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Non-Contact

 
 

Why non-contact sensors?

Many temperature sensing applications can benefit from non-contact sensing methods. The measurement target may be: moving, above the melting point of other sensors, mechanically of chemically hostile or simply at too great a distance to run wires. Some non-contact methods can also be faster than traditional contact methods of temperature measurement. Most practical non-contact sensors use infrared (IR) radiation and are referred to as IR Temperatures Sensors or IR Detectors

Ho do non-contact IR sensors function?

Non-contact or radiation temperature sensors function by measuring components of the electromagnetic radiation naturally radiated by all objects above absolute zero. Only a small proportion of this radiation is visible to the human eye, as the following power spectra diagram indicates!

The vertical axis in this diagram represents the intensity of the radiation from a blackbody. The horizontal axis represents the wavelength of the radiation. Due to the wide range of values, especially on the vertical axis, logarithmic scales have been used to show detail over a wide range of temperatures. Each of the curves represents the spectra of radiation from a black object at a particular temperature.

If the more common linear scales are applied, the shape of the curves differ with the lower temperature curves tending to bunch close to the horizontal axis

Plank's Equation

As can be seen from these diagrams, as the temperature of the object increases, the curve moves to the left (shorter wavelengths) and up (more energy is radiated). For the theoretically minded, this phenomena is described by Plank's equation:

E = 2.Pi.c2.h /(L5(ehc/LkT - 1) )

Where:

Pi = 3.1416
c is the speed of light = 2.9979264.108 m / s
h is Plank's constant = 6.6262.10-34 j - s
k is Boltzmann's constant = 1.3805.10-23 j / K
L is the radiation wavelength in meters
T is the absolute temperature of the object in K

The wavelength of the peak energy has a simple relationship (Wien's Displacement Law) with the object temperature:

Lpk = 2.898 / T    (µm)

You can experiment with Plank's equation using a spreadsheet or other application. The plots on this page were generated with Mathcad.

The human eye as a radiant temperature sensor

In the visible region, the radiant energy cannot be easily seen until the temperature reaches about 550°C, when it will appear as a dull red. (While you can feel the radiate heat on the face from objects below this temperature, it is difficult to judge the temperature).

As the temperature is increased the red progressively turns to white and then to a bright bluish white. The brightness progressively increases. These effects can be explained from the intensity spectra curves, the visible region of which has been enlarged in the figure below:

The colour of the radiated light depends on the proportion of each of the different constituent colours (of the rainbow). If there are more with a long wavelength, then the colour will be reddish. If the balance is approximately even the colour will be white and if there is a dominance of shorter wavelengths, then the colour will be bluish. The slope of the temperature curves indicates the balance of colours.

This phenomena allows us to approximately gauge the temperature of a hot object from its colour. I say approximately, because the human eye is very poor at determining absolute colour - you could expect an accuracy of ±200°C if you're lucky!

However, the human eye is very good at comparing colours, so if you are provided with a calibration palette of colours to match to the hot object's colour, your estimate of temperature is likely to be better than ±50°C.

As an interesting sideline, 'white' light can be given a colour temperature - it is literally the temperature of an object required to produce a light of the same whiteness or warmth. This is particularly important with colour photography as photographic film and the sensor array in electronic cameras detect light in an absolute way. The result is their output can often be tinted creating a warmer or cooler picture than was originally perceived by the human eye.

The human eye - brain combination is a wonderfully adaptable sensor, able to automatically correct for colour balance and a huge million to one range of intensity levels.

How to measure temperature with radiation sensors?

The operating principal of the eye can clearly form the basis of radiation temperature sensors. The eye contains a lens that focuses the radiation onto the retina or the radiation detector of the human system. The brain processes the colour and intensity signals, and with a little training can produce a temperature result. This is much the same process that a radiation temperature sensor must undertake to yield temperature.

Practical systems are both more complex and accurate. It is now possible to use long wavelength detectors and filters to maintain an accuracy to within a few degrees and with 0.1°C resolution.

There are two basic ways to convert the radiation an object radiated to a temperature.

1. The simplest is to measure the radiant energy over a spectral band. This gives a single intensity reading that is then converted to temperature. For this method to provide accurate readings the entire field of view of the radiation detector must include only the object being measured. This is because the signal is proportional to the total radiation received in the solid angle defined by the sensors optics. Obviously the narrower this angle the easier it is to ensure only the object is included. Some sensors include a laser pointer to assist in aiming.

2. The second method of determining the temperature is to use colour - that is measure the radiation in two spectral bands and then take the ratio of the two. This two colour ratio is then converted to a temperature. This method is an extension of the technique used by the manual colour matching pyrometer. While more expensive than the first method, it has one important advantage: it is less influenced by the surface emissivity characteristics of the object. Some smart sensors employ a hybrid of the two methods, using both colour and intensity information to compute temperature.

Selecting the spectral bands in which to measure temperature is not a simple task. Inevitably the selection is a compromise between competing requirements. Things that need to be considered include:

Radiance vs. Wavelength: The rate at which radiance increases with temperature is greater at shorter wavelengths.

Radiance vs. Temperature: Lower temperature objects radiate at longer wavelengths, so the spectral range of the detector can be matched to the temperature range to be measured. For example to measure temperatures from 600°C to 3000°C a detector centred on 1µm is fine, but for a -30°C to 1200°C range a 8-14µm band may be used.

Object Emissivity, Reflectance and Transmittance: The optical characteristics of the object being measured are important. Take glass as an example below 2.5µm it is transparent, but above that figure becomes increasingly opaque. Selecting a measurement wavelength in the transparent area will obviously look through the object. Beyond 4µm glass is completely opaque.

Atmospheric Transmission: The atmosphere is far from a perfect transmitter of radiation and this is especially the case in the infrared. There are strong CO2 and H2O absorption bands at 1.35-1.45, 1.7-2.1, 2.5-2.9, 4.2-4.5, 5.1-7.5 and >14 µm. If the length of the atmospheric path is short (say less than a meter), these are not likely to be a problem. In addition the absorption due to water vapour is more likely to be an issue because it varies with humidity.

Cost vs. Performance of the Detector: A practical sensor can only employ reasonably priced detectors and support electronics. Generally it is easier and cheaper to produce high temperature sensors. This can be implemented without moving parts or cooled detectors. As the temperature range is lowered, the difficulty and cost increases.

Detecting radiation

An important characteristic of radiation detectors is their spectral response and how filters can modify it. There are two very broad categories of radiation detectors:

Quantum detector: This is essentially a photon counter that is sensitive to all photons that have a certain minimum energy. The sensing element is usually a semiconductor crystal or ceramic-like material. Detectors of this type can have a reasonably uniform response up to a particular wavelength, but beyond that point their response drops off rapidly. Intrinsically, the quantum detectors can respond quickly to fluctuations in radiation.

Thermal detector: This relies on the heating effect of absorbed radiation. The small temperature rise can be measured by a conventional temperature sensor, typically a thermopile or thermistor. The spectral characteristics of the absorber's surface finish will determine the sensors characteristics. In general, the response will be wide and flat but lacking the sensitivity and speed of the quantum detectors.

Some of the many radiation detectors used include:

Sensor
Type Spectral Response Sensitivity Comments
Thermal
very wide
low
Response can be modified by surface finishes
Silicon Photodiode
Quantum
0.4 to 1.1µm
low
Response only adequate for high temperature radiometers
Lead Sulfide (PbS)
Quantum
1.0 to 3.0µm
good
Cooling to say 77°K extends range to 4µm and reduces noise
Lead Selinide (PbSe)
Quantum
1.0 to 4.5µm
good
Cooling to say 77°K extends range to 4µm and reduces noise
Pyroelectric plastic films
Thermal
8 to 11µm without surface finish
good sensitivity to changes
Polyvinylidene fluoride (PVDF) such as Kynar film from Ampec or Solef film from Solvay & Cie
Pyroelectric ceramics
Thermal
to 14µm
ok
Range of performances available

The EIA (Electronics Industry Association of USA) has standardised a number of spectral responses with S-designations (e.g. S-1, S16, S-25) which tend to be based on particular detector materials. However, this level of detail is beyond the scope of this document.

For optimum performance, some radiation sensors must be cooled to a low temperature (e.g. liquid nitrogen). Much of the work done in developing radiation sensors has been to improve performance at normal ambient temperatures, because of the inconvenience of cooling to such low temperatures.

One interesting problem associated with non-contact measurement of temperature around ambient or lower, is the impact of local radiation emitted by components within the sensor. This radiation may be far greater than that from the object being measured. One solution is to cool the offending components so that they emit less radiation.

An alternate and generally preferred solution is to convert the incoming signal into an AC (alternating current) signal by mechanically chopping the signal beam with a slotted rotating disc. The signal is then amplified with an AC amplifier that rejects the local DC signal. This technique is commonly used in the lower temperature radiation sensors, but because of cost and the reliability issues associated with the moving parts, such sensors are only used if the low temperature range is really necessary.

Some instruments use the chopper disc to insert an additional filter into the optical line. This enables the two colour ratio method of temperature determination to be made.

Blackbody, emissivity and errors

Emissivity is a term for the energy emitting characteristics of different materials. It is the function of wavelength, temperature and angle of view. Emissivity is defined as the ratio of the energy radiated by an object at a given temperature to the energy emitted by a blackbody at the same temperature.

A blackbody neither transmits nor reflects energy. A blackbody absorbs and re-emits all energy incident upon them. The emissivity of a blackbody is 1.0, and all objects other than blackbodies have an emissivity of less than 1.0. For example, an object may have an emissivity of 0.85. That means that the object emits only 85% of the energy emitted by a blackbody.

In practice it is not possible to construct a perfect blackbody. However, close approximations can be built that perform well over a limited spectral range. A classic blackbody construction is an insulated hollow metal ball or box with a blackened inside surface and a small hole (say 10% of width of ball or box). This hole will behave like a blackbody. Another black body design consists a stack of razor blades

Blackbodies are ideal surfaces for IR measurement and are used to calibrate IR measuring devices. Correction factors and adjustments are usually built in to IR sensors, so that they may be calibrated for specific emissivities. If the correction factors are not applied, the temperature reading will be lower than the actual temperature of the object being monitored.

The emissivity of an object may change with time as its surface corrodes or oxidizes. In some applications it is possible to attach a stable target to the object being measured to minimise this effect.

Incident radiation from other sources can reflect off an object and be registered by a radiation detector. The source may be incandescent lighting, nearby hot objects or sunlight.

Commercial non-contact instrument manufacturers

Manufacturers of Infrared or Non-contact IR Temperature Sensors include: IRCON Inc, Mikron Instrument Co. Inc. and Raytek Corp.