For the biochemist, a recording spectrophotometer
is probably of greatest value for following
chemical reactions by plotting changes of extinction
('absorbance' or 'optical density') against
time at a fixed wavelength. Many biochemical
reactions, however, involve starting materials with
a fairly high extinction and produce relatively small
changes in this extinction. For such cases it is of
great value to be able to expand at will any section
of the extinction scale (e.g. the section 0.6-0.7) to
fill the whole height of the chart, so as to increase
the slope of the reaction curve.
Some commercial spectrophotometers can be
fitted with scale-expansion attachments for this
purpose, but they tend to be somewhat complicated
and expensive, and to require several adjustments
to bring them into operation. Some cover only a
limited range of the scale.