A red dot scope, also referred to as a reflex sight or a reflex scope serves the purpose of projecting a visible red dot, or pattern, onto a lens that the viewer can see from any angle and match up to the background as seen through their off-side eye. In contrast to conventional scopes, its main purpose is not to magnify. In reality, this feature would detract from the overall usefulness of the red dot scope. And manufacturers are aware of this -- typically red dot scopes provide no magnification at all. The main purpose of these scopes is to project a reticle through the optics that the user can use to take aim.

The reticle seen on the lens is not projected directly on it; it is reflected from a system of optics which allow the user to see both the backdrop of what they are seeing and the red dot superimposed over it. This is done via a method called optical collimation in which light is passed through a directional filter that ensures it is traveling at the same angle and direction.

This filtering process alone makes the red dot scope incredibly functional in situations where the user cannot afford to have their vision obscured by a standard sight. The collimation is used to allow the user of the red dot scope to hold it at any distance or angle from the their eye with little to no deformation or fading.

Many of the higher quality red dot scopes also balance the angular movement in the user's eye with relation to the red dot scope's position. That means the user doesn't have to be at an angle directly behind the line of sight of the scope for the reticle to be projected accuratley. If they are looking through it at an arbitrary angle, the optics are designed to automatically compensate. This happens with as great degree of accuracy near 180 degrees, and to a lesser amount the further out the viewing angle is moved from base. This is a very useful feature to have since the user is often moving around quickly.

Early tactical gear red dot scopes were called occluded scopes, which means that they were not clear. The red dot scope was set up in a manner that allowed the users' brain to place the reticle pattern over the image that was being seen with the user's off eye. Put another way, the user would look at the reticle with their right eye while keeping their right eye open, and their optical processing would automatically superimpose both images. This is no longer a mainstream design practice.

Common current designs incorporate either a full sight tube for the optics or a single vertical ring to encompass and support the lenses. Neither offers any large advantage over the other except in times where a polarizing filter or anti-glare device is used. The open ring design obviously is not favorable to this equipment. Many users feel that the open ring design is less awkward to the users' line of sight, but this is really an individual comfort issue that each user must decide for themselves.