Holographic collimators are objects made of a tube, or a base, which holds a surface treated glass to have the maximum transmittance compatible with adequate reflectance, on which a point is projected, usually of red color for which they are known as red -dot, but lately also green. Holographic sights can therefore be open, cheaper but more fragile. The light spot is usually generated by a LED, but this is not the only possible light source: there are some for use in variable light where the light spot is an optical fiber that takes light from a special window in the body of the light. Instrument and others in which the light spot is generated by a tritium element, just as there are combinations of the various systems; even someone is equipped with a laser diode. The light beam, projected from the lower part of the instrument, is reflected from the glass towards the eye of the shooter, on the extension of a straight line parallel to the barrel. It is therefore said that the red dot is projected to infinity; even if the definition is not correct, however, helps to understand even those who have no knowledge in the field of optics.
Although the holographic collimator can be equipped with magnifications, usually of low value, the tendency is to separate the two functions, so there are special tools equipped with quick release attachment to be placed behind the red dot, which remains at 1x. This allows you to aim with both eyes open, without losing the peripheral vision as would be achieved through a spyglass; in this case the light spot as if it were transparent on the target. Furthermore, the red marker is only seen by the shooter, with a great advantage on the laser pointer that is perceived by the subject illuminated by it. The red dot usually covers a five-minute arc of an angle, but there are also some with a major coverage, up to 15 or 20 MOA, as well as there are lower coverage, either 4 or 2 MOAs. Considering that the shooting with the red dot takes place at short distances, usually at less than 100 meters representing more or less the maximum limit for operational use, the coverage of a visual angle much broader than that which can be obtained with the reticle of a pointing dog, it is not a problem. Moreover, the collimator can be placed in any position on the barrel without necessarily having to be near the eye, which is an undoubted advantage when working with rifles with a supported recoil.
Finally, the presence of a single point makes the collimation on the target faster than an optical lattice, which can have a low contrast on the target itself while a bright spot is always clearly visible, also because its brightness is generally adjustable.
Some models, destined to an unlikely pull at greater distances, are also equipped with a parallax corrector, to avoid that the movements of the shooter’s head can generate a deviation from the sight line. Moreover, considering the usually small distances and the lower precision of the holographic viewfinder compared to a traditional optical system, the parallax correction generally does not make much sense, especially considering that this kind of pointer is intended for instinctive shooting and in any case the fast response. At distances in the order of tens of meters, each parallax error is not influential on the acquisition of the target. Recently, holographic viewfinders, usually of the open type, have been developed for smooth-bore weapons to operate in low light conditions.