Introduction

Collimators are optical systems designed to produce a reasonable quality image of a target (or light source or some other object) at optical infinity. The angular size of the image is usually small; therefore the field-of-view is small and thus the system is relatively simple. Since the target has to be imaged at optical infinity, it must be placed at the front focal point of the collimator lens.

Collimated light is often referred to incorrectly as parallel light. No doubt, the term arises because paraxial or unaberrated real rays from a single point in the object or target are all parallel to each other in image space. This term often leads to the misunderstanding that a collimated beam has parallel sides. If this were true, a collimated beam would have zero divergence. In reality, a collimated beam diverges and there are three causes of this divergence: (1) the finite size of the source or target, (2) aberrations and (3) diffraction. Diffraction usually only dominates the divergence if the beam has a small diameter, say several millimetres or less. The diameter of collimators used in visual optics is usually much wider than this and therefore source size and aberrations are the dominant causes of beam divergence. Let us look at these in turn.

Effect of source size

In Gaussian optics, the beam must diverge and the amount of divergence is proportional to the size of the source or target. This can be easily demonstrated using Figure 23.1, which shows a source or target of radius η at the front focal point F of the collimating lens.