Collimation

Collimation, in the context of night vision, refers to the method of aligning the output image(s) produced by a night vision device with each other and/or the real field of view.

Due to manufacturing inaccuracies, image intensifier tubes usually have imperfect alignment between the housing and the electro-optical assembly.

To counteract this, many devices allow some form of collimation.

For monocular devices, collimation is not as important, as the user doesn't notice a slight offset of the image in comparison to the surrounding dark environment.

For binocular devices, collimation between the two pods is important, with collimation relative to the real field of view being only of secondary importance for users requiring highly accurate observation capabilities, e.g. pilots. If the two pods are misaligned, the best case is some degree of discomfort as the user's eyes have to adjust to looking into two different directions, and in the worst case simply cause the user to see double.

Eyepiece Collimation

The simplest form of collimation uses a rotating eyepiece with a slightly off-axis optical assembly. By rotating the eyepiece, the output image can be moved circularly in the field of vision.

This method has very limited use in aligning the image with the real world, as the offset is always constant. A skewed output image can potentially be aligned better by counteracting the offset to some degree. Additionally, users might prefer an offset on the vertical axis rather than an offset on the horizontal axis.

For alignment between the two pods of a binocular device, this method is much more useful, as in most cases (as long as the offset in the image intensifier tubes is not too severe) the two circles of offset have two intersections, meaning one can find two positions at which both pods have the exact same amount and direction of offset.

There are multiple ways to practically collimate such devices without an expensive collimation device.

The simplest method simply requires opening and closing each eye quickly and observing the offset of a distant light source in both images, while rotating the eyepieces until the light point is aligned.

Alternatively, one can use a camera mounted in a fixed position, and holding the pods in front of it, yet again observing the offset. This method can be extended by also noting where the light point is without any pod held in front of the camera.

True Collimation

There exist factory-collimated devices where the image intensifiers are (often permanently) mounted into a shoe inside of the housing which perfectly aligns them with the optical axis. These housings have the clear disadvantage of making it impossible to field-swap the image intensifier tubes.

Most new head mountable devices from L3 / Insight achieve true collimation by having the eyepiece fixed to the main housing, having a tube to which alignment features are glued in factory which cancel the image offset mostly caused by the inverting fiber optic output, and having secondary alignment features on the eyepiece and on the objective lens, so that when assembled together, the factory aligned features on the tube guarantee a collimated end result, including correct diopter setting and potentially alignment of the eyepiece optics to the "focal point" of the concave fiber optic output of the tube. This design allows to field-swap an image intensifier without the need for collimation on a test bench after assembling the device.

True collimated monoculars have been popular outside of the US, most likely due to the ability to randomly pair two such monoculars as a binocular. This is not possible with an AN/PVS-14, although a usable solution exists, by having a binocular bridge with having the AN/PVS-14's oriented 180deg rotated compared the each other allows one to adjust the eyepiece collimation level vertically, and so that it is always horizontally converging.