GB2112131A - Apparatus for measuring the temperature of an object form the IR radiation emitted thereby - Google Patents

Abstract

Apparatus for measuring the temperatures of an IR radiation emissive object (12) comprises a housing (8) having an aperture at which is located a body (4) of IR- transmissive material. The body (4) is thermally controlled by a Peltier-effect device (6) and has a first portion (4A) which acts as a window for transmitting radiation from the object (12) to a detector (1) via an optical arrangement (2, 3). A second portion (4B) of body (4) has a covering (5) which prevents transmission of radiation from the object (12). Optical arrangement (2, 3) is arranged to switch the detector (1) alternately to receive radiation from portions (4A) and (4B) of body (4) and the detector output drives a closed loop temperature controller (10) coupled to device (6) in order to provide a zero differential detector output signal, whereupon the temperature of the body (4) is an accurate measure of the temperature of the object (12). The temperature of body (4) is measured by any suitable sensor. <IMAGE>

Description

SPECIFICATION Apparatus for measuring the temperature of an object from the IR radiation emitting thereby This invention relates to apparatus for measuring the temperature of an object from the infrared (IR) radiation emitted therein by detecting the radiation from the object and comparing it with that from a reference source of known temperature.

Various forms of apparatus for performing this task are currently manufactured -- some use two reference sources to calibrate gain of the detector/amplifier system; some compare signal levels of the received radiation at two or more wavelengths; some use a single temperature reference and rely on constancy of system sensitivity; and at least one apparatus controls the temperature of the reference source using the detector output until the differential signal between the reference and the object being examined is zero, at which time the temperature of the reference is equal to that of the object, and the temperature of the latter (which is usually inaccessible) is determined by measurement of the reference temperature.All these forms of apparatus are satisfactory in operation under ideal conditions, but a serious difficulty arises when, under practical conditions, the apparatus is used in a hostile environment such that a window must be used to isolate the delicate components of the apparatus from the environment. In this case the surface of the window tends to become contaminated by deposition of dirt, chemicals, water and the like and this contamination modifies the level of radiation from the object prior to its reception by the detector, the modified radiation then being compared with that from the reference source and consequently the correct temperature of the object is no longer measured.

The effect of a contaminated window may be partly overcome by placing the temperature reference outside the window at a distance from the window, since the radiation from the reference is then attenuated by the same amount as the radiation from the object. With this arrangement however the surface of the temperature reference tends to become contaminated thereby changing surface emissivity which affects calibration of the apparatus.

It is an object of the present invention to provide apparatus for measuring the temperature of an object frm the IR radiation emitted thereby and which is unaffected by moderate degrees of contamination by dirt, chemicals, water and the like.

According to the present invention there is provided apparatus for measuring the temperature of an object from the infra red (IR) radiation emitted thereby, said apparatus comprising a housing with an aperture for reception of IR radiation from said object, a body of IR-transmissive material secured to said housing at said aperture, said body having an uncovered first portion which is open to reception of IR radiation from the object and a covered second portion which is prevented-from receiving IR radiation from the object, means for varying the temperature of said body whereby the IR radiation emitted thereby is varied, an IR detection and means for observing the object through said first body portion and for guiding IR radiation to said detector alternately from said first and second portions of said body, servo-drive means connected between the detector output and the temperature-variation means to drive the temperature of said body until the detector produces a zero differential output signal, and means for determining the temperature of said body as a measure of the temperature of the object when the detector differential output signal is zero.

It will be understood that in the present invention the covered second portion of the body of IR-transmissive material functions as a temperature reference whilst the uncovered first portion of the body functions as a window for radiation from the object. Furthermore the apparatus of the present invention is unaffected by contamination on the window because the radiation from the object which is absorbed by the window and its contamination is exactly compensated by the emission from the window and its contamination both of which effectively are temperature controlled when there is zero differential output signal from the detector.It should be noted that the amplitude of this differential signal, when present, is dependent on the degree of contamination of the window and on detector sensitivity drift, but neither factor affects the temperature of the body at which a null differential output signal is obtained from the detector.

The present invention is based on the assumption that any contamination which adheres to the window is rapidly brought to the same temperature as the window by thermal conduction therefrom which is the case for moderate degrees of contamination and where the contamination is of finite thickness such as may occur with water droplets only the surface of the contaminant in contact with the window need be at window temperature. Clearly if the level of contamination of the window is such as to obscure it completely a null differential output signal will be obtained independently of the object temperature and to avoid this it may be necessary to clean the exterior surface of the window periodically for example by using an automatic wiper device. With such an arrangement the self radiation of the wiper device, when detected by the detector, can be used as a confidence check.The same wiper device can be used to eliminate the principal sources of difficulty in use of the apparatus which arise due to the presence of low emissivity contaminants. That is, contaminants which are either highly transparent or highly reflective. The former may induce unwanted refractions so that the detector receives radiation from an unintended object, whereas the latter causes the detector to receive unwanted radiation from within the apparatus. If however the detector is sensitive to radiation only within the 8-14 ,um range these difficulties are greatly reduced, if not eliminated, because in this spectral range most common contaminants are highly emissive.

The present apparatus provides exact compensation for contaminants where the four radiation sources (i.e. the object, the contaminant, the window, and the reference source) are each of the same emissivity or are each black body radiation sources but an acceptable level of compensation can be achieved even when these conditions do not apply.

So far as thermal gradients in the body of IRtransmissive material are concerned it is very desirable that none exist as between the first and second portions thereof which can be achieved by selecting a suitably large capacity of temperaturevariation means but gradients through the thickness of these portions does not affect operation of the apparatus. Conveniently the body of material is germanium but various other known IR-transmissives materials can be used and the means for determining the temperature of the body may be in the form of a thermal sensor embedded therein, for example a platinum resistance wire.

An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings in which: Fig. 1 schematically illustrates apparatus according to the present invention; and Fig. 2 illustrates a modified form of a detail.

The operation of the embodiment can best be understood if the detector 1 is regarded as the radiation source rather than the receiver of radiation as is actually the case. The detector is placed at the focus of a first concave mirror 2 which collimates the radiation and a second concave mirror 3 forms a real image of the detector near the outer surface of a body or block of material 4 the temperature of which is controlled by a Peltier effect device 6. The angular area containing the object 1 2 under examination is determined by the cone angle produced by the concave mirror 3.When the mirror 2 is tilted about its rotation axis 9 the image of the detector is deflected from the right hand half 4A of the body 4 and which acts as a window to the left hand half 4B of the body 4 which has been treated at 5 by deposition to render it opaque and of high emissivity so that it acts as a temperature reference.

The surface of the oscillating mirror 2 is located substantially at the focus of mirror 3, so that the cone of radiation from the detector incident on the body 4 moves in a telecentric manner (i.e. without change of cone angle) as the mirror 2 oscillates.

The oscillation of mirror 2 then causes the detector 1 to generate a differential output signal the amplitude of which is dependent on the temperature difference between the reference 5 and the object. This signal is passed into an electronic servodrive 10 along with a phase signal from an anglular pick-off 11 at the oscillating mirror 2, the latter signal being needed to determine the sense of the differential signal. The drive 10 then controls the Peltier device 6 until a null is achieved in the detector differential output signal. The temperature of the reference is then read off from a sensor (not shown) attached to the body 4. The sensor may be any of a number of known devices, (e.g. thermacouple, thermistor) which is capable of accurate calibration.To avoid temperature gradients the window should have high thermal conductivity, and germanium is probably the best choice for most applications.

It will be appreciated that should the temperature of the object under examination change rapidly the thermal time constant of the controlled body 4 will be such that the servo system will be unable to maintain a null differential signal for some time during and after the change. For many applications this would not matter, but if the signal level from the detector is measured it gives an indication of the temperature error and may be used to correct the reading. This correction signal is proportional to detector sensitivity and transmittance of body 4, but since in most applications the correction would be small, the errors inthe correction signal would have a small effect on the estimated temperature of the object under examination.

The present invention is by no means restricted to the configuration shown in Fig. 1. For example, the detector 1 may be imaged by a lens or lenses rather than mirrors, and the periodic deflection of the detector image may be accomplished by rotation or translation of a reflection or refractive component, or even by translation of the detector itself or the aperture stop 7.

Although the apparatus shown is a "staring" instrument, which measures the average temperature of a relatively large area, the same principle can be applied to a scanning instrument.

In the latter case the entrance pupil of the system would be placed on the window, and the thermal reference would consist of a emissive pattern on the window. This reference would be adjusted until the mean detector signal arranged over the entire scan is zero, the error signals from different parts of the scanned field giving a measure of the deviation from the mean temperature. This error signal would, of course, be affected by window contamination, but the mean temperature would be accurately known.

Fig. 2 illustrates a modified form of the body 4 in greater detail, In this case the housing 8 is provided with aperture 16 in which is fitted the body 4 surrounded by thermal insulation 1 7 (e.g.

PTFE). Body 4 is thermally bonded to a pair of Peltier-effect devices 6A, 6B, by means of copper conductors 1 7A, 1 7B. The temperature reference 5 is formed by a black deposition or coating on the outer surface of the body 4 which is made of germanium.

Claims (7)

1. Apparatus for measuring the temperature of an object from the infra red (IR) radiation emitted thereby, said apparatus comprising a housing with an aperture for reception of it radiation from said object, a body of IR-transmissive material secured to said housing at said aperture, said body having an uncovered first portion which is open to reception of IR radiation from the object and a covered second portion which is prevented from receiving IR radiation from the object, means for varying the temperature of said body whereby the IR radiation emitted thereby is varied, an IR detector and means for observing the object through said first body portion and for guiding IR radiation to said detector alternately from said first and second portions of said body, servo-drive means connected between the detector output and the temperature-variation means to drive the temperature of said body until the detector produces a zero differential output signal, and means for determining the temperature of said body as a measure of the temperature of the object when the detector differential output signal is zero.

2. Apparatus as claimed in claim 1, wherein the body and the covering which defines the covered second portion thereof have substantially the same emissivity characteristics.

3. Apparatus as claimed in claim 1 or claim 2, wherein said temperature varying means have .sufficient thermal capacity that no thermal gradients exist between said first and second body portions.

4. Apparatus as claimed in any preceding claim, wherein said IR detector is sensitive only to radiation in the 8-14 ym range.

5. Apparatus as claimed in any preceding claim, wherein said means for alternately guiding radiation to said detector comprises a pivotal member, and an angular-position sensor is provided thereat the output of which is delivered to said servo-drive means to determine the sense of any non-zero differential signal produced by the detector.

6. Apparatus as claimed in any preceding claim, wherein said detector and said body are located at conjugate focii of said means for alternately guiding radiation.

7. Apparatus for measuring the temperature of an object substantially as hereinbefore described with reference to Fig.1 or as modified by Fig. 2 of the drawing.