Abstract: An ink jet printer comprising an ink jet print head having at least one row of a plurality of ink ejecting ports for ejecting ink droplets along a plurality of ink droplet paths, the ink jet print head residing at a first elevation; a collimated light source and a detector each residing at a second elevation that is lower than the first elevation, the detector positioned opposite the collimated light source, the ink jet print head being movable to a test position where the at least one row of a plurality of ink ejecting ports can fire non-printing droplets, the collimated light source directing light at the detector along a light path that intersects the plurality of ink droplet paths when the print head resides in the test position; and an aperture located in between the collimated light source and detector and proximate to the detector to limit a field of view of the detector and increase an optical signal-to-noise ratio of the detector.