ISO 14490-5:2021 pdf download – Optics and photonics — Test methods for telescopic systems — Part 5: Test methods for transmittance.
5.3? Monochromator? or? set? of? filters The monochromator or the set of filters can be omitted if the signal processing unit comprises a spectral detector. Grating or prism monochromators can be used to select the wavelength. The smallest adjustable wavelength interval shall be less than 2 % of the dominant wavelength (usually 0,55 µm) of the respective measurement. The necessary spectral bandwidth depends on the sample. It should be ensured that a steep change in the transmission curve is detected correctly. Thus, the bandwidth should be selected such that the transmittance across the band changes by less than 4 %. This condition cannot always be satisfied because of technical measurement and energetical reasons or because the time/cost effort is not adequate. In these cases, a maximum bandwidth of 4 % of the wavelength is allowable. A bandwidth of less than 2 % of the wavelength is necessary if the colour contribution indices are to be calculated. Instead of a monochromator, a set of narrow-band filters (full width half maximum <20 nm) may be used. They are especially useful with flat-shaped transmission curves. The number of measuring points shall allow for a definite curve fitting. A measurement with spectral filters instead of a monochromator is also appropriate if only single measurements are required. 5.4 Collimator The collimator may contain a refracting lens or a mirror. The collimator shall be adjusted to the aligned components in such a way that full and uniform illumination of the following aperture stop is assured. The focal length of the collimator shall be long enough that in relation to the field stop of the collimator the spot diameter is small enough to pass the image plane of the test specimen and is not obscured by target marks and internal structures. One-third of the image plane diameter should not be exceeded.
5.5 Aperture stop The aperture stop should be circular and located close to the objective lens of the test specimen if possible. The diameter should be ≤ 50 % of the entrance pupil of the test specimen, as well as smaller than the opening of the integrating sphere. Auxiliary systems can be used for beam forming to realize these requirements. These systems shall stay in the ray path during the measurement with and without test specimen. Generally, the smallest possible aperture stop should be used which is compatible with the signal-to- noise requirements of the detector. Special care should be taken when measuring telescopic systems with variable magnification where at some magnification settings the entrance pupil can be considerably smaller than the free objective lens diameter.