Dynamic range

The usable interval or ratio between the smallest and largest signals of a measurement system.

An instrument's ability to display very low or very high readings does not mean that every reading is suitable for quantification. Dynamic range concerns which regions, from the weakest to the strongest signal, still meet defined quality requirements.

A range needs a criterion for usability

Dynamic range can be expressed as the ratio or interval between the largest and smallest usable signals. The method must define usable: for example, acceptable signal-to-noise ratio and uncertainty at the low end, and no saturation with the specified photometric response maintained at the high end.

The linear dynamic range is narrower: it includes only the region where response and the target quantity follow the specified linear relationship. A reading beyond this region does not justify continuing to use the original linear conversion.

The two ends of an absorbance measurement have different limits

Low absorbance is usually limited by Blank differences, baseline drift and random noise. High absorbance corresponds to extremely weak transmitted light and is susceptible to stray light, detector response at low signal and photometric nonlinearity. A single metric cannot describe both ends.

Shortening the path length can lower the absorbance of a concentrated sample, allowing more of the intended light to reach the detector. Lengthening it may amplify absorption changes in a dilute sample. Changing path length also changes the calculation and calibration conditions.

Distinguish sample range from instrument range

The measurable concentration range for nucleic acids, proteins or other samples also depends on absorption coefficient, matrix, purity, scattering and dilution. An instrument's photometric dynamic range cannot be converted directly into a concentration range applicable to every sample.

A complete assessment specifies wavelength, bandwidth, path length, reference materials, the linear model, allowable error and Blank conditions. Reporting only the number of orders of magnitude, without these boundaries, cannot support the choice of an experimental method.

Dynamic range is not an isolated parameter for which bigger is always better. It describes the quality of results that a method can deliver at both the low-signal and high-absorbance ends.