The present invention relates to an apparatus for measuring the temperature of a body, comprising:

- a first laser light source for radiating the body with a laser light beam with a first wavelength such that the body undergoes a measurable temperature increase;

- a second laser light source for radiating the body with a laser light beam with a second wavelength such that the body undergoes a measurable temperature in¬ crease; - a first radiation detector for detecting radiation of the first wavelength coming from the object;

- a second radiation detector for detecting radia¬ tion of the second wavelength coming from the object;

- a control circuit which is adapted to form alter- nating time windows of the first type and the second type, wherein the control device activates the first laser light source and the second radiation detector during windows of the first type and activates the second laser light source and the first radiation detector during windows of the second type.

Such an apparatus is known from the international patent application with publication number WO 93/10426.

This known apparatus provides in elegant manner the contactless measuring of the temperature of a body, in that the influence of poorly measurable coefficients subject to change is eliminated. The body of which the temperature has to be measured is radiated by means of a laser light beam of a first wavelength, which results in a measurable increase in temperature, thereby causing a change in the amount of radiation emitted by the body.

The change in the intensity of the radiation at a second wavelength is detected by the radiation detector at a

point in time after the radiation with laser light of the first wavelength has taken place.

The temperature increase caused by the radiation with the first wavelength is a function of a relevant coefficient, while the radiation emitted at the second wavelength is a function of another coefficient. By then performing the measurement in reverse order the influence of both coefficients can be eliminated. For further elucidation reference is made to said publication. Although this prior art apparatus allows a reproduc¬ ible measurement, there exists a need in the art for measuring the temperature of rapidly moving bodies, for instance bodies which perform a periodic movement. An example of such bodies are the blades of a gas turbine. It is pointed out here that the blades of a gas turbine are components which are subjected to a very critical load, wherein the lifespan is largely dependent on the temperatures occurring during operation. It is thus of great importance to measure the temperature of the tur- bine blades of a gas turbine with the greatest possible accuracy. It is hereby possible to prevent the blades being replaced prematurely in case of too cautious an approach, which would involve waste of capital, while it is on the other hand prevented that the lifespan of the blades is exceeded and defects occur, whereby the gas turbine must be shut down, which generally entails even greater cost.

This object is achieved in that the apparatus is adapted to measure the temperature of a body performing a periodic movement, wherein the control device is adapted to determine the time windows such that between a time window of the first type and a time window of the second type the body has travelled through at least a part of its periodic movement. This offers the possibility of also measuring the temperature of rapidly moving bodies in reliable manner; the measurement is hereby as it were divided into two stages, whereby each part of the measurement is performed

in a very short time span. It will be apparent that in the case of such rapidly moving components the effects of the first measurement will not yet have disappeared when the second measurement is performed, so that this problem is obviated by performing the measurements at a time-lag such that they do not affect each other and wherein the measurement are always performed when the moving body is situated in the same position. The measuring apparatus is in any case situated in this position. It is also noted that it is also possible to divide the measuring apparatus into two parts and to cause the measurement of the first time window to be performed at one particular location in the periodic path and the second measurement at a different location in the period- ic path. Such a configuration is however not generally to be recommended.

It is assumed in the above that only two measure¬ ments have to be performed for reliable measurement of a body,- however, due to influences of for instance noise it may very well be possible that it is sensible to carry out more than the minimum number of two measurements. Influences from noise can then be eliminated in statisti¬ cal manner, which enhances the accuracy of the measure¬ ment. As a result of these steps it is possible to carry out an accurate temperature measurement of for instance blades of a gas turbine, so that the lifespan of a blade can be estimated more precisely.

It will be apparent that the invention is not limit- ed to this application; it is likewise applicable to other components which move at a high speed and which are exposed to extreme temperatures.

According to a preferred embodiment the laser light sources are adapted to generate a laser light pulse of very short duration relative to that of the time window. According to yet another preferred embodiment the control device is adapted to activate the relevant detec-

tor during the same window some time after generating of the laser light pulse.

According to a preferred embodiment already eluci¬ dated in the foregoing, the time duration of windows is chosen such that during a second window subsequent to a first window the body has travelled through its complete movement at least once. As explained above, this offers the possibility of accommodating the measuring apparatus at only a single location and of using particular compo- nents thereof for both measurements.

Other preferred embodiments are specified in the sub-claims.

The present invention will be elucidated hereinbelow with reference to the annexed drawings, in which: figure 1 shows a schematic perspective view of an apparatus according to the invention; figure 2 shows a diagram explaining the operation of the apparatus according to the invention; and figure 3 shows a schematic cross-sectional view of a variant of the embodiment shown in figure 1.

Shown in figure 1 is a gas turbine designated in its entirety with 1 which is formed by a blade wheel 2 which is mounted on a shaft (not shown) and to which are fixed blades 3. The whole unit is accommodated in a housing 4. It will be apparent that in reality a larger number of blade wheels is connected in cascade. The operation of only a single blade wheel is of importance in explaining the invention.

In the housing 4 is arranged a first aperture 5 in which an optical output 6 is arranged. This optical output 6 is connected by means of a glass fibre cable 7 to a double-action optical input 8. This latter is con¬ nected to two laser light sources 9 respectively 10, with interposing of a safety device 11. The laser sources 9,10 are coupled to the double optical input 8 by means of optical means which may be formed by air, a vacuum chan¬ nel or for instance again by glass fibres. Included in

this path is a safety device 11, the operation of which will be elucidated later.

Further arranged in housing 4 is a second aperture 12 in which an optical input 13 is arranged which is connected by means of a glass fibre cable 14 to an opti¬ cal output 15, which is connected to two radiation detec¬ tors 16 respectively 17. The optical path between the optical output 15 and the two radiation detectors 16,17 likewise extends through the safety device 11. A control device 18 is further arranged for control¬ ling the relevant components.

The operation of this apparatus will subsequently be elucidated, also with reference to figure 2.

At the point in time t ₁ the first laser source 9 is activated by control device 18 such that it generates a laser pulse P . This laser pulse P has the wavelength 1 determined by the laser source 9. The time duration of this laser pulse is particularly short; in the order of magnitude of a few nanoseconds. This laser pulse is fed via the optical input to the glass fibre cable 7 and fed by means of the optical output 6 to blade 3 ' at which the optical output 6 is directed. The time duration of this laser pulse 9 is so short that even at the high rotation¬ al speed of the blade wheel 2 the blade 3 has barely moved. As a result of the laser pulse striking the blade 3' this latter is subjected locally to an increase in temperature, whereby the radiation intensity of this blade will change and blade 3' will emit a radiation peak according to the curve 19 within the reception spectrum of the first radiation detector.

At the point in time t ₂ the first radiation detector 16 performs a measurement and feeds the relevant result to the control device 18. Between the points in time t _χ and t ₂ the blade 3' has moved little. This means that the displacement is negligible.

Subsequently, one period T further, the reverse procedure takes place; the laser light source 9 emits a laser light pulse P ₂ under the control of control device

18, this at the point in time t ₃ , while at the point in time t ₄ a radiation measurement is performed by radiation detector 17.

The control device 18 processes both measurement results, therein taking into account the theory as ex¬ plained in the above mentioned international patent application.

The safety device 11 prevents radiation emitted by the first laser light source 9 reaching the first radia- tion detector 16; this would result in destruction of the detector due to the fact that the radiation intensity of the radiation emitted by the first laser light source 9 is many orders of magnitude greater than the intensity for which the detector is designed. Conversely, this also applies for the second radiation source and detector.

It is noted here that between the points in time t ₁ and t ₂ a full rotation of the blade wheel takes place.

It is likewise possible however to cause more than one full rotation to take place between these points in time. It is even possible to cause less than a single rotation to take place,- the measurements at the points in time t _χ and t ₂ and at the points in time t ₃ and t ₄ will then however have to be performed at different locations along the path of the blade wheel. Finally, it is pointed out that it is indeed possi¬ ble to choose the points in time t _χ and t ₂ such that a measurable displacement of the blade 3' has occurred therebetween; the two measurements will then have to be performed at different locations, for instance at a short distance from one another.

It is further pointed out that the radiation area of the optical output for radiating the blade 3' with laser light and the interception area of the optical input for conducting the secondary radiation generated through the input area do not have to be exactly the same,- the radia¬ tion area of the laser source will generally be slightly smaller and lie in the centre of the radiation area. It is also possible, taking into account the displacement of

the blade 3' in time, for the radiation area of the laser to lie in the middle of the interception area.

Finally, it is pointed out that an embodiment is shown in figure 3, wherein the optical input and optical output 13 and 6 are combined to a combined optical in¬ put/output 20. This combined input/output 20 is arranged in a single aperture 21 arranged in housing 4 and com¬ prises a dichroic mirror 22 and two lenses 23,24, of which one, i.e. 24, is arranged inside the housing and the other, i.e. 23, is arranged outside the housing. By making use of the dichroic mirror 22 it is possible to use the optical input/output both to supply the light coming from the laser source and to discharge the radia¬ tion emitted by the body 3. Particularly the fact that a part of the optical input/output is arranged inside the housing and another part of the optical input/output is arranged outside the housing herein results in an exceptionally attractive and compact construction; it is noted herein that little space is available, particularly inside the housing.

It will be apparent that diverse modifications can be made to the above described apparatus without depart¬ ing from the scope of protection of the appended claims.