Linear integrating cavity light source for infra-red illumination of sensitized media

摘要

A highly uniform infrared illumination source for illuminating a stripe of a moving sensitized web (14) for line scan imaging of imperfections in the coating on the web by a CCD imaging camera (18). A light integrator (30) having an elongated housing formed with side (32) and end walls (34,36) defines a linear light integrating cavity (38) having diffusely reflecting interior wall surfaces. An elongated array (60) of infrared LEDs is spaced along the side wall for emitting light into the cavity (38) for integration within the cavity (38). A longitudinally extending slit (44) is formed in the side wall (32) through which a diffuse, linear light beam exits the elongated slit (44) having a varying longitudinal intensity profile. The intensity of the light emitted by the LEDs (701-70n) is modulated in an intensity pattern that alters the varying longitudinal intensity profile of the linear light beam to provide a desired longitudinal intensity profile of the stripe of diffuse illumination. Preferably, the intensity modulation renders more uniform the longitudinal intensity profile of the linear light beam illuminating the web as viewed by the imaging camera (18). The desired light intensity profile along the length of the emitted light beam is achieved by clustering the LEDs (701-70n) with LED drive circuits (801-805) operated at different drive currents as a function of a set point control signal appropriate to the characteristics of the imaging camera (18) and a transmitted light intensity feedback signal to maintain the intensity pattern of the LEDs and the uniform intensity profile as viewed by the imaging camera (18) regardless of the optical density of the web (14).