Source size measurement and calculation of visual angle

Abstract

Military and law enforcement seek to assess different light technologies and devices for potential operational use. Because small source lasers are bound by the point-source maximum permissible exposure (MPE), they have limited visual effectiveness at range. By introducing a diffusing optical element, an extended source’s image size increases with approaching distance, and because the image progressively increases on the retina, the MPE increases as well. A higher MPE at the aperture permits the use of a more powerful laser and therefore greater fluence downrange. The visual angle subtended by an extended source can be measured by imaging the output of the laser near the focal length of a lens located in the beam in place of the observer. An image is formed near the focal length of the lens. The image of the laser source subtends an angle equal to that of the visual angle, calculated by dividing the image diameter by the distance from the focusing lens to the image plane.Military and law enforcement seek to assess different light technologies and devices for potential operational use. Because small source lasers are bound by the point-source maximum permissible exposure (MPE), they have limited visual effectiveness at range. By introducing a diffusing optical element, an extended source’s image size increases with approaching distance, and because the image progressively increases on the retina, the MPE increases as well. A higher MPE at the aperture permits the use of a more powerful laser and therefore greater fluence downrange. The visual angle subtended by an extended source can be measured by imaging the output of the laser near the focal length of a lens located in the beam in place of the observer. An image is formed near the focal length of the lens. The image of the laser source subtends an angle equal to that of the visual angle, calculated by dividing the image diameter by the distance from the focusing lens to the image plane.