A compact optical spectrometer for measuring hemoglobin parameters in whole blood includes an enclosed spectrometer housing having a light entrance port, a light input slit disposed on one side of a circuit board substrate and positioned adjacent to and aligned with the light entrance port, a light-array detector disposed on the one side of the circuit board substrate adjacent the light input slit, a light dispersing element disposed downstream from the light input slit and an achromatic lens disposed between the light input slit and the light dispersing element to direct the light from the input slit to the light dispersing element and to direct the dispersed light from the light dispersing element to the light-array detector.