An image is a projection of a three-dimensional scene in the object space to a
two-dimensional plane in the image space. An ideal imaging system should
map every point in the object space to a dened point in the image plane,
keeping the relative distances between the points in the image plane the
same as those in the object space. An extended object can be regarded as an
array of point sources. The image so formed should be a faithful reproduc-
tion of the features (size, location, orientation, etc.) of the targets in the object
space to the image space, except for a reduction in the size; that is, the image
should have geometric delity. The imaging optics does this transformation
from object space to image space.
The three basic conditions that an imaging system should satisfy to have a
geometrically perfect image are (Wetherell 1980) as follows:
1. All rays from an object point (x, y) that traverse through the imaging
system should pass through the image point (x’, y’). That is, all rays
from an object point converge precisely to a point in the image plane.
The imaging is then said to be stigmatic.
2. Every element in the object space that lies on a plane normal to the
optical axis must be imaged as an element on a plane normal to the
optical axis in the image space. This implies that an object that lies in
a plane normal to the optical axis will be imaged on a plane normal
to the optical axis in the image space.
3. The image height h must be a constant multiple of the object height,
no matter where the object (x, y) is located in the object plane.
The violation of the rst condition causes image degradations, which
are termed aberrations. The violation of the second condition produces eld
curvature, and the violation of the third condition introduces distortions.
Consequences of these deviations will be explained in Section 2.4. In addi-
tion, the image should faithfully reproduce the relative radiance distribution
of the object space, that is, radiometric delity.