Modulation transfer function (MTF) is considered a standard of optical system characterization. Effective MTF measurement allows designers and optical metrology engineers to ensure electro-optical systems perform as expected with high accuracy and resolution.
While there are a handful of methods available to measure the performance of optical systems, some are better and offer more actionable metrics than others. This is partly influenced by the equipment and technology used in the testing.
This article explores some of these effective MTF measurement methods that can help engineers and manufacturers keep up with the demand for high standards in optical systems. It will also touch upon an MTF test station by CI Systems that can be used for effective testing of infrared (IR) elements.
Why is Modulation Transfer Function (MTF) Measurement Essential?
MTF is a variant of optical transfer function (OTF) and is widely used as a benchmark test today to measure the performance of lenses and other electro-optical systems such as infrared (IR) imagers and virtual reality (VR) optics.
There are several reasons why MTF measurement is essential:
1. MTF is a quantitative measure of image quality
The best way to test out an optical system is to measure the quality of the image it records. It can be anything from as common as contact lenses used by individuals to imagers used in automotive industries. Out of all the technical parameters of such an optical system, its ability to transfer various levels of detail from an object to a recorded image within an expected range of resolution is considered the foremost. This ability is measured in terms of modulation, or in the context of images, contrast.
MTF as a measurement system considers the spatial frequencies of an image structure and presents simple, direct information. This is akin to audio frequency response, which is why optical systems are often measured for their modulation.
MTF measurement gives an easy picture of how well an optical system was built and how accurate it will be in its applications. This is more important in fields of science where high precision is the bare minimum requirement.
2. MTF measurement can be tailored for specific applications
With MTF measurement, engineers can correlate the resulting information with the corresponding attributes of the end application. For example, if a photography system has a specific range of pixel resolution, MTF can allow testing within that range. This will ensure that the optical system is tested and rated according to its specifications. There is no need to use a standard one-size-fits-all measurement technique. Although, it should be noted that MTF is a system-agnostic measurement standard.
A corollary to this advantage is that MTF also allows modeling concatenated systems i.e., systems that use two or more optical or optics-related equipment. The product of MTF measurement of each of those systems can be correlated with that of the system as one. This is beneficial for testing complex systems that are made of different types of optical test devices such as collimators and integrating spheres.
3. Lens MTF measurement is possible in real-time
Another advantage of using the MTF testing standard is that it can be applied directly in an application environment. The MTF testing environment can be customized according to the nature of the equipment and setup of the entire system. This allows in-process optics testing regardless of the type of optical system that needs testing.
MTF measurement also acts as a universal testing method for optical systems. Using the MTF system prevents the influence of any kind of human error or other external forces in the designs or their testing. Lastly, MTF measurement offers additional parameters of image characterization for engineers to work on. Field curvature is just one such element that can be gauged using an MTF test system.
All of this makes the MTF measurement standard an indispensable part of any optical system management. Investing in methods or test stations that apply MTF measurement in real time can thus be considered a wise use of one’s capital, time, and efforts.
How to Measure MTF?
Before getting down to the different methods of MTF measurement, here’s a quick consideration of MTF as a tool. With resolution and contrast as its two prime components, MTF offers an understanding of what happens when a lens or an optical system is unable to transfer the expected levels of details from an object to an image. In other words, the decrease or increase in MTF as a function of modulation or contrast versus resolution – which is called the MTF curve – is the critical takeaway here.
The key to measuring MTF is to know that image quality is measured in terms of contrast. As a principle, MTF can only be applied to systems where an input results in an output or response. Camera sensors, IR imagers, and seismometers are a few examples of such a system.
There are four to five major methodologies of MTF measurement, namely frequency, slit scanning, video capture, and interferometry. Slanted-edge testing is the fifth method. The methods available today at the disposal of testing engineers are all variants of these techniques, thanks to advances in optical technologies. Here’s a quick look at these MTF measurement methodologies.
1. Using Scanning Methods
Scanning utilizes the linear system theory, in which an image captured by the test lens (with a known input) is studied to derive MTF parameters. Determining the spatial frequency of the image captured by the test lens will give the spatial profile, which in turn will help calculate the spread function. Executing Fourier transform of this derived function will help determine the quality of the optical system under test.
The peculiarity of scanning methods is in its choice of a tiny light source like a small pinhole. Any optical system will only capture a blur of this light source, helping engineers detect the variation in the image’s spatial profile.
It’s the most cost-effective yet time-consuming way of measuring optical systems. The OptiShop by CI Systems is capable of testing using the scanning methods.
2. Using Video Capture
Video capture MTF testing is the most thorough and versatile method of MTF measurement as it allows testing at higher throughput rates and can test a wide range of optical systems. It works on the linear system theory similar to the scanning methods with the exception that a solid-state array is placed at the focal plane of the lens under test instead of using a tiny light source.
Video MTF measurement is faster than scanning and has the following benefits:
- Continuous feedback loop function in an application.
- Real-time monitoring of any optical system.
- Sagittal and tangential MTF measurement in tandem.
- MTF measurement of low-resolution optics.
OptiShop can also handle video capture more effectively as it is designed for a wide range of IR Imagers and other cameras. The video method is known to give much richer measurement data including additional parameters such as distortion and field curvature, which makes OptiShop a preferred test station for large organizations that handle different types of optical systems.
Here’s a quick questionnaire to further break down the use cases of OptiShop as an effective MTF measurement tool. Engineers can use this questionnaire to pinpoint the type of information they hope to capture using MTF testing.
MTF Station - OptiShop by CI Systems
3. Using Frequency Generation
Frequency generation is the simplest method in this list as it deals with a single spatial frequency. Manual measurement of contrast of the captured image is noted and plotted in a graph over a range of spatial frequencies. The variance in image contrast and source frequencies gives the required MTF information to ascertain the quality of the lens under test.
While it is the simplest, the frequency generation method is highly prone to human errors. Another disadvantage is the need for simultaneous manipulation of sources and detectors to vary the frequencies and contrast, which may not be viable for large optical systems. Furthermore, calibration of the test equipment, including of instruments such as black bodies and other sources also poses a hurdle in this method. It is no longer used as a defining MTF measurement method and is always supported by iterations or cross-checking using other methods.
4. Using Interferometry
In this lens MTF measurement method, the pupil function of the lens under test is auto-correlated or its point spread function is calculated by Fourier transforming the pupil wavefront. The advantage of this method is that it can be linearly tested with certain applications that operate in limited wavelengths. Unlike video capture, interferometry is not capable of handling systems with wide ranges.
5. Using Slanted-Edge Method
This is a relatively newer model and is applicable to closed test environments due to its aversion to noise. The method analyzes the image of a slanted knife-edge target, which can be one-dimensional or multi-dimensional as per the requirements.
Tips to Use MTF Measurement Effectively
While applying one of these MTF measurement methods is essential to studying the performance of a lens, additional parameters can be recorded and analyzed to get more information on the quality.
For instance, an MTF chart can be used to derive the Bokeh effect, which is used to measure the quality of the out-of-focus areas of an image that a lens produces. The Bokeh effect is influenced primarily by the quality of the lens materials and the number of aperture blades. According to various studies on lenses and their quality, more blades promise a higher Bokeh.
MTF is also a great standard to use to measure two or more identical optical systems. Even though a manufacturer might guarantee identical composition in its systems, vendors and engineers can still use MTF charts to confirm whether the systems are indeed identical. This is widely used in ophthalmic measurements as that field of medicine requires high-precision systems.
MTF measurement can also be customized according to the type and size of the lens under test. For example, in 2010, a team of physicists developed a flexible MTF measurement system to measure micro lenses. The team used a “conventional optical microscope with an optimized approach for lens illumination”, after which the measurement data was correlated with that of a commercial MTF measurement system.
Conclusion
MTF measurement for lenses and other optical systems is an important testing method to gauge the quality of a system’s ability to record an image. The data given out by tools like the OptiShop is instrumental in ensuring quality control, especially for demanding systems where high accuracy and resolution are the norm. MTF measurement finds routine application in ophthalmology, advanced photography, and photo-lithography.
The fact that such MTF test stations can be operated by anyone without a high level of optical training is an added advantage. The tool can be plugged into any in-progress system for quick testing, which can then command a course correction if required.
MTF is also not restricted by wavelength ranges. There is no need for adjustments in measuring parameters either as MTF works as a one-size-fits-all testing device for any and all types of optical systems.
Want to learn more about OptiShop as a cost-efficient optical test unit? Contact CI Systems