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Optical System and Design
Published in Shen-En Qian, Hyperspectral Satellites and System Design, 2020
A beam splitter is an optical device for dividing a beam into two or more separate beams. A simple beam splitter may be a very thin sheet of glass inserted in the beam at an angle to divert a portion of the beam in a different direction. A more sophisticated type consists of two right-angled prisms cemented together at their hypotenuse faces. A dichroic mirror, also referred to as a dichroic filter, is the optical device commonly used as a beam splitter in imaging spectrometers to divert the beam to VNIR and SWIR spectrometers. It spectrally separates light by transmitting and reflecting light as a function of wavelength. A long-pass dichroic mirror is highly reflective below the cutoff wavelength and highly transmissive above it, while a short-pass dichroic mirror is highly transmissive below the cutoff wavelength and highly reflective above it.
Light
Published in J. R. Coaton, A. M. Marsden, Lamps and Lighting, 2012
Interference is exhibited when a screen is illuminated by two separate but mutually coherent sources of light. Mutual coherence means that both sources are radiating light of exactly the same wavelength, and have a constant phase relation (section 5.1.3). The result of combining the light from both sources is that at some places on the screen the light waves are in phase and add together; at other places the waves are out of phase and cancel each other. Interference between the two sets of waves is normally seen as a pattern of light and dark bands on the screen. In practice mutually coherent sources of light are produced by splitting a beam from a single source, and this is usually achieved by using partially reflecting films on glass. One application of interference in present-day lighting technology is in the dichroic filters which are used to reflect or transmit certain selected parts of the spectrum (section 6.9). These also make use of the fact that a beam of light, reflected at normal incidence from the surface of a medium of higher refractive index, suffers a phase change of 180° (Ditchburn 1976).
Other Passive Devices
Published in David R. Goff, Kimberly Hansen, Michelle K. Stull, Fiber Optic Reference Guide, 2002
David R. Goff, Kimberly Hansen, Michelle K. Stull
Figure 9.4 illustrates a bulk optics WDM. Constructed from discrete lenses and filters, a dichroic filter lies at the heart of this type of WDM. Dichroic filters, based on interferometric techniques, reflect the light that they do not transmit. Referring to the figure, imagine that Fiber 1 carries two wavelengths, 850 nm and 1310 nm. Also imagine that the dichroic filter passes wavelengths longer than 1100 nm, known as long-wave pass (LWP) filter. As the light exits Fiber 1 it first passes through the lens which focuses the light at a point. As the light hits the filter, the 1310 nm light passes through the filter and is collected by Fiber 3. The 850 nm light exiting Fiber 1 on the other hand reflects off of the filter and is collected by Fiber 2. Thus the two wavelengths have been effectively separated, and the information carried on each wavelength can be independently decoded. The dichroic filter can offer a great deal of isolation in the transmission mode, but has poor isolation in the reflection mode. Usually these types of WDM's feature both short-wave pass (SWP) and LWP filters, and these filters combine to achieve the best system performance.
Simultaneous PIV/PLIF and Pulsed Shadowgraphy Measurement of Gas-Liquid Flows in a Swirling Separator
Published in Nuclear Technology, 2019
Yalan Qian, Tingting Zhang, Jingjing Li, Yuchen Song, Junlian Yin, Dezhong Wang, Hua Li, Wei Liu
Figure 5a depicts the measurement system consisting of a flow channel, a laser setup, an LED array, a beam splitter, two CCD cameras, and a synchronization unit. The origin of our coordinate axes was defined to be the same as the position of the swirl vane’s outlet. The upward direction was assigned as the positive z-axis, and the transverse direction was assigned as the y-axis. The laser sheet entered at x/R = 0 and illuminated the y-z plane, i.e., the midplane of the test section. The arrangement of the two CCD cameras is one for detecting the air core shape (the lower camera in Fig. 5b) and the other set perpendicularly for PIV/PLIF. A beam splitter was set in front of the two cameras and the internal dichroic filter with different reflection or transmission properties at two different wavelengths.12 The typical dichroic filter used in our work passed red LED light and reflected green laser light and orange fluorescence light. A square array of red LEDs, placed behind the test section, served as the light source for the shadowgraphy camera. In order to capture the air core shapes and flow field simultaneously, the triggering of the laser, the LEDs, and the two CCD cameras was synchronized.