@@ The ND3 Laser Designator is a precision optical lighting instrument using advanced green laser technology. The patented Rotary Optical Collimator allows full adjustment and control of beam diameter and intensity to focus light where you need it most. Rotating the collimator ring to minimum creates a bright micro beam of green laser light visible up to 3 miles. Rotating the collimator to more open settings provides plenty of illumination to light a trail at night or paint a target at up to 250 yards. Battery life allows you to handle any emergency with over 7 hours of contniuous use. Why Green Light? @@ Green light is the most visible to the human eye and requires the least amount of eye adjustment at night. An eye that has adapted to daytime conditions generally has a maximum sensitivity around 555nm. Compared to the color red which optical spectrum is around 630 - 650nm our eye is closer to the Green region of the optical spectrum at 510 - 530nm.