Photometric measuring apparatus

Claim 1

In photometric apparatus for the type wherein absorbance through a sample cell in one of two radiation paths is determined by directing light from a source of predetermined wavelength contemporaneously along both paths, the improvement comprising: a first PIN diode disposed at the end of the path with the sample cell to detect intensity of radiation thereat; a second PIN diode disposed at the end of the oher path to detect intensity of radiation thereat; a field stop, a lens, and an aperture stop sequentially aligned along an axis to receive light from the source and to pass a beam of uniform intensity over the cross-sectional area thereof; means for proportionally distributing said beam to the paths; means for focusing light from said beam distributing means to symmetrically image said field and aperture stops substantially at the entrance and exit respectively of the sample cell; means for focusing light that passes from the sample cell to symmetrically image said aperture stop wholly on the sensitive area of said first PIN diode; means for focusing light from said beam distributing means to symmetrically image one of said field and aperture stops wholly on the sensitive area of said second PIN diode; first and second parametric amplifiers separately disposed to receive the outputs from said first and second PIN diodes respectively; and circuit means for deriving signals logarithmically proportional to the output from each said parametric amplifier and for determining the difference between said logarithmic signals.