.bio.net>
=> Followup-To: sci.optics
=>
=> I am interested in finding out if there exist alternative definitions
=> of "optical density" in different disciplines. That is, does a
=> physicist mean the same thing as a biologist when she says "optical
=> density". I'll start by stating what I mean when I say "optical
=> density". If a beam of light has an initial intensity I0 (in watts,
=> for example, or watts/(unit area)), and then the beam passes through a
=> medium of optical density 'od', then the intensity of the beam of
=> light after passing through the medium is:
=>
=>      I = I0 * 10^(-od)                       (1)
=>
=> Taking the base ten logarithm of equation (1) then yields an
=> expression for 'od' in terms of I/I0:
=>
=>      od = -log(I/I0)                         (2)
=>
=> I am, for the record, a physicist.
=>

Ed,
Biochemists use "optical density" to mean exactly what you have
given above.  To elaborate by quoting from "Lehninger's Biochemistry"

"The fraction of the incident light absorbed by a solution at a
given wavelength is related to the thickness of the absorbing layer and
to the concentration of the absorbing species.  These two relationships
are combined into the Lambert-Beer law, given in integrated form as:

log(I0/I) = acl			(1)

where:	I0 is the intensity of the incident light,
I is the intensity of the transmitted light,
a is the molar absorbancy index (also given as epslon or molar
extinction coefficient),
c is the concentration of the absorbing species (moles/liter),
l is the thickness of the light-absorbing sample (generally
arbitrarily set at 1.0 cm).

The Lambert-Beer law assumes that the incident light is parallel and
monochromatic and that the solvent and solute molecules are randomly
oriented.  The expression:

log(I0/I) = O.D. = Absorbance	(2)

is called the absorbancy (A) or optical density (O.D.);
where absorbancy is prefered."

Most nucleic acid oriented biochemists and molecular biologists
also accept the following:

An absorbance unit (or O.D. unit) is directly proportional
to the concentration of the absorbing solute, when the absorbing
layer is a fixed thickness.  We also have a tendency to talk about
a test tube (epp. tube now a days) containing say 2 OD's of RNA.
That means if we were to dissolve the RNA in 1 ml and measure the
absorbance in a 1 cm path length cuvette, we would obtain a A260
reading of 2.000 .  In years past we had to define this in our
publications, but now this is generally accepted.

Cheers,

Bruce A. Roe
Professor of Chemistry and Biochemistry
INTERNET: BROE at aardvark.ucs.uoknor.edu
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```