Optical Sensors Questions and Answers

Answer: C

CCD stands for Charge-Coupled Device.

A CCD is an array of closely spaced MOS capacitors. Light falling on each pixel generates charge in proportion to its intensity, and by clocking the gate voltages in sequence the charge packets are passed from one capacitor to the next, like buckets in a line, until they reach an output amplifier that converts each packet to a voltage. Invented at Bell Labs in 1969, it was the image sensor in most digital cameras, scanners and telescopes before CMOS sensors took over.

The other expansions are invented. Control Circuit Design, Central Core Database and Composite Camera Device are not standard terms in electronics, and none describes the charge-transfer principle that gives the device its name.

Answer: B

The shift register in a CCD performs serial transfer of charge.

In a charge-coupled device, each pixel collects photo-generated charge. To read the image, the charge packets are moved step by step from one capacitor to the next by clocked gate voltages, row by row into a horizontal shift register, which then shifts them one at a time to the output amplifier. This bucket-brigade movement is serial charge transfer.

The CCD does not store data in the digital sense, and the analog-to-digital conversion happens after the output amplifier, in separate circuitry. Image processing is done later by a processor. The shift register's job is only to move charge to the output.

Answer: C

A solar cell is a transducer that generates electrical power: it is a photovoltaic device that converts incident light directly into a voltage and current without an external supply. A photoconductor changes its resistance under light, and a photodiode and a phototransistor are normally used with an external bias to give a current that varies with light, so these are not power-generating transducers in the sense the question asks. The answer is option C.

Answer: D

All four statements are valid, option (D). A liquid crystal display behaves essentially as a capacitor, its decay time is slow, it does not emit light and so is invisible in darkness, and a twisted nematic segment looks black when it is switched on.

Options A, B and C each name only some of the statements, so they leave out a true one. Since (i), (ii), (iii) and (iv) are all accepted, only option D is complete.