Key Components related to MTF Testing

Does your company need MTF testing? Before determining that, it’s important to know the main components of modulation transfer function (MTF). MTF measures the transfer of modulation (contrast) from a subject to an image. To put it another way, it measures how well the lens transfers details from an object to the images that the lens projects. Here are the main parts:

1. Contrast/Modulation

You can normalize how intense a bar target is by giving a max value to the white bars, as well as a value of zero to the black bars. Plotting out these values produces the result of a square wave. The idea of contrast is then visible.

When this idea is used in an example involving imaging, the intensity pattern that takes place before and action image is visible. Contrast/modulation then can be defined as how well the minimum/maximum intensity values are moved from the object plane to the image plane.

In various imaging applications the camera sensor, imaging lenses, and illumination are important in definite the image contrast that results. The lens contrast is usually defined regarding the object contrast’s percentage that’s produced. The ability of the sensor to reproduce contrast is usually defined using decibels (dB) in old-school cameras, and data bits in digital cameras.

2. Resolution

This is the ability of the imaging system to pick up the object’s details. IT’s use it in different ways including line-pairs per millimeter. This is also referred to as “frequency.” The opposite of the frequency results the spacing measured in millimeters between 2 lines.

Bar targets with a group of alternative black/white bars are good for testing the performance of systems. One practical way of understanding line-pairs is to imagine them as pixels contained on a camera sensor. One line-pair links to 2 pixels.

A pair of camera sensor pixels is required for every line-pair of the solution. One of the pixels is used for the red line, and the other one is used for the black space in-between the pixels. The camera’s image resolution can then be defined as equal to two times its pixel size. 

How about the MTF itself? A lens’ MTF measures how well the lens can transfer contrast at a certain resolution. That’s from an object to an image. To put it another way, MTF uses resolution and contrast in one specification. As line spacing drops, it becomes more difficult for lenses to transfer the decrease effectively in contrast. This causes MTF to decrease.

Why is MTF important? In traditional systems integration of the performance of the system is estimated using the weakest link’s principle. The weakest link proposes that the resolution of a system is limited only by the component that has the lowest resolution.

This works for fast estimates. However, it isn’t accurate because the component in the system causes an error to the image. This results in worse image quality than the weakest link itself.  

All of this information is critical to understand how MTF testing is done.