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The Basics of Testing Modulation Transfer Function (MTF) in Electro-Optical Systems

19 October, 2017

The theory behind modulation transfer function and how it relates to electro-optical systems is already well described in all the relevant textbooks. Being that it is broadly speaking the measure of the ability of an optic system to transfer different levels of detail from real-life object to on-screen image, it is vital that this quality is thoroughly examined before any electro-optical system is permitted to be used in the field. Be sure to learn as much as you can about this kind of testing and you’ll understand its importance in military avionics.

The Crucial Nature of This Function

You’ll often find modulation transfer function testing simply referred to as an MTF test. It is widely considered to be a very useful optical function to test as modulation transfer is what is partially responsible for the generation of sharp, detailed images often seen and acted upon in military avionics.

It can be used to ascertain the quality of many aspects of a given electro-optical system, including all sorts of different lenses from the simplest you can imagine all the way up to multi-element lens assemblies. Ultimately, making sure that equipment passes an MTF Test is what allows us to benefit from a single specification that includes high resolution at the same time as high contrast.

Typically, each of the following components has associated transfer functions that need to be checked using simulation and testing boards.

  • Lens
  • Camera sensor
  • Image capture boards
  • Video cables

How MTF Relates to Other Functions

Transfer functions in general exist when a response is related to a stimulus, or in other words when an output is generated as a result of a particular input. As such, these functions are commonly found in fields such as seismology and, of course, military avionics.

An overall function known as optical transfer function is what gives us a description of how optical systems like infrared cameras respond to a range of pre-defined stimuli. This function is itself composed of two parts, where MTF corresponds to its magnitude and PTF describes the phase components.

Methods and Instruments to Use

There are several ways in which you can test the function of your infrared optical system to check the contrast and resolution produced. Ultimately there are four main methods you need to be aware of.

  • Discrete or continuous frequency generation
  • Image scanning
  • Video-based methods
  • Interferometric approaches

The accurate measurement of this vital parameter as part of a thorough quality control test means that your equipment will be able to fulfil its intended functions in full.

Building an Efficient Approach

When it comes to testing approaches, one of the most important considerations is standardization. You want each stage of the process to be repeatable in a reliable fashion that corresponds to real applications. Simulation protocols can be developed with the help of specially designed testing boards that utilize the principles of black body radiation to boost the reliability of your tests.

When testing equipment that captures infrared and near-visible radiation, it’s incredibly difficult to use sources in these wavelengths as a calibration standard that can be reliably used. The main reason is because they cannot be spatially extended and radiating uniformly in all direction within a given hemisphere. However, integrating spheres are built perfectly for this function.

Typically, integrating spheres are constructed in such a manner as to allow the use of various filters to limit certain wavelengths at particular times. The end result is that you can examine the qualities of modulation transfer function under a variety of conditions subsequently applicable to real-life scenarios.

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