In many optical systems, an imaging or image converter element, such as a lens or image tube, is used to convert light received from an object into a focused, magnified, or amplified image as required for desired optical applications. However, transmission through the imaging element often results in a field curvature in the output image that must be corrected. For example, in the uncorrected system of FIG. 1, transmission of light from an object along an optical axis OX through an imaging element A is shown to have a curvature distortion in the output image plane A'. For simplicity, imaging element A is shown and referred to as a single lens; generally, it can be an optical system of arbitrary complexity.
Furthermore, some optical systems have two or more imaging elements (again, of arbitrary complexity) in which an image from one imaging element provides an object for a second imaging element. As shown in the uncorrected system of FIG. 3, an image plane A' is output from the imaging element A with a curvature distortion, and becomes the input object to a second imaging element B, resulting in a further distorted, output image plane B'. The compounded distortion effects can render the resulting image in poor quality or unsatisfactory condition for the desired optical application.
Merely inserting the fiber optic element FO of FIG. 2 between the imaging elements A and B would not prevent the curvature distortion at the output image plane B' due to imaging element B. This is because the flat plane of the fiber optic element FO would not adequately compensate for the differences in lens radii or curvature, for example, of the different imaging elements A and B.
Efforts to correct field curvature distortions of optical systems have generally been accomplished by the addition of corrective elements, such as a field-flattening lens. However, the additional corrective elements add to the cost, weight, and volume of such systems, and can introduce unwanted other effects into the optical system, such as internal reflection, depth of field alteration, or refraction. Moreover, in compound systems where a reversal of the field curvature of the image plane from the first imaging element is desired prior to entry through the second imaging element in order to minimize complication of the second imaging element and to yield a correct output image, the field-flattening lens does not provide an adequate solution.
It is therefore a principal object of the invention to provide an optical system having means for correcting a field curvature in an image transmitted through an imaging element. It is a further object to provide such means which will reverse the field curvature of an image so that it can be transmitted through a second imaging element and result in a correct output image. In particular, the invention seeks to provide for field curvature correction which does not introduce unwanted other effects into the optical system.