The difference between the NR Lite(2) and CNR 2 lies in the material used to cover the thermopiles.
CNR 2 uses glass domes for the pyranometers (that measure short wave radiation) that have a bandwidth of 300 nm to 2800 nm. It uses silicon domes for the pyrgeometers (that measure long wave radiation) that have a bandwidth of 4500 nm to 42000 nm. This leaves a gap between 2800 nm and 4500 nm. This is the so called atmospheric window where very little radiation comes in (see picture below).
The NR Lite(2) uses NO domes. It uses two detectors with a Teflon coating which have a bandwidth of 200 nm to 100.000 nm.