A close-up of the light bar design, showing a full LED PCB unit, as viewed from the HPD array. 

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Context 1

... basic requirement on the MDCS system is to remove and each is collimated in order to project a small light spot onto the potentially severe distortions from the array of HPDs. The the HPD focal plane. The light bar is mounted as a gantry be- corrected photon position should contribute an error small in tween two linear-motion control stages. The movement of a comparison with the intrinsic spatial resolution of the HPD. given light spot in the direction of the stages is very fine and The strategy for the MDCS is implemented in two steps. nearly continuous, while the positioning of the light spot in the First, a light spot is projected at a precisely-known position on direction along the light bar is discrete, e ff ected by powering the quartz window of a given HPD in the array. The spot has a adjacent LEDs in turn. This choice is dictated by the spatial small size in comparison to the pixel size, and is oriented front- constraints on the system. on to minimize refractive e ff ects at the window and shadowing by the magnetic shields. The spot is moved to scan the HPD 3. System Description array in two dimensions, with small enough step sizes to map out the distorted image of the pixels. The data taken from this Two identical and independent MDCS systems were de- scan form a “direct” mapping of the distortion at a given mag- signed and constructed for RICH1, one for each of the upper netic field. Second, an “inverse” mapping is created, assigning and lower HPD enclosures. In this section, the description per- a given distorted pixel hit to its real position on the photocath- tains to only one of the two systems. ode. This may be done by a parameterized functional fit to the data if the distortion is su ffi ciently smooth to do so with a small 3.1. Light Bar enough reconstruction error, or alternately by a look-up table The light bar consists of two long carbon fiber side rails on if the distortion is too severe or non-uniform. With proper sur- which are mounted a series of 19 LED PCB units, custom de- vey information, this strategy can provide an absolute positional signed for this application 3 . calibration of each pixel in the HPD array. Each LED unit consists of a six-layer PCB, on which are Implementing this strategy for the MDCS system required mounted four LED arrays, an on-board addressable microcon- consideration of the tight physical constraints on the system in troller, a driver circuit to control each LED, and a collimator terms of overall area (1283 × 540 mm 2 ), form factor ( < 40 mm unit. This unit is shown in Fig. 3. high), and maintenance of a clear photon aperture for the HPD The LED arrays 4 each consist of a 5 × 7 matrix of green LEDs array. Additional considerations were made due to operation in on 2.54 mm centers. The LEDs have dominant wavelength 569 a magnetic and radiation environment. nm and half-width 30 nm. The four LED arrays are tilted as a The basic system design is shown schematically in Fig. 2. Instead of a single light spot moving in two dimensions, spatial 3 Design and fabrication of these LED PCB units (and controller) was made constraints on the system size forced a solution having a long in conjunction with SenSyr LLC, 111 College Place, Syracuse NY 13244, bar with many individual light sources (LEDs), termed a “light www.sensyr.com . bar.” Each LED in the light bar can be individually powered, 4 Lite-On LTP-757G LED arrays. ...

Context 2

... LED unit consists of a six-layer PCB, on which are mounted four LED arrays, an on-board addressable microcon- troller, a driver circuit to control each LED, and a collimator unit. This unit is shown in Fig. ...