ebstokes@maxwell.crd.ge.com (03/31/91)
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 Stokes ebstokes@crd.ge.com
BROE@AARDVARK.UCS.UOKNOR.EDU (Bruce Roe) (03/31/91)
Ed Stokes wrote:
=> Message-Id: <9103302101.AA11433@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@aardvark.ucs.uoknor.edu
BITNET: BROE@uokucsvx
AT&TNET: 405-325-4912 or 405-325-7610
SnailNet: Department of Chemistry and Biochemistry
University of Oklahoma
620 Parrington Oval, Rm 208
Norman, Oklahoma 73019
FAXnet: 405-325-6111
ICBMnet: 35 deg 14 min North, 97 deg 27 min Westdbk@aberystwyth.ac.uk (04/02/91)
OD and Absorbance are *NOT* the same. Absorbance refers to the absorption of light *BY A CHROMOPHORE*. OD refers to the amount of light that sets out from the source but doesn't reach the detecctor. In biology, the usual reason is that it is *SCATTERED* (mainly elastically), not that it is absorbed. Hence the difference in terminology. As Michael Caine might say, "not many people know this".