chapter 3, Applications

Excerpt

In this section, we present more details about imaging physics, what criteria describe best the image quality, and what makes aspheres so attractive to the community of designers.

3.1 Physical Considerations

The design of an optical system has to ensure that the specifications for image quality are fulfilled inside the 3D working volume at the object side. This volume is defined as the product of the field of view (FoV) and the usable depth of focus (DoF). The image quality for all object points, often expressed by spatial and spectral aberration values, must be controlled within the spectral bandwidth of the received light.

The aberrations can be physically understood as optical wavefront deformations of an object point source. Such a source emits a spherical wavefront, which is picked up by the entrance pupil of the optics. When traveling through the system, the wavefront is unavoidably deformed by diffraction but also by imperfect design or by manufacturing errors. Then a distorted spherical wavefront leaves the exit pupil, causing an aberrated or blurred intensity spot in the image plane. Because diffraction is a physical phenomenon of light as information carrier, we must accept, in any case, a degraded image. An optical system unavoidably acts as a low pass filter, which cuts off higher spatial frequencies, that is, higher spatial structures.

To minimize the aberrations, the designer needs a minimum number of optical parameters to vary, such as surface shape, lens thickness, and the glass values of all the lenses. But what are the quality criteria for which an optical system has to be optimized?

3.2 Image Quality

A good image must look sharp, brilliant, and must be stable.