Turbid samples have large backgrounds which make obtaining accurate absorbance readings very difficult. The more turbid the sample, the more incorrect the answer obtained by an ordinary spectrophotometer will be. And, one cannot know that the results are incorrect with the ordinary spectrophotometer without further quantitative measurements.
With a scattering sample, the measuring light is deflected (scattered) away from the detector, so close positioning of a detector to the sample is critical. Most absorbance spectrophotometers use small face detectors positioned many centimeters from the sample. The DW-2/2000 uses a large face detector a few millimeters from the sample.
However, the "DW-2" gets its name from its more obvious difference from normal spectrophotometers: its use of two monochromators to produce a reference wavelength which is subtracted from the (artificially high) absorbance returned by the sample.
The monochromator is motorized, so that it can scan wavelengths. This "Mono 1" is used to scan the spectrum of interest, i.e. 520 580 nm. "Mono 2" is set to the reference wavelength. When the difference between the returned values is taken, one has a "difference spectrum" which is as close to "correct" as one can obtain from a scattering sample. (One is still contending with two effects: the reference beam is not at the same wavelength as the measuring beam, and, without extraordinary measures, one cannot collect all the scattered light and, therefore, cannot get a perfect answer.)
When the absorbance value is calculated using the Mono 2 reference, in effect, one produces a "Difference Spectrum".