Optical Density – explained

Optical Density (OD) is a measurement used in biotech to quantify DNA, RNA, and proteins.

But what is it exactly?

In simple terms, OD is the amount of light absorbed at a specific wavelength divided by the distance the light travels through the sample (optical pathlength or OPL) in centimeters. It represents the absorption you would get if the OPL was 1 cm.

Why is it useful?

Absorbance alone isn’t very useful without knowing the pathlength. OD is valuable because it removes the pathlength variable from the absorption measurement, allowing for comparisons between different analyzers with varying pathlengths.

Example:

If the absorbance is 20 for a cell with a 1 cm OPL, the OD is 20.

For a cell with a 0.1 cm OPL and an absorbance of 2, the OD is still 20.

This consistency makes comparisons possible and helps keep measurements within the absorbance range of the analyzer.

Convenient, right?