an indication of whether or not a target object is present at a site of a collet assembly is derived by monitoring light reflected from the site . the use of reflected light enables the presence of objects to be detected , especially transparent objects . in a preferred arrangement for determining if a sapphire die is present in a die - handling collet assembly , light from a source is collimated or focused into a narrow beam and directed through a beam splitter at the target site of the collet assembly . light reflected from the surface of the die is further reflected by the beam splitter toward a photo - sensor . the reflected light is measured when the collet passes over a dark background while on the way to deliver a die at a bonding position and again while returning after attempting to place the die at the bonding position . a determination of a die present in the former and absent in the latter is indicative of a well - placed die at the bonding position .