Meaning
Performance metrics quantify the ability of an imaging system to transfer contrast from an object to an image at various spatial frequencies. The modulation transfer function provides a complete description of the resolution limits and the image quality of a detector or an optical lens. It is expressed as a ratio of the output contrast to the input contrast, ranging from one at low frequencies to zero at the resolution limit.
This curve allows engineers to compare different hardware configurations and predict the visibility of small features in a final scan.
Contrast Resolution
Large features are usually transferred with high fidelity, while the contrast drops as the features become smaller. The modulation transfer function shows how much of the original signal is preserved as the spatial frequency increases. A sharp drop in the curve indicates a system that loses detail quickly as the magnification or feature density rises.
Frequency Response
Testing these systems involves scanning a resolution target with known line pairs per millimeter. The modulation transfer function is calculated by measuring the amplitude of the recorded sine waves for each frequency. This objective measurement replaces subjective evaluations of image sharpness.
Optical Quality
Factors such as the X-ray focal spot size and the detector pixel pitch limit the maximum achievable resolution. A high modulation transfer function across a wide range of frequencies is preferred for industrial inspection where both bulk defects and fine cracks must be detected. Improving this metric often requires upgrading the hardware or improving the alignment of the system components.
