Symbol reader using differentiating circuit for light beam focusing

DETAILED DESCRIPTION An object of the present invention is to provide an optical device which accurately reads symbols over a broad read range without increasing its cost and complexity over known devices.
To achieve this objective, the present invention provides a symbol reader for reading symbols such as bar codes, numbers, letters and other such characters.
The symbol reader comprises a light source, a condensing lens for condensing light beams emitted from the light source, a mirror which reflects the light emitted from the light source onto the symbol, and an optoelectric converting element for converting the light beam reflected from the symbol surface into an electric signal.
Means are provided for obtaining a differential coefficient of an amount of reflected light from an electric signal derived from the optoelectric transducing element.
Control means are provided for adjusting the position of the light source and/or the condensing lens along an optical axis of the condensing lens so as to maximize the amplitude of the differential coefficient.
A symbol affixed to an object a finite distance away from the inventive symbol reader is scanned by the light beam emitted from the light source and transmitted through the condensing lens.
Light transmitted through the condensing lens forms essentially a circular spot with a finite diameter on the surface to which a symbol is attached.
As the reflecting mirror rotates, the spot of light also moves.
When the spot crosses from the surface portion (without the symbol) to the symbol itself, the magnitude of the reflected light also changes.
This change is caused by the differences in reflectivity of the surface and the symbol attached to the surface.
This change in the amount of reflected light is detected by the optoelectric scanning element.
As the diameter of the spot formed by the light beam increases, the time it takes for the magnitude of reflected light to change increases as well.
Conversely, when the diameter of the spot decreases, the change in the amount of light reflected takes place in a shorter period of time