Radiometer

The radiometer is an instrument for detecting and measuring the intensity of radiant thermal energy, especially infrared rays.

Description

A radiometer is a glass or quartz tube in which a partial vacuum has been drawn; Inside the tube is a shaft with four very light blades. One side of the blades is blackened, while the other is made of polished metal. When receiving external radiation the black side of one vane absorbs more radiation than the polished side of the opposite vane, causing the first vane to move away from the radiation source. This effect produces a constant rotation of the blades -the black side moving away from the light source-, with a speed that depends on the intensity of the radiant energy.

Previously used in meteorological instruments to make measurements high in the atmosphere, these mechanical radiometers have been almost completely replaced by solid-state electronic devices that measure radiant energy more directly and accurately.

Origin

The radiometer is a device designed by the English chemist Sir. William Crookes in 1873, which was built when he was trying to measure the molecular mass of thallium (an element discovered by himself). When weighing it on an analytical balance, Crookes noticed that the buoyancy of the air slightly reduced the weight, so he set out to eliminate the source of the error, building a vacuum chamber to weigh his samples. But instead of stabilizing his measurements as he had hoped, his arrangement in the vacuum chamber resulted in strange readings that were influenced by the temperature of the material he was weighing. Hot objects seemed to be repelled by cold objects, as in some experiments he observed that an object placed on a scale changed its weight when a warm body was placed near it.

Operation

It consists of a windlass, whose fins are white or metallic on one side and black on the other, arranged so that it can rotate with minimal friction, inside a glass vial in which a vacuum has been made partial. The light rays are reflected by the light surface and absorbed by the black one, which consequently heats up. Thus, the residual air in the bulb expands upon contact with the black surfaces and pushes the blades, causing the pinwheel to rotate, the greater the more intense the light. These instruments have been almost completely replaced by solid-state electronic devices, which measure radiant energy more directly and accurately.

Light sources used in Radiometry

The sources of general use in Radiometry are of vital importance as auxiliary systems for the calibration of the detectors that constitute the optical power meters for optical fibers. Semiconductor laser sources are the elements that generate and inject light as a carrier signal for fiber optic communications, a signal that must be measured with fiber power meters such as the one that is intended to be developed.

Desirable characteristics of a radiation source

The desirable characteristics of an optical source will depend on the specific application to which it is to be applied, but in general, they are the following:

• Temporal stability: The flow that is radiated must remain constant. Ideally, it should always be the same, that is, that the flow of radiated energy does not vary over time, even when the source is turned off and on.

• Spectral stability: The spectral distribution must always remain constant, and also its relative shape should not change, ideally never.

• Sufficient radiant flux: It is important that the signal/noise ratio is as high as possible to obtain the lowest uncertainty levels.

• Reliability and ease of use.

• Immunity against external environmental changes.

• Reduced size.

• Light weight.

• Little consumption.

• High performance: Getting all these characteristics in a single device is practically impossible, and for this reason, depending on the application in which our optical source is going to be present, we will look for some characteristics or others.