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Crystal Filters
Published in John T. Taylor, Qiuting Huang, CRC Handbook of ELECTRICAL FILTERS, 2020
The traditional “discrete” crystal filter is typically a hybrid-lattice-type design and consists of an assembly of crystals, capacitors, and RF transformers mounted on a PC board. Each crystal is individually manufactured to a specific frequency and mounted in a hermetically sealed container. Because the crystals can be built to any desired frequency, a wide variety of filter designs is possible. Designs have been developed which can provide filters with Butterworth, Chebyshev, Bessel, or elliptic function responses as well as skewed (single-sideband) characteristics. In addition, a variety of techniques is available for building filters with linear-phase or phase-compensated characteristics. This exceptional design flexibility makes the discrete crystal filter very attractive for specialized requirements. Also, because the filter is essentially a PC-board assembly of individual components, the cost of tooling is minimal, an important factor for small-quantity orders. However, the cost to fabricate crystals is relatively high, and discrete crystal filters are typically the most expensive of the various electromechanical filters. For frequencies below 1 MHz there is extensive overlap between the capabilities of crystal and mechanical filters. In general, the cost of mechanical filters is lower particularly for larger-quantity requirements where the cost of tooling can be amortized.
Thermal neutron scattering properties of Bismuth crystal filter
Published in Journal of Nuclear Science and Technology, 2021
Lipeng Wang, Liangzhi Cao, Hongchun Wu, Lingti Kong, Yongqiang Tang
Table 4 gives the thermal neutron beam quality parameters at the back end of Bi filter with different methods. It can be seen that the thermal neutron fraction in the phonon spectrum model is higher than that in the free gas model. The main reason is that the inelastic scattering cross-sections in the phonon spectrum model is smaller than that in the free gas model in the low-energy region. Similarly, the thermal neutron fraction in the Method 2 and Method 3 without considering the coherent elastic scattering are larger than that in the Method 4 which considers coherent elastic scattering, it is mainly due to the increase of the total cross-sections caused by the sharp increase of coherent elastic scattering above the Bragg energy threshold. The comparison of energy spectrum results is shown in Figure 9, it shows that the filtering effect of Bi crystal with free gas model cannot be appropriately captured. The treatment of Methods 2, 3 and 4 can further increase the thermal neutron fraction in the beam. The introduction of coherent elastic scattering can reduce the thermal neutron fraction. Besides, the energy spectrum of Method 2 is the nearest that of Method 3, which is the reason for using proper semi-empirical formulas for Bi’s thermal scattering to neglect coherent elastic (i.e., ‘Bragg’) scatterings, which assumes the use of a single Bi crystal filter with a preferential orientation relative to the neutron beam.
High-photon-throughput snapshot colour imaging using a monochromatic digital camera and a pupil-domain diffuser
Published in Journal of Modern Optics, 2019
Jonathan Hauser, Michael A. Golub, Amir Averbuch, Menachem Nathan, Valery A. Zheludev, Omer Inbar, Shay Gurevitch
Existing red–green–blue (RGB) imaging and photographic methods are based on an additive colour model of machine representation of three primary colours from spectral data in the visible range. A straightforward approach for the electro-optical implementation of colour vision is based on time-sequential colour filtering that resorts either to a set of mechanically exchangeable colour filters (1–4) or to an electronically tuneable colour filter, e.g. a liquid crystal filter (5) or Fabry–Perot interferometer (6). However, time-sequential colour filtering is not applicable to single snapshot photography of dynamic fast changing objects. Furthermore, each of the three RGB colour filters transmits about one-third of the incident light flux, while the other two-thirds of light are absorbed, causing substantial light flux losses. The widely used Bayer RGB colour filter array (CFA) (7) includes a spatial pattern of a periodically repeating pattern of one red, two green and one blue filters facing pixels at the image sensor, as shown in Figure 1(a). The Bayer CFA enables snapshot digital colour imaging, but leads to resolution loss and inherits the substantial light flux losses of time-sequential colour filters. While resolution loss in redundant optical images is customarily compensated by an interpolation process called ‘demosaicing’, the lower than 40% light throughput remains an intrinsic disadvantage of the RGB CFA. The white-RGB (WRGB) CFA (8), in which one of the green filters in the Bayer pattern is replaced with a ‘white’ (transparent or ‘monochromatic’) one, as shown in Figure 1(b), increases light throughput only up to 50%. Furthermore, colour reconstruction in WRGB is sensitive to noise. Some improvement in light throughput can be achieved by resorting to a limited number of RGB pixels at a sparse set of locations on the image sensor with mostly ‘white’ pixels (9), albeit at the expense of colour rendering quality reduction between the sparse RGB pixels. In an alternative approach with a Foveon array (10), each pixel of the image sensor consists of 3 stacked layers of photodetectors, where the top layer is sensitive to blue light, the middle layer is sensitive to green light and the bottom layer is sensitive to red light. While delivering the snapshot mode at full spatial resolution, the Foveon array still suffers from unacceptable losses in light flux at the layers of the image sensor. Another off-the-shelf architecture for RGB imaging which does not include a Bayer CFA is suggested by JAI’s Apex series (11), by performing separate imaging to each colour band using prisms, dichroic filters and multiple sensors. This architecture suggests improved spatial resolution and shows less optical losses compared to a conventional Bayer CFA; however, it includes a quite complicated optical layout.