The collimator is an optical instrument consisting of a well corrected objective lens with
an illuminated reticle at its focal plane or close to it. The reticle position determines whether the outgoing beam is collimated, or a real or virtual image is formed.
The reticle is placed at the lens focal plane. The emerging beam is collimated, meaning that all rays originated from the same point on the reticle plane are parallel to each other. The image of the reticle is said to be projected to infinity because parallel rays will not cross at a finite distance.
The reticle is placed beyond the focal plane
of the objective lens. The emerging beam is convergent, meaning that rays originated from the same point on the reticle plane meet again at a single point on the right side of the lens, leading to the formation of a real image at a finite distance.
The reticle is placed between the lens and its focal plane. The emerging beam is divergent and there exists an apparent crossing point on the left side of the lens which is equivalent to an object placed at that point from where the divergent rays are originated. This image is said to be virtual.
Fig. 2 shows a schematic representation of the cross section of a collimator. Reticle and objective lens are the most important components and are mounted in a tube. On the left side there is a light source, followed by a diffuser and a set of condenser lenses that fulfill the purpose of obtaining a more homogeneous intensity distribution across the reticle pattern.