Optical system for a large depth-of-field bar code scanner

DETAILED DESCRIPTION OF THE INVENTION A simplified diagram illustrating the optical principal of the optical system of the invention is shown in FIG.
2.
The lens 21 forms an image 25a of a photodetector array 25 that defines the working range of the bar code reader in a plane that intersects the bar code tag 23 to be read.
The principal plane of the lens A, the plane of the detector array B, and the working range plane C have a common intersection line X, and thus are arranged in what is known as the Scheimpflug condition.
With this arrangement, the lens 21 forms an image 23a of the bar code tag 23 on the detector array 25, such that when the bar code tag 23 is within the working range 27, a horizontal slice of the image 23a of the bar code will be substantially in focus on at least one of the elements of the detector array.
In contrast to the conventional optical arrangement illustrated in FIG.
1, the plane of the image 23a of the bar code tag does not coincide with the plane of the detector 25.
The optical properties of an optical system operating in the Scheimpflug condition are illustrated in FIG.
3.
In this arrangement, the object O is tilted with respect to the optical axis of the lens L.
The lens L forms an image I of the object, but that image is in focus only on an image plane that is also tilted with respect to the optical axis of the lens.
The geometry of the Scheimpflug condition is such that the object plane, the lens plane and the image plane intersect at a common line X.
Typically, a Scheimpflug arrangement would be used in a camera to focus a scene located in a large, tilted object plane (the face of a tall building from near its base) onto an image plane.
The object plane and the image plane must be carefully aligned with respect to the lens plane to properly focus the entire image onto the image plane.
Referring again to FIG.
2, it can be seen that the optical system of the invention varies significantly from the classic Scheimpflug arrangement.
One important difference is that the image 23a of the bar code tag is mostly out of focus on the plane of detector 25