UNLOCKING THE DARK: NIGHT VISION DEVICES AND CRUCIAL PERFORMANCE METRICS

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Categories: Night Vision

Night vision devices offer unique advantages in the absence of light. Understanding the intricacies of these devices is paramount, and key performance parameters, such as the Figure of Merit (FOM), Signal-to-Noise Ratio (SNR), resolution, sensitivity, and spectral response, illuminate the path to enhanced night vision.

Figure of Merit

FOM is an abstract measure of image tube performance, derived from the number of line pairs per millimeter (resolution) multiplied by the tube’s signal-to-noise ratio. Tubes with a very high signal to noise value and high resolution (lp/mm) will result in the highest FOM value and thus, higher the performance of the image intensifier tubes and devices.

Signal-to-noise ratio (SNR)

The SNR determines the capability of night vision devices during the low light condition as it defines the Image Intensifier output brightness divided by the root mean square of the variations in output brightness and is usually measured at a light level of 108 μlx. The signal-to-noise ratio (SNR) is the key parameter to predict the performance of an II tube. Hence higher SNR means objects can be seen with better contrast and enhances the ability of the tube, under low illumination conditions. The systematic arrangement of SNR is shown below.

Image Intensifier Tube’s SNR determines the ability to amplify low light signals during dark conditions. Hence, the higher is the figure of SNR, the less noise/more signal you get in low-light environments. 

Night Vision Devices Resolution

Resolution of Night Vision Devices signifies details the human eye is able to pick in the image when seen through the device. It is more accurately known as limiting resolution, tube resolution is measured in line pairs per millimeter or lp/mm. The higher value of resolution means that the device can generate recognizable images of smaller targets separated by smaller distances Thus, resulting in better quality images.

Night Vision Devices Sensitivity 

Photocathode sensitivity is a measure of how well the image intensifier tube converts light into an electronic signal so it can be amplified. The measuring units of photocathode sensitivity are micro- amps/lumen (µA/lm) or microamperes per lumen.

Photocathode sensitivity is a measure of how well the image intensifier tube converts light into an electronic signal so it can be amplified. The measuring units of photocathode sensitivity are micro-amps/lumen (µA/lm) or microamperes per lumen. This criterion specifies the number of electrons released by the Photocathode (PC). PC response is always measured in isolation with no amplification stage or ion barrier (film). Therefore, tube data sheets (which always carry this “raw” figure) do not reflect the fact that over 50% of those electrons are lost in the ion barrier. While for most latest 3rd generation image intensifiers the photo response is in the 1800 µA/lm (2000 µA/lm, the actual number is more like 900 µA/lm.

Night Vision Devices Spectral response

The diagram below shows the spectral response curves for different types of photocathods.

NIGHT VISION DEVICES LIMITATIONS

  • The equipment requires some night light (moonlight, starlight, etc.) to operate. The level of performance depends upon the level of light.
  • Night light is reduced by passing through the clouds, while operating under trees, under the shadows of buildings etc.
  • Under starlight conditions low contrast environments (such as snow-covered territory, sandy deserts, large bodies of water or grassy hills) degrade visibility thereby disguising or masking changes in terrain.
  • Under too low-light conditions the goggles will lose some of the resolution that it has under full moon.
  • The equipment is less effective for looking into shadows and other darkened areas.
  • The equipment is less effective through rain, fog, sleet, snow, or smoke.
  • The equipment will not “see” through dense smoke.
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