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Circuit controls intensity of reflex optical sights

-February 21, 2002

A popular category of aiming/pointing aids is the reflex, or "red-dot," sight. This system finds use in such diverse applications as astronomy, archery, and shooting. In the reflex sight, light from an internal source—typically a high-intensity red LED—reflects from a curved, transparent optical (reflex) element through which you view the target. The result of this geometry is that the image of the LED (the red dot) appears superimposed on the target image, thus indicating the point of aim. When you correctly adjust the aiming point of the telescope, bow, or gun, the target and LED images coincide. The reflex sight offers several advantages over competing pointing technologies, such as telescopic and open sights. These benefits include rapid and intuitive target acquisition, noncritical eye positioning, and a wide field of view.

For best sight performance, the intensity of the red-dot light source must at least roughly match the illumination level of the target. Otherwise, if the source is too dim, the aim-point dot loses itself in the brightness of the target. If too bright, the dot flares, and its apparent size increases, obscuring the point of aim and making precise pointing difficult or impossible. For this reason, most reflex sights require manual adjustment of the source intensity. Although this adjustment is effective enough, the time and attention needed to optimize intensity with a manual control detracts from the fast and intuitive target-acquisition capabilities of the red dot. The circuit in Figure 1 uses phototransistor Q1 to sense target brightness and automatically adjust the LED output. The circuit maintains near-constant dot size over a wide range of ambient-light levels.

Potentiometer R1 divides Q1's photocurrent, IP, between the LED driver, Q2, and the bias transistor, Q3 (connected as a diode). The adjustment of R1 therefore determines the ratio between drive current, IL, and ambient (target) intensity over the range of 1 to B, where B is the beta of Q2 (greater than 100). The prototype of the intensity-control circuit was packaged in a small plastic enclosure attached to the side of a Compasseco Inc (www.compasseco.com) Tech Force model 90 30-mm objective reflex sight. The light shield mimics the field of view of the sight, so the light that Q1 samples represents the target intensity visible through the sight. Proper adjustment of R1 results in good compensation of dot intensity for a wide range of both incandescent and natural light. The circuit effectively maintains a constant angular dot diameter of 4 minutes of arc under outdoor ambient lighting ranging from dark overcast to full sunlight. The circuit also delivers similar performance under indoor incandescent-lighting conditions. Compensation with fluorescent lamps, however, is less satisfactory because of the absence of an adequate near-infrared component in the spectrum of these light sources. You could probably fix this shortcoming by using a suitable visible-light filter in front of Q1.

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