radiometer, instrument for detection or measurement of electromagnetic radiation; the term is applied in particular to devices used to measure infrared radiation. One of the earliest experiments in radiometry was performed c.1800 by W. Herschel, who observed the heating of a mercury thermometer by sunlight; he was also able to detect heat radiated from hot but not incandescent bodies. E. Becquerel was able (c.1843) to detect near-infrared radiation by photographic means. Radiometers that function by an increase in the temperature of the device, such as Herschel's thermometer, are called thermal detectors. Commonly used thermal detectors include the thermocouple, which produces a voltage when heated, and the bolometer, which changes in electrical resistance when heated. Devices that can, in principle, detect a single quantum of radiant energy, such as Becquerel's photographic plate, are called quantum detectors. Many current quantum detectors are based on the photoelectric cell. The term radiometer is often used to refer specifically to a type of thermal detector invented by Sir William Crookes (c.1874). Because his device was somewhat insensitive and not readily calibrated, it is rarely used today as a scientific instrument. A Crookes radiometer consists essentially of two parts. The first part is a glass bulb from which most of the air has been removed, creating a partial vacuum. The second part is a rotor that is mounted on a vertical support inside the bulb. The rotor consists of four light, horizontal arms mounted at right angles to one another on a central pivot; the rotor can turn freely in the horizontal plane. At the outer end of each arm is mounted a metal vane, placed vertically. Each vane has one side polished and the other blackened; the vanes are arranged so that the polished side of one faces the blackened side of the next. When radiant energy strikes the polished surfaces, most of it is reflected away, but when it strikes the blackened surfaces, most of it is absorbed, raising the temperature of the surfaces. The air near a blackened surface thus becomes hotter, exerts a greater pressure on the blackened surface, and causes the rotor to turn. The rate of rotation provides an indication of the intensity of the radiation.

A radiometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Generally, the term “radiometer” denotes an infrared radiation detector, yet it also comprises detectors operating on any electromagnetic wavelength, e.g. spectroradiometer.

A radiometer's characteristics are:

• spectral range
• spectral sensitivity
• angle of view
• directional response

Radiometers detect and measure radiation as thermal (convert absorbed energy to a signal) and photodiode (photons absorbed at a constant response/quantum). The radiometer's radiation-detecting bolometer absorbs radiation falling upon it, raising its temperature, then is measured with a thermometer. The higher temperature might be related to the incident radiation's power.

The Crookes radiometer is an early-model infrared-radiation and light-detector. A variant type of is the Nichols radiometer that operates on a different principle, and is more sensitive than the Crookes type.

A Microwave radiometer operates in the Microwave wavelengths. The radiometer contains argon gas to enable it to rotate. Keely said that when you vibrate an atomic substance (e.g., argon) with an atomic vibration (e.g., microwave) you get rotation. When you put a radiometer containing an atomic elemental gas, such as argon, into a microwave, it causes it to spin, thus the microwave interaction with the argon gas is creating rotational raleigh waves.

As an eponym, radiometer usually denotes a Crookes radiometer, a device wherein a rotor (with dark and light vanes) spins when exposed to light in a partial vacuum.

See also

• Net radiometer (instrument)
• Pyranometer (instrument)
• Photometry (optics) Main article - explains technical terms and units

External links

• Use of Radiometer as an Alternative Energy Source
• Sympathetic Vibratory Physics - John Worrell Keely's Sacred Science