A lung water computer system utilizing a single optical detection path. A single beam of visible (VIS) and infra-red (IR) radiation is chopped and filtered in at least two bandwidths centered on the absorption peaks of tracers such as indocyanine green (ICG) and deuterated water (DHO). The chopped, filtered beam is transmitted along the optical detection path to a densitometer cell provided with intersecting fluid and optical channels. The optical channels are milled to provide an accurate optical path length (OPL) across the fluid channel. The chopped filtered radiation traverses blood containing ICG and DHO in the fluid channel and is detected by a two color (VIS and IR) detector at the end of the optical detection path. The VIS and IR outputs of the detector are processed in separate electronic channels. The channels are sampled and the sample data is used to compute optical density, tracer concentration, cardiac output, tracer transit time, recovery factor and lung water according to well-known algorithms. Although not necessary in computing lung water, the channels may be sampled adaptively based on the sample levels or signal to noise ratio (S/N).