Detector open aperture

Figure 9.1 A typical Z-scan set-up. A sample is scanned near the focusing point Zq within 30 mm. Transmitted light after the sample is separated into two beams one beam is detected by an open aperture detector to get the open aperture signal Topenfz) that includes...

While self-focusing and self-defocusing are manifestations of the refractive part of the degenerate cubic optical nonlinearity, the absorptive part results in variation of the total power of the beam transmitted through the sample as a function of z. This can be monitored with a detector that integrates the power in the whole beam, and the changes of such power as a function of z (in the so-called open-aperture scan) can be directly related to the nonlinear absorption coefficient of the sample, a2. 

The excitation lamp is enclosed within a compartment which is designed to shield the detector and its associated electronic circuits from the heat generated by the lamp. A small fan flushes cool air into the upper compartment that houses the detection devices. The trigger mounted at the instrument s handle is used to operate a dual shutter that opens and closes the excitation and emission apertures simultaneously. The hand-held instrument is low cost (a/ 2,000), simple to operate, and weighs only a little over 1 kg without the power supply ( 5 kg). A photograph of the prototype instrument is shown in Figure 2. 

The 3-to-2 photon technique, simple counting setups and, possibly mean lifetime measurements could fulfill these criteria. A simple setup, shown schematically in Figure 7.28, is suitable for the first two applications. Positrons are implanted into the sample. Focusing into micron-sized areas is possible. Positronium forms, traps in pores and annihilates in closed pores or escapes through open porosity. Two detectors, one behind the sample and a second with an aperture on the side, observe all positronium (and positron) annihilations and only those from within the sample, respectively. The former detector is also set up to provide 3-to-2 photon ratios. 

The selection of the microscopic area for FTIR microspectroscopy is achieved by a remote aperture located between the objective and detector. The remote aperture commonly has a rectangular opening with two pairs of knife-edged blades. The blades are often made from a material that is transparent to visible light but opaque to infrared light. 

The aperture is about 50% lai-ger than the detector, to ensure a large FOV within the spatial resources. A one-dimensional coded aperture is defined by four geometrical parameters the element size, the number of elements, the fraction of open elements and the pattern of open and closed elements. We consider the optimum configuration of these four parameters for detecting GRBs with the WXM. 

Coded arrays were originally conceived for applications in X-ray imaging (Mertz and Young 1961 and Dicke 1968). A coded array is defined to be a pattern on a periodic two-dimensional lattice which associates with each lattice point a 0 or a 1 indicating whether the lattice point is open or closed . In coded-aperture imaging, the open and closed lattice points become open and closed cells in an opaque mask which casts a shadow of the photon source on a position-sensitive detector. For a review see Caroli et al. (1987). A uniformly redundant array, or URA (Fenimore and Cannon 1978), is a particular form of coded array. For a URA, each possible vector displacement between pairs of inequivalent open lattice points occurs a uniform number of times. (Equivalent lattice points are separated by a period of the array.)... 

---