Hach LANGE Pocket Colorimeter II User Manual page 13

The combination of an LED light source and an interference filter sets
the measurement wavelength.
Select the best wavelength
The single wavelength instruments each have a different LED and
interference filter to measure at a specific wavelength.
The wavelength (color) of light used is typically selected so that it has a
maximum absorption, but other wavelengths can be selected to
minimize interferences or other factors. For the best results, select the
instrument wavelength with knowledge about the absorbance spectra
of the species of interest, as well as the spectra of other colored
species that could be in the sample. Figure 6 shows a typical
absorption spectrum.
Refer to Table 1 to select the best instrument wavelengths to use for
testing. Do not use this table for samples that have more than one
absorption region that adds to the visible color. For example, a green
solution can have a yellow and a blue absorption peak. One or the
other peak can be used for measurements if both have different
analyte concentration. Other samples can look brown because there
are several spectra that add to the visible color.
English 13