We propose an optical design process that significantly reduces the time and costs in direct backlight unit (BLU) development. In it, the basic system specifications are derived from the optical characteristics of RGB light-emitting diodes (LEDs) comprising the BLU. The driving currents are estimated to determine the theoretical RGB flux ratio for a desired white point. The number of LEDs needed to produce the target luminance is then calculated from the combined optical efficiencies of the components. Last, an appropriate array configuration is sought based on the illuminance distribution function for meeting the target uniformity. To showcase the design process we built two 42-inch triangular cluster arrays of 40 x 16 LED elements. When a flat reflective sheet was used, the minimum thickness required of the system to satisfy the target uniformity was 30 mm. Introducing a patterned reflective sheet removed hotspots that resulted from reducing the system thickness without the aid of additional optical components. Using an optimized patterned reflective sheet, reduction in system thickness as much as 5 mm was possible.

A class of metasurfaces for underwater sound absorption, based on a design principle that maximizes thermoviscous loss, is presented. When a sound meets a solid surface, it leaves a footprint in the form of thermoviscous boundary layers in which energy loss takes place. Considered to be a nuisance, this acoustic to vorticity/entropy mode conversion and the subsequent loss are often ignored in the existing designs of acoustic metamaterials and metasurfaces. The metasurface created is made of a series of topographic meta-atoms, i.e., intaglios and reliefs engraved directly on the solid object to be concealed. The metasurface is acoustically sticky in that it rather facilitates the conversion of the incident sound to vorticity and entropy modes, hence the thermoviscous loss, leading to the desired anechoic property. A prototype metasurface machined on a brass object is tested for its anechoicity, and shows a multitude of absorption peaks as large as unity in the 2-5 MHz range. Computations also indicate that a topographic metasurface is robust to hydrostatic pressure variation, a quality much sought-after in underwater applications.

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Despite its introduction more than a decade ago, a two-dimensional ultrasonic array remains a luxury in nondestructive evaluation because of the complexity and cost associated with its fabrication and operation. This paper describes the construction and performance of a two-dimensional virtual array that solves these problems. The virtual array consists of only two transducers (one each for transmit and receive) and an aluminum chaotic cavity, augmented by a 10  ×  10 matrix array of rectangular rods. Each rod, serving as an elastic waveguide, is calibrated to emit a collimated pulsed sound beam centered at 2.5 MHz using the reciprocal time reversal. The resulting virtual array is capable of pulse-echo interrogation of a solid sample in direct contact along 10  ×  10 scan lines. Three-dimensional imaging of an aluminum test piece, the nominal thickness of which is in the order of 1 cm, is successfully carried out using the virtual array.

The performance of a jagged-edge noise barrier (thin, infinitely long) is investigated numerically. The source is a spherical N wave. The theoretical approach is to treat the incident field in the plane above the barrier as a plane piston radiator (Kirchhoff approximation). Numerical calculation gives the radiated field (pressure time waveform) at any point in the shadow zone. A discrete Fourier transform then yields the spectrum. The numerical result for a straight barrier compares favorably with the asymptotic result of U. J. Kurze [J. Acoust. Soc. Am. 55, 504–518 (1974)]. Because of the intrinsic limitation imposed by the Kirchhoff approximation, however, the numerical method does not give accurate results when the listener is deep in the shadow zone. For the jagged-edge barrier, the numerical simulation confirms the experimental findings of Ho et al. [J. Acoust. Soc. Am. 101 (to be published 1997)] in that making the barrier edge jagged (1) increases the insertion loss at high frequencies, but (2) degrades the performance somewhat at low frequencies. Suggestions for improving the insertion loss formula of Ho et al. for a jagged-edge barrier are also made. [Work supported by the Texas Advanced Technology Program.]