Scout Master

Night vision equipment work in the near-infrared band at a wavelength of about 1 micrometer. Unlike thermal imaging systems, which operate in complete darkness by detecting heat radiation signatures in infrared wavelengths beyond 3 micrometers, night vision works in near darkness by detecting ordinary ambient light, usually from the moon and stars, that is reflected by objects in the scene being viewed. Night vision contains an image intensifier tube that uses the photoelectric effect to amplify very weak light. As each photon of incoming light collides with a detector plate inside the intensifier tube, the plate ejects several electrons that are further amplified into a cascade of electrons. These electrons are accelerated by a strong electric field towards a phosphor screen which emits light at the point of impact of the electrons. A bright image is thus formed on the phosphor screen. Outdoor environments that are illuminated only by star light can be easily viewed using night vision devices.

Most night vision devices do not detect color information, and hence a monochromatic phosphor screen is sufficient. A green phosphor display is generally used because the human eye is most sensitive to the color green, which falls in the middle of the visible light spectrum.

Night glasses are telescopes or binoculars with a large diameter objective. Large lenses can gather and concentrate light, thus intensifying light with purely optical means and enabling the user to see better in the dark than with naked eye alone. Often night glasses also have a fairly large exit pupil of 7 mm or more to let all gathered light into the user’s eye.

However, many people can’t take advantage of this because of the limited dilation of the human pupil. To overcome this, soldiers were sometimes issued atropine eye drops to dilate pupils. Before the introduction of image intensifiers, night glasses were the only method of night vision, and thus were widely utilized, especially at sea. Second World War era night glasses usually had a lens diameter of 56 mm or more with magnification of seven or eight. Major drawbacks of night glasses are their large size and weight.

The most popular method of achieving night vision is through the use of image intensifiers. Image intensifiers are commonly used in night vision goggles, night scopes and cameras. So, how do they work? They amplify the available light to achieve better vision. An objective lens focuses available light (photons) on the photocathode of an image intensifier. The light energy then causes electrons to be released from the cathode, which are accelerated by an electric field to increase their speed. The electrons enter holes in a microchannel plate and bounce off the internal specially-coated wall, which generate more electrons as the electrons bounce through, creating a dense “Cloud” of electrons representing an intensified version of the original image. Then, the energy of the electrons makes a phosphor screen glow. The visual light shows the desired view to the user. The reason a green phosphor is used in these applications is because the human eye can differentiate more shades of green than any other color.

Some advantages of Image Intensifiers:

• Excellent low-light sensitivity

• Enhanced visible imaging yields the best possible recognition and identification performance.

• High resolution

• Low power and cost

• Ability to identify people