An optical instrument for measuring characteristics of a specimen comprises a light source and an axial reflective member spaced from the light source to define an optical axis for the instrument along a line between the light source and the reflective member. A non-axial reflector is positioned laterally of the reflective member and laterally of the optical axis to receive a beam reflected from the axial reflective member. The non-axial reflector is oriented to reflect the beam onto the specimen. The instrument also contains a segmented photosensor and a second non-axial reflector to receive the beam reflected from the specimen and reflect it onto the segmented photosensor. The photosensor is positioned in the instrument to receive the beam from the second non-axial reflector. A conductor connected to each segment of the photosensor is provided for carrying current to signal conditioning hardware used to compare the current from the segments of the photosensor to provide information concerning the specimen.