Optical density (OD) is old but effective and a widely used term everywhere...but yet it has no units. Should such confusing terms be introduced in science?
Nice explanation Lindsay! though I already know it...actually I have no problem with dimensionless quantities... I just wanna ask why it is termed as density, as the term "Density" always has some physical bulky meanings ;-) ... e.g., we have mass density, charge density, energy density, package density, population density, traffic density and so forth....so why we use the word "Optical Density" though it does not make sense of density... hope you can understand my problem with this term....
Hang on a second why is having no units (or being dimensionless) confusing?
Optical density could also be thought of how well a material can block a particular light. How we could derive this is by ratio of the light "illuminating" the material and the resultant light transmitted through the material, both the "components" we use to calculate the optical density have units, however they cancel causing optical density to become dimensionless.
We have a number of dimensionless quantities in science and they are perfectly acceptable. http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Dimensionless_quantity.html
Nice explanation Lindsay! though I already know it...actually I have no problem with dimensionless quantities... I just wanna ask why it is termed as density, as the term "Density" always has some physical bulky meanings ;-) ... e.g., we have mass density, charge density, energy density, package density, population density, traffic density and so forth....so why we use the word "Optical Density" though it does not make sense of density... hope you can understand my problem with this term....
And how does optical density of film correlate with "area density" of silver halide on film's surface? Does anyone know? That would be interesting.
In spectroscopy, the absorbance (also called optical density) of a material is a logarithmic ratio of the radiation falling upon a material, to the radiation transmitted through a material. Absorbance measurements are often carried out in analytical chemistry.
It is a ratio: you are dividing light units over light units and receive 1, thus mathematically you cannot have units.
Hi Loredana, the OD is a negative index of power with base of power equal to 10. Mathematically it must be unitless. It will obtain units (e.g. cm-1) when you convert OD to absorption coefficient by dividing on film thickness.
really , it was a very excellent explanation from all of you .....thanks allot
Dear Samaneh Torbati ,
yes, you are right.
Optical density is measure of absorbance.
Please see:
https://www.photonics.com/EDU/optical_density/d5788
https://www.sciencedirect.com/topics/engineering/optical-density
and the answer of Dr Panikumar Durga Anumolu above.
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