ISO 9336-3:2020 Optics and photonics — Optical transfer function — Application — Part 3: Telescopes.
5.3 Collimators
The object collimator shall he a well-corrected achromat with a focal length at least twice that of the objective of the specimen and a working aperture diameter at least 10 mm greater than the objective of the specimen. Reflective (off-axis) or catadioptric collimators may be preferred, especially for tests at different wavelengths, thus providing a constant apparent test object distance without the requirement for refocusing when changing the wavelength.
For the image analyser collimator, a convenient focal length would be 100 mm as this would ensure that the movement of the image analyser along its focus slideway would he within the range of a readily available (e.g. 25 mm) micrometer movement if the field curvature reached 2 rn-1. However, there may be circumstances where the resolution of the image analyser may require a longer focal length to be used. Alternatively, an image analyser collimator with fixed focal length in combination with well corrected microscope objectives of different lateral magnifications can be used.
5.4 Spectral response
Unless otherwise specified, the spectral response of the test system shall match that of an observer using the specimen in its normal viewing role or that of the detector if the specimen is intended for non- visual use (e.g. infrared systems). This may be achieved by using a specially designed filter combination to give the desired match in conjunction with the source emission and the detector spectral sensitivity (see notes to Table 2).
Ideally, measurements shall be carried out with narrow bandwidth quasi-monochromatic) radiation, preferably at the dominant wavelength of the eye or the detector spectral response. If more than one wavelength or a wavelength range is of interest, it is advisable to perform quasi-monochromatic tests in succession to ensure the separation of chromatic and resolution deficiencies.
The most effective position for filters is after the image analysing element as the effect of stray radiation is reduced. However, in good laboratory conditions, it is quite practicable to position the filter within the test pattern unit.
5.5 Spatial frequency range
To a large extent, the test specimen will be the controlling influence on spatial frequency ranges as derived in object space. In image space, the range might be limited by the resolution of the eye or the detector. The lower and upper bound of the spatial frequency range shall be defined in the imaging state table.
The corresponding frequency range in object space will he given by M times the lower and upper bounds, where M is the magnification of the telescope.ISO 9336-3 pdf download.