optical density

optical density

a spectrophotometric measurement of light scattered by a suspension at a particular wavelength. Optical density measurements can be used to determine the BIOMASS concentration in a suspension, when, for instance, monitoring the GROWTH of a culture of MICROORGANISMS. It should be noted, however, that the microorganisms in the suspension will both scatter and, probably to a lesser extent, absorb the incident light, and it is the sum of these properties that is measured. It is therefore debatable what term should be applied. ABSORBANCE is sometimes used interchangeably with optical density, but this is not strictly correct. Optical density is generally accepted as the more appropriate term when measuring microbial growth.

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

density, optical

A term applied to optical filters. It is equal to the logarithm to the base 10 of the reciprocal of the transmission factor T thus,

where D is the symbol for optical density. Syn. absorbance. See absorbance; absorption; spectrophotometer; transmittance.

Table D1 Density of optical lens materials
		n		density (g/cm3)
glass
spectacle crown		1.523		2.54
dense barium crown		1.620		3.71
dense flint		1.706		3.20
dense barium flint		1.700		4.10
titanium oxide		1.701		2.99
plastics
PMMA		1.490		1.19
CR-39		1.498		1.32
polycarbonate		1.586		1.2
cellulose acetate butyrate		1.48		1.2
cellulose propionate		1.46		1.2

Table D2 Relationship between optical density D and light transmission T of optical filters
D	T (%)
0.0	100
0.1	79.4
0.2	63.1
0.3	50.1
0.4	39.8
0.5	31.6
0.6	25.1
0.7	20.0
0.8	15.8
0.9	12.6
1.0	10.0
1.5	3.16
2.0	1.0
2.5	0.32
3.0	0.1
4.0	0.01

Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann