chapter 6, Metrology

Excerpt

High-precision fabrication technologies for spherical, but for aspherical surfaces in particular, have been significantly improved within the last few years, mainly driven by developments in the semiconductor market. Aspheric surfaces would be even more attractive if production costs could be further reduced. Their main advantages in optical systems are a better image quality or a reduced number of optical surfaces while maintaining image quality. An improved metrology is necessary, and an inprocess metrology for production and quality assurance would be desirable.

6.1 Measurement of Optical System Performance

For the measurement of optical system performance, classical methods are available. The most frequently used will be mentioned briefly.

The optical transfer function (OTF) is frequently used for image quality analysis and is preferable for sensor systems. Both parts of the complex OTF, the amplitude term (called the modulation transfer function, MTF) and the phase term (called the phase transfer function, PTF), should be measured. The OTF is mostly obtained as a Fourier transform of the measured point or line spread functions at different field angles. The MTF gives the contrast ratio of a line pattern in the image versus object space for different spatial frequencies and therefore indicates whether the optical system fulfills the resolution specifications. The MTF, as a function of the spatial frequency, gives more useful information about the performance of the optical system, compared to the classical resolution test. The PTF itself is extremely important for the designer to be able to judge comatic errors in his layout. It is also very important for the assembly process. In both cases of a perfect optical system, the PTF should be zero. Any nonzero PTF values result from asymmetries (for example, asymmetric point spread functions) and are serious hints for decentered lens elements.

Another test method used in astronomy and microscopy is the “star test,” where the image of a pinhole is analysed in shape and position [3] (Sec. 16.2.4).