Low-Light Imaging

Today, the most popular and well known method of performing night vision is based on the use of image intensifiers. Image intensifiers are commonly used in night vision goggles and night scopes. More recently, on-chip gain multiplication CCD cameras have become popularized for performing low-light security, surveillance and astronomical observation.

Image Intensifiers

HOW THEY WORK: This method of night vision amplifies the available light to achieve better vision. An objective lens focuses available light (photons) on the photocathode of an image intensifier. The light energy causes electrons to be released from the cathode which are accelerated by an electric field to increase their speed (energy level). These electrons enter holes in a microchannel plate and bounce off the internal specially-coated walls which generate more electrons as the electrons bounce through. This creates a denser "cloud" of electrons representing an intensified version of the original image.

All image intensifiers operate in the above fashion. Technological differences over the past 40 years have resulted in substantial improvement to the performance of these devices. The different paradigms of technology have been commonly identified by distinct generations of image intensifiers. Intensified camera systems usually incorporate an image intensifier to create a brighter image of the low-light scene which is then viewed by a traditional camera.

HOW THEY WORK: In order to overcome some of the disadvantages of image intensifiers, CCD image detector manufacturers have substantially improved the sensitivity of certain CCD detectors by incorporating an on-chip multiplication gain technology to multiply photon-generated charge above the detector's noise levels. The multiplication gain takes place after photons have been detected in the device's active area but before one of the detector's primary noise sources (e.g. readout noise). In a new multiplication register, electrons are accelerated from pixel-to-pixel by applying high CCD clock voltages. As a result, secondary electrons are generated via an impact-ionization process. Gain can be controlled by varying the clock voltages.

Different from low-light imaging methods of night vision (which require some ambient light in order to produce an image), thermal imaging night vision methods do not require any ambient light at all. They operate on the principal that all objects emit infrared energy as a function of their temperature. In general, the hotter an object is, the more radiation it emits. A thermal imager is a product that collects the infrared radiation from objects in the scene and creates an electronic image. Since they do not rely on reflected ambient light, thermal imagers are entirely ambient light-level independent. In addition, they also are able to penetrate obscurants such as smoke, fog and haze. There are two types of thermal imaging detectors: cooled and uncooled. Cooled detector infrared cameras require cryogenic cooling to very cold temperatures (below 200K). Uncooled detector infrared cameras are normally either temperature stabilized (at room temperatures) or entirely unstabilized.

Thermal images are normally black and white in nature, where black objects are cold and white objects are hot. Some thermal cameras show images in color. This false color is an excellent way of better distinguishing between objects at different temperatures. 

Cooled-detector Infrared Cameras

HOW THEY WORK: Cooled infrared detectors are typically housed in a vacuum-sealed case and cryogenically cooled. The detector designs are similar to other more common imaging detectors and use semiconductor materials. However, it is the effect of absorbed infrared energy that causes changes to detector carrier concentrations which in turn affect the detector's electrical properties. Cooling the detectors (typically to temperatures below 110K, a value much lower than the temperature of objects being detected) greatly increases their sensitivity. Without cooling, the detectors would be flooded by their own self-radiation.

Materials used for infrared detection include a wide range of narrow gap semiconductor devices, where mercury cadmium telluride (HgCdTe) and indium antimonide (InSb) are the most common.

Uncooled-detector Cameras

HOW THEY WORK: Unlike the cryogenically cooled detectors described above, uncooled infrared detectors operate at or near room temperature rather than being cooled to extremely low temperatures by bulky and expensive cryogenic coolers. When infrared radiation from night-time scenes are focused onto uncooled detectors, the heat absorbed causes changes to the electrical properties of the detector material. These changes are then compared to baseline values and a thermal image is created. Despite lower image quality than cooled detectors, uncooled detector technology makes infrared cameras smaller and less costly and opens many viable commercial applications.

Uncooled thermal imaging camera (portable)

A popular and sometimes inexpensive method for performing night vision is by near infrared illumination. In this method, a device that is sensitive to invisible near infrared radiation is used in conjunction with an infrared illuminator. The Sony Night Shot camcorder popularized this method. Because of the IR sensitivity of the camcorder's CCD detector and since Sony installed an infrared light source in the camcorder, infrared illumination was available to augment otherwise low-light video scenes and produce reasonable image quality in low-light situations.

The method of near-infrared illumination has been used in a variety of night vision applications including perimeter protection where, by integrating with video motion detection and intelligent scene analysis devices, a reliable low-light video security system can be developed.

IR Illumination

• Filtered incandescent lamps: A standard high power lamp that is covered by an infrared filter designed to pass the lamp's near infrared radiation and block the visible light component. These devices typically need good heat transfer properties since the intense visible light is internally absorbed and dissipated as heat.
• LED type illuminators: These illuminators utilize an array of standard infrared emitting LEDs.
• Laser type: The most efficient infrared illuminator, these devices are based on an infrared laser diode that emits near infrared energy.