http://www.trammellphoto.com/dichroic-filter/
How Reflex Sights Work
The optical collimator makes the reflex sight possible. Collimators align rays of light from a source in parallel. Refractive and reflective collimators are employed. The collimator creates an image of a dot or reticle illuminated by an LED, radioactive or fiber optic light source and projects it to infinity. The image is projected onto a specially coated dichroic mirror or beam splitter which lets all other wavelengths pass to let the firer see the target, but reflects the colour employed by the reticle. This results in a perfect image of the target with the aiming mark projected to infinity.
Holographic reflex sights use a laser hologram to project the image of the reticle into the target plane. This results in an aiming mark that is optically centred in its entirety. This avoids parallax problems caused by variations in eye position relative to the aiming window. It also means partial obscuration of the aiming window will not prevent the shooter seeing the reticle in its entirety. The main disadvantage of the holographic sight is its laser diode’s greater power consumption than an LED – battery lives being measured in hundreds rather than tens of thousands of hours.
The location of the collimator dictates the configuration of the sight. If it is mounted underneath you can have a sight that is less bulky, with the sighting screen taking the form of a small head up display. This can put the sight line high above the bore – a perfect configuration with M16s or other rifles or support weapons with an inline configuration, but less advantageous with others. These configuration can also be less robust. Side mounting makes tube-construction possible. Tubes make for extremely rugged sights and are inherently ambidextrous as you can simply rotate the sight 90 degrees in its rings to switch the adjusters from azimuth to elevation.
Most reflex sights are non magnifying as this is much better for both-eyes open operation and fast shooting. There is a trade off in reticle size, the larger the aiming mark the faster it can be acquired, but the more it obscures the target making long range accuracy problematic. However, magnifying red dot sights exist which take advantage of reflex operation to combine a large illuminated aiming point for fast target engagement with a traditional ballistically calibrated ladder reticle for long range precision. A bright illuminated aiming mark is used for close ranged shooting, backed by a traditional reticle for longer ranged engagements. This philosophy is exemplified by the Trijicon ACOG, adopted as standard across the US armed forces. 1X magnification sights can be turned into low powered scopes by the simple addition of a detachable or flip to side magnifier. Trijicon have gone the opposite route by attaching peep battlesights and/or small red dot sights to the top of their most recent ACOGs.
Most sights use internal adjusters, the sight housing remaining firmly mounted and the optics inside moving to move the point of impact. The adjustments are calibrated and usually take the form of audible or tactile clicks. The calibrations vary from manufacturer to manufacturer and model to model. The fly fishing tackle (the most recent incarnation of the US Army M68) for example employs clicks that are 16mm at 100 meters or 1/2″ at 80 yards whilst the USMC’s AN/PVQ-31B / TA31RCO has clicks that move the point of impact 0.33″ at 100 yards.
Amost all red dots sights feature variable brightness levels for the reticle. Some have night vision compatible ranges of settings. Adjustment is accomplished automatically in some models, either electronically or by the amount of light the fiber-optic element can pull in from the environment. The latter system can cause problems when shooting from bunkers or rooms that are not illuminated into brightly lit terrain. The brightness of the image can be varied in some models by rotating a polarised filter against another one – this results in anything from full brightness to total black-out. As the sight itself often incorporates a polarising filter, mounting a single adjustable polarising filter can have the same effect. This could be a problem with polarised glasses, but the leading makes all now select polarisations that do not conflict with sunglasses.
It has become commonplace for shooters to ‘co-witness’ their iron sights with their non magnifying reflex sight. It is unnecessary to align the red dot to sit on top of the foresight or anywhere else – it only matters that both systems are zeroed on the target. A popular method is to have the iron sights in the bottom third of the sight picture and manufacturers like Eotech are starting to make sights with optional risers to enable this; for example their 557.AR223.
Copyright Chris Pieterman 2010
UPDATE: Canon 5D Mark II / 50D Fluorine sensor filter cleaning
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Tech Lighting 140DICBLU Blue Contemporary / Modern Dichroic Lens Filter Tech Lighting Dichroic Lens FilterDichroic lenses filter for a pure effect. Requires a lens holder or an accessory that accepts lenses or louvers.Features:ETL ListedSpecifications:Height: 2″Length: 0.1″Width: 0.1″Weight: 0.05… |
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Tech Lighting 140DICYEL Yellow Contemporary / Modern Dichroic Lens Filter Tech Lighting Dichroic Lens FilterDichroic lenses filter for a pure effect. Requires a lens holder or an accessory that accepts lenses or louvers.Features:ETL ListedSpecifications:Height: 4″Length: 0.1″Width: 0.1″Weight: 0.05… |
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Tech Lighting 140DICRED Red Contemporary / Modern Dichroic Lens Filter Tech Lighting Dichroic Lens FilterDichroic lenses filter for a pure effect. Requires a lens holder or an accessory that accepts lenses or louvers.Features:ETL ListedSpecifications:Height: 1.4″Length: 0.1″Width: 0.1″Weight: 0.05… |
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Flood Light Bulb with Dichroic Filter in Blue $10.89 BO-23-BLUE Features: -Flood light.-For all ages. Construction: -Glass and steel construction. Color/Finish: -Blue color. Specifications: -50W MR-16 bulb. Dimensions: -Overall dimensions: 2” H x 2” W…. |
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Flood Light Bulb with Dichroic Filter in Green $10.89 BO-23-GRN Features: -Flood light.-For all ages. Construction: -Glass and steel construction. Color/Finish: -Green color. Specifications: -50W MR-16 bulb. Dimensions: -Overall dimensions: 2” H x 2” W…. |
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Flood Light Bulb with Dichroic Filter in Red $10.89 BO-23-RED Features: -Flood light.-For all ages. Construction: -Glass and steel construction. Color/Finish: -Red color. Specifications: -50W MR-16 bulb. Dimensions: -Overall dimensions: 2” H x 2” W…. |
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Smith Victor 401150 Camera-Mounting Video Light SV950 Professional DC $179.95 Smith Victors professional on-camera video light is perfect for lighting interview subjects. The light housing is constructed of lightweight aluminum-fiber material. Both the built-in diffused soft-light filter and dichroic daylight filter rotate independently for optimal light control. Control color temperature from 3200°K to 5600°K by simply rotating dichroic filter. 4-leaf barndoors can be re… |
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Lowel Blue Dichroic Filter with Holder for the Pro and i-Light System. $40.97 Converts 3200° K light to average daylight. Light loss approximately one stop…. |
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Dichroic Glass Filters are 2-1/2 x 2 x 1/8 Thick Dichroic Filter These aren’t take-outs, they’re nevwis (Never Went Ins) from a projector manufacturer. The dichroic glass filters are 2-1/2″ x 2″ x 1/8″ thick. The reflective surfaces morph from bright blue to pink to, say, pale green, depending on the light you’re looking at. But then that’s what dichroic filters do. Made in the USA…. |