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A holographic weapon sight or holographic diffraction sight is an optic that allows the user to look through a glass optical window and see a reticle image superimposed at a distance on the field of view. The hologram of the reticle is built into the window and is illuminated by a laser diode.
Holographic weapon sights use a laser transmission hologram of a reticle image that is recorded in three-dimensional space onto holographic film at the time of manufacture. This image is part of the optical viewing window. The recorded hologram is illuminated by the collimated light of a laser diode built into the sight. The sight can be adjusted for range and windage by simply tilting or pivoting the optical window.  To compensate for any change in the laser wavelength due to temperature, the sight employs a holography grating that disperses the laser light by an equal amount but in the opposite direction as the hologram forming the aiming reticle. Like the reflector sight, the holographic sight is not "parallax free", having an aim-point that can move with eye position. This can be compensated for by having a holographic image that is set at a finite distance with parallax due to eye movement being size of to getbthe optical window at close range and diminishing to zero at the set distance (usually around a desired target range of 100 yards).
Since the hologram is illuminated by a laser there is no need for the sight "window" to be partially blocked by a semi-silvered or dielectric dichroic coating found in standard reflex sights. The optical window in a holographic weapon sight looks like a piece of clear glass with an illuminated reticle in the middle. The aiming reticle can be an infinitely small dot whose perceived size is given by the acuity of the eye. For someone with 20/20 vision, it is about 1 MoA.
One drawback of a holographic sight is shorter battery life when compared to reflex sights that use LEDs, such as red dot sights. The laser diode in a holographic sight uses more power and has more complex driving electronics than a standard LED of an equivalent brightness, reducing the amount of time a holographic sight can run on a single set of batteries.
- ↑ Red Dot Sights / Reflex Sights & Holosights Explained -Electronic Sights; A look at why they exist, how they work, and how you use them.
- ↑ “Compact Holographic Sight” - EOTech company Patent #5,483,362 issued January 9, 1996
- ↑ ar15.com " Parrallax on an Eotech?" - Tech support question - Parallax issues with 550 series- "The sights do have parallax error of +/- 1.2 " or +/- 0.6" (1.2 " side to side). The sight is designed to be parallax free at long distance 100yds to infinity. At close range, there will be a parallax error equaling to the width of the window which is 33mm or 1.3". A perfectly aligned sight will have parallax error of 1.3" at 10 yds and at 17 ft. As you move further away from 10 to 40 yards parallax becomes less and is almost zero at 50 yards."
- ↑ Jane's international defense review: IDR.: Volume 34, page 76