[5] It rapidly became an industry standard[7] due to its low cost, durability and ease of use,[8] and has been referred to as an "iconic lab spectrophotometer".

Absorption at any particular wavelength in the ultraviolet visible range is proportional to the concentration of the substances in the solution or other medium, in accord with the Beer–Lambert relationship.

This type of standard curve relates the concentration of the solution (on the x-axis) to measures of its absorbance (y-axis).

[13] In this plot, the slope of the line is the product ε x l. By measuring a series of standards and creating the standard curve, it is possible to quantify the amount or concentration of a substance within a sample by determining the absorbance on the Spec 20 and finding the corresponding concentration on the calibration curve.

Alternatively, the logarithm of percent transmittance can be plotted versus concentration to create a standard curve using the same procedure.

[16] Likewise the turbidity of water suspensions of clays and other particles of size suitable for light scattering can be quantitatively determined by means of a Spectronic 20.

The instrument can also optionally be calibrated with a stock solution of a sample at a concentration known to have an absorbance of 2 or else vendor supplied standards, using the light absorption knob in the diagram shown at right.

To measure wavelengths above 650 nm, the bottom of the instrument is opened, and a red filter and a red-sensitive photocell is installed.

Using the original instrument requires manual setting of the wavelength[2] and making readings from a moving-needle analog display.

Software modes described in the Spectronic 200's specifications include scanning, four wavelength simultaneous measurement, and quantitative analysis with up to four standards,[21] in contrast to the SPEC 20D+ which offered only single point calibration.

In 2003 the product line was moved to Madison, WI and the brand renamed to Thermo Electron.

[22] The Spectronic 20 is apparently one of the few lab instruments to remain intact after the destruction of the laboratory in the movie Back to the Future.