A rotatable grating is used for selecting a wavelength. A beam splitter splits the beam into two: half of the intensity goes to the reference diode, which measures the intensity without absorption, which is I0. The other half passes the flow cell, where it is partially absorbed by the sample. The sample diode measures the light intensity after absorption.
A beam is diffracted by a grating not only once but many times. In zero order diffraction, the grating works like a mirror and simply reflects the beam. Wavelengths are not separated for the 0th order and most intensity is diffracted by this order. The first order diffraction is used by detectors for separating wavelengths. Higher level orders occur as well but are usually not wanted. Cut-off filters like the one shown above can be used for removing these diffractions. The 0th order maximum is used for setting one of the two calibration angles.
The light generated by the deuterium lamp has no unifom intensity for all wavelengths but a characteristic distribution.