High-quality optical systems can only be produced with cutting-edge, flexible, and precise lens measuring equipment in today’s innovative technologies. Measurement of the optical system’s capacity to transmit different degrees of information from object to image is known as the MTF (Modulation Transfer Function).
It is possible to measure MTF using various techniques, including discrete or continuous frequency generation and picture scanning. MTF can now be measured with great efficiency and accuracy because of recent improvements in precision mechanics and electro-optics technology. Instrumentation may be divided into four main categories: frequency generation, scanning, video, and interferometric.
- Frequency Generation
An object photographed by the Lens under Measure and composed of a single spatial frequency pattern is the most straightforward MTF measurement method. The operator directly measures the contrast of the picture. Because the output can be readily monitored, frequency generating techniques have a significant advantage over other approaches.
An infinitesimal pinhole may be used as an input to a scanning system, and this information is used to calculate the MTF of the picture. The optical analogue of assessing an audio speaker’s frequency response is this way of measuring MTF. An indefinitely tiny light source will generate a blurred picture, similar to the total output of a speaker with a single input audio frequency. The characteristics of the blur also reveal the lens’s frequency response.
The same theoretical issues apply to video approaches as they do to scanning methods. The speed at which video MTF measurement may be performed is a major benefit. To update the MTF, a solid-state array must be sampled, and the Fourier transform computed electrically. This gives an updated spread function and MTF curve continually.
There are two ways to determine the MTF of a system using an interferometer: by measuring the PSF computed by Fourier processing the pupil wavefront or by auto-correlating the pupil function of the lens-under-test. This is ideal for systems that can be tested in an interferometer; do not display major chromatic aberrations, and whose wavefront errors do not fluctuate significantly throughout the wavelength of interest. Broadband sources and spectrum filters may be used with scanning, video, or discrete frequency techniques for polychromatic testing. If a monochromatic source generates MTF, it can only be used at these wavelengths.