Figure of Merit (FOM) in Night Vision Devices - Embrace the Darkness

02/11/2021 by Raghav Jain


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In military applications, Night Vision Devices are widely used in search, rescue, and surveillance operations and have become crucial equipment for armed forces across the globe. Night vision devices are developed with the ability to provide a clear image at night without additional illuminators. These Devices are today useful to provide enormous benefits to the soldiers to step ahead in the nighttime combat missions.   
While choosing NVDs, few important performance parameters that need to be considered are:

  1. Figure of Merit,  
  2. Signal-to-Noise ratio,  
  3. Resolution

A figure of Merit (FOM):

FOM (figure of merit) is used to determine the overall performance of the image intensifier tube and is calculated as a product of the image intensifier’s resolution and a signal-to-noise ratio. The value of FOM is easily calculated for all types of tubes from known measured values, it is a quantity that is used for measuring the effectiveness of a device, and for characterizing the output reliability of empirical models, and determine the functioning of the night vision devices.

The FOM value is calculated in the following manner:

FOM ∝ Resolution (lp/mm)

Currently, the FOM value reaches a rate of 2100 by Gen II+, and Gen III devices. Tubes with a very high S/N value and high resolution (lp/mm) will result in the highest FOM value and thus, higher 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.

II 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. 

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.
Often the resolution in the same night vision device is very different when measured at the center of the image and the periphery of the image. 

The above is experimentally determined as described in the image shown below-

Thus, Higher resolution means higher detailing of images.

Benefits of NVD’s with Higher FOM:

  • It helps in providing a clearer image,  
  • In pitch dark conditions higher FOM makes target detection and recognition easily achievable. 
  • It also helps to differentiate between generations of II Tube. 

Based on the above study, it is important to consider FOM while selecting the device as per the user's requirements.  MKU’s technically advanced Night Vision Devices provide a greater advantage to the soldiers in pitch dark conditions through higher FOM in the devices, this game-changing technology has effectively transformed the battlefield of the 21st century and given its user a clear-cut advantage over their adversary.

Read the similar article: Which one is right for you - nvd or thermal device


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