Night Vision Definitions Explained

Night Vision Definitions:
Do you want to know what all those specifications on your tube data sheet mean and more? Here is a list of the most important night vision terms related to your night vision system's performance and features.

Automatic Brightness Control(ABC)- An electronic feature that automatically reduces voltages to the microchannel plate to keep the image intensifier's brightness within optimal limits and protect the tube. The effect of this can be seen when rapidly changing from low-light to high-light conditions; the image gets brighter and then, after a momentary delay, suddenly dims to a constant level.

Black Spots - These are cosmetic blemishes in the image intensifier or can be dirt or debris between the lenses. Black spots that are in the image intensifier do not affect the performance or reliability of a night vision device and are inherent in the manufaxturing processes.

Blooming -  Momentary loss of the night vision image due to intensifier tube overloading by a bright light source. When such a bright light source comes into the night vision device's view, the entire night vision scene becomes much brighter, "whiting out" objects  within the field of view. Blooming is common in Generation 0 and 1 devices.

Bright-Source Protection(BSP) - An electronic function that reduces the voltage to the photocathode when the night vision device is exposed to bright light sources such as room lights or car lights. BSP protects the image tube from damage and enhances its life; however, it also has the effect of lowering resolution when functioning.

Diopter - The unit of measure used to define eye correction or the refractive power of a lens. Usually, adjustments to an optical eyepiece accomodate for differences in individual eyesight. Most systems provide a +2 to -6 diopter range.

Distortion - There are two types of distortion found in night vision systems. One type is caused by the design of the optics, or image intensifier tube, and is classical optical distortion. The other type is associated with manufacturing flaws in the fiber optics used in the image intensifier tube.

     Classical Optical Distortion - Classical optical distortion occurs when the design of the optics or image intensifier tube causes straight lines at the edge of the field of view to curve inward or outward. This curving of straight lines at the edge will cause a square grid pattern to start to look like a pin cushion or barrel. This distortion is the same for all systems with the same model number. Good optical design normally makes this distortion so low that the typical user will not see the curving of the lines.

     Fiber Optics Manufacturing Distortions - Two types of fiber optics distortions are most significant to night vision devices: S-distortion and shear distortion.

          S-Distortion - Results from the twisting operation in manufacturing fiber-optic inverters. Usually S-Distortion is very small and is difficult to detect with the unaided eye.

          Shear Distortion - Can occur in any image tube that uses fiber-optic bundles for the phosphor screen. It appears as a cleavage or dislocation in a straight line viewed in the image area, as though the line were "sheared".

Equivalent Background Illumination (EBI) - This is the amount of light you see through a night vision device when an image tube is turned on but no light is on the photocathode. EBI is affected by temperature; the warmer the night vision device, the brighter the background illumination. EBI is measure in lumens per square centimeter (lm/cm2). The lower the value the better. The EBI level determines the lowest light level at which an image can be detected. Below this light level, objects will be masked by the EBI.

Emission Point - A steady or fluctuating pinpoint of bright light in the image area that does not go away when all light is blocked from the objective lens.

Photocathode Sensitivity - Is the raw conversion efficiency of light into electrons at the photocathode surface, directly influencing the performance and usability of light detection sources.

Halo - a Halo appears as a circular glow or ring around bright light sources. The lower the halo value the better, typically one or less is considered a good halo value. This phenomenon occurs due to the way image intensifier tubes and optical components handle incoming light.

Resolution - is measured in line pair per millimeter(LP/MM). A higher resolution means sharper and more detailed images, which allows for better identification of distant objects. 64 lp/mm or better is considered good resolution for night vision devices.

Signal to Noise Ratio - The higher this value the better. The signal to noise measures the ratio of useful image signal to internal electronic noise. Higher values will not have as much static in the image or less grain. 24 and up is considered a good ratio for this vlaue.

Figure of Merit - Signal to Noise Ratio x Resolution is how to calculate the FOM(figure of merit) and is a quick way to determine tube performance. A value of 1200 or less is considered poor. 1400 to 1800 is standard quality while 1800+ is considered premium quality 3rd gen night vision.

For custom night vision systems to meet your application contact Aurora Tactical Group, LLC. at chase@auroratactical.com or call 479-751-8136. LE/MIL quotes available.