The measure of the maximum angle between the true direction and the apparent direction of a celestial body as observed from earth due to aberration. It has a value of 20.496 seconds of arc. The aberration angle depends upon the ratio of the velocity of the earth in its orbit and the velocity of light in addition to the angle between the direction of the light and the direction of motion of the observing telescope. The maximum value is obtained when the celestial body is at the pole of the ecliptic.
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POLARIZING PYROMETER
A type of pyrometer, such as the Wanner optical pyrometer, in which monochromatic light from the source under investigation and light from a lamp with filament maintained at a constant but unknown temperature are both polarized and their intensities compared.
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PLANISHING
Smoothing the surface of a metal by a rapid series of overlapping, light hammerlike blows or by rolling in a planishing mill.
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PLAIN CONCRETE
Concrete without reinforcement but often with light steel to reduce shrinkage and temperature cracking.
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PHOTONICS
The electronic technology involved with the practical generation, manipulation, analysis, transmission, and reception of electromagnetic energy in the visible, infrared, and ultraviolet portions of the light spectrum. It contributes to many fields, including astronomy, biomedicine, data communications and storage, fiber optics, imaging, optical computing, optoelectronics, sensing, and telecommunications. Also known as optoelectronics.
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PHOTONEPHELOMETER
A nephelometer that uses a photocell or phototube to measure the amount of light transmitted by a suspension of particles.
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PHOTOVOLTAIC EFFECT
The production of a voltage in a nonhomogeneous semiconductor, such as silicon, or at a junction between two types of material, by the absorption of light or other electromagnetic radiation.
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PHOTOTUBE CURRENT METER
A device for measuring the speed of water currents in which a perforated disk, which rotates with the current by means of a propeller, is placed in the path of a beam of light that is then reflected from a mirror onto a phototube.
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PHOTOELECTRIC LIQUID-LEVEL INDICATOR
A level indicator in which rising liquid interrupts the light beam of a photoelectric control system; used in a tank or process vessel.
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PHOTOFLASH BOMB
A missile dropped from aircraft; it contains a photoflash mixture and a means for ignition at a distance above the ground, to produce a brilliant light of short photoelectric effect See photoelectricity.
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PHOTOELECTRIC TURBIDIMETER
Device for measurement of solution turbidity by use of photocells to detect the loss of intensity of light beamed through the solution.
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