U.S. Pat. No. 6,207,946 provides an adaptive lighting system for use with a machine vision apparatus for capturing high contrast images of an article to be inspected. The adaptive lighting system includes an array of plural optical fiber light emitting elements for projecting light onto an article to be inspected, a camera for recording an image of the article, a light intensity control circuit for varying intensity of the light emitted by the array to adjust the illumination of the article viewed by the camera, and a processor programmed to calculate a median gray value from a predetermined number of camera pixel intensity values of the article to re-adjust the intensity of the light until a setting with suitable contrast is reached.
However, the adaptive lighting system can only adjust the intensity of the light emitted by the array of plural optical fiber light emitting elements according to the contrast of the image, and, in practice, it is insufficient to consider the contrast alone. Moreover, besides the intensity of the light, other factors such as pattern, tone, etc., of the light impinging on the article are also important, and may affect the contrast and other image properties of the image of the article illuminated by the light.