Patent ID: 7262844

Claim:
A spectrophotometer, comprising: a sealed housing lacking an internal light source; a reference cell mounted in said housing; a sample cell mounted in said housing; an optical system within the housing to split an externally generated light beam into reference and sample beams and to deliver the reference and sample beams through separate paths, one path passing the reference beam into the reference cell and another path passing the sample beam into said sample cell; reference cell interaction means for interacting the reference beam with a reference and reflecting at least a portion of the reference beam; sample cell interaction means for interacting the sample beam with a sample and reflecting at least a portion of the sample beam; a reference light detector for detecting at least a portion of light reflected by said reference cell interaction means; a sample light detector for detecting at least a portion of light reflected by said sample cell interaction means; a light guide for delivering the externally generated light beam into said optical system from a light source external to said sealed housing; and a detector circuit receiving a reference current from said reference light detector and a sample current from said sample light detector, said detector circuit producing a difference voltage proportional to the difference between the reference current and the sample current by a current mode subtraction at a summing point having feedback to balance said reference current and said sample current to produce a difference current that is then converted to the difference voltage, said detector circuit simultaneously producing the difference voltage and at least one of a voltage proportional to said reference current and a voltage proportional to said sample current.