System and method for employing infrared illumination for machine vision

This invention provides a machine vision device adapted to read inscribed symbology on the surface of an object, such as a wafer, covered in photoresist that employs both bright field and dark field illumination in the infrared region. Using illumination with light in this spectral band, an inscribed symbol can be read by a camera sensor substantially unaffected by the presence of and/or number of layers of photoresist covering the symbol. The camera sensor is tuned to receive such illumination, and is thereby provided with an image that distinguishes the symbol's scribe lines on the underlying wafer surface from the surrounding specular wafer surface. The device includes a housing that supports the imager and imager lens below an array of IR LEDs. The sensor has an optical axis that is reflected from horizontal to vertical by a mirror and then back to horizontal by a beam splitter that is aligned with two spherical lenses and an outlet window at the front of the housing. The array is located in line with lenticular arrays behind the beam splitter, along the central optical axis of the lenses and window.