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Night Vision

 
An image intensifier fits between the lens and camera body on a digital SLR. Courtesy of Electro
physics.com
 
 
A microchannel plate using a scanning electron microscope. Photo courtesy of NTMDT Co at www.ntmdt.ru.
 
 
A cluster of monarch butterflies hanging from a eucalyptus tree in their winter gathering place at night captured with an Electrophysics Astroscope on a Canon camera. Courtesy of Electro
physics.com
 
 
Night vision trail cameras such as this Trail Scout from Bushnell are used by both hunters and naturalists. They are triggered by movement and use an infrared flash to capture photos in the dark. Courtesy of bushnell.com

Night vision refers to the ability to see in low light or total darkness. This is made possible by a variety of devices and technologies. Some of the technologies are low cost and available to consumers. Others are very expensive and used primarily by law enforcement, the military, naturalists and scientists. The technologies in use include amplifying existing light, capturing thermal radiation, and capturing infrared light.

Amplifying Existing Light

Images can be captured in low light, but not total darkness, using an image intensifier, a device also found in night vision goggles and night scopes. In these devices a lens focuses the dim light from stars, the moon, or other sources onto the photocathode of the image intensifier. This converts the light's photons into electrons which are then accelerated by an electric field to increase their energy level. The electrons then pass through holes in a microchannel plate where they bounce off specially coated walls to generate thousands more electrons. When these electrons hit a green phosphor screen they change back to photons recreating an image of the original scene. It's this image that is then captured by the digital camera. Green phosphor is used because the human eye can differentiate more shades of green than any other color, allowing for greater differentiation of objects in the picture.

In fog or heavy rain you may find that too much reflected light degrades the systems performance. In addition images often have a faint hexagonal pattern, caused by the microchannel plate and black spots throughout the image area.

As improvements in image intensifiers have been made over the years and are referred to as generations (also known as "Gen"). The improvements have been in the ability to capture images at lower light levels and the reduction of distortion. Generation 3, currently providing the clearest and sharpest images, use a photocathode coated with sensitive gallium arsenide which is more efficient at converting light to electrical energy at extremely low levels of light.

A boy walking his pet (rabbit) at night captured with an Electrophysics Astroscope on a Canon camera. Courtesy of Electrophysics.com

Instead of using an image intensifier, there are cameras that use CCD detectors that boost the signal on a chip—called on-chip multiplication gain cameras. This is similar to increasing the ISO on your camera but it boosts the captured signal before it reaches one of the detector's primary noise sources—readout noise. However, since temperature affects noise, these devices work better when their temperature is stabilized at or below room temperature.

Thermal Imaging

Thermal imaging is the only technology that can take pictures in total darkness. Instead of light, these cameras capture infrared energy emitted in proportion to a subject's temperature. If you have ever watched cop shows on TV, you may have seen one of these cameras in a helicopter letting the pilot guide police on the ground to a fleeing subject in total darkness. These devices are also able to penetrate smoke, fog and haze.

Thermal images are normally black and white with cold subjects shown in black and warm or hot items in white. Some thermal cameras capture images in false colors that help you better distinguish objects at different temperatures.

For improved performance some devices using this technology house the detector in a vacuum-sealed case so it can be cooled to a low temperature. This adds to their size, weight, purchase price and operating cost.

Near Infrared Illumination

The only low cost night vision system uses an infrared illuminator to light the scene and a device sensitive to infrared to capture a picture. This approach, used by Sony in their Night Shot cameras and in many trail/deer cameras can capture images in low-light situations. They can also capture images through night-time fog, mist, rain and snowfall as well as windows. Illuminators are generally standard high power lamps covered with an infrared filter that passes the lamp's near infrared radiation and blocks the visible light component. Other illuminators use infrared emitting LEDs or infrared laser diodes.

Vegetation on the edge of a pond in Santa Barbara, California captured with an Electrophysics Astroscope on a Canon camera. Courtesy of Electrophysics.com


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