GB2235604A

GB2235604A - Transient thermography

Info

Publication number: GB2235604A
Application number: GB9013415A
Authority: GB
Inventors: Alan John Basil Travis; Brooke Armitage Ward; John Michael Green
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1989-06-23
Filing date: 1990-06-15
Publication date: 1991-03-06
Anticipated expiration: 2010-06-15
Also published as: GB2235604B; GB8914437D0; GB9013415D0

Abstract

A transient thermography apparatus including means 1-5 for producing a spot of high intensity radiation upon a region of a surface of a stationary body 6 under test, means 10 for detecting thermal radiation emitted from the region of the surface of the body under test as a result of the irradiation of the region of the surface of the body under test by the spot of high-intensity radiation, at a predetermined time delta t after the application of the spot of high intensity radiation, and means for deriving from the output signal of the radiation detector a thermal image of the region of the body under test for display on a monitor 18. The raster scanning of the detector means 10 over the body 8 is orthogonal to that of the radiation spot so that for each complete raster scan of the body a vertical line of spots 19 is displayed on the monitor. In a second embodiment the radiation spot and the detector means are scanned over the body by two orthogonally arranged mirror drums. <IMAGE>

Description

TRANSIENT THERMOGRAPHY The present invention relates to transient thermography.

Transient thermography is a technique for detecting inhomogeneities or discontinuities within a body by irradiating it with an intense pulse of radiant energy and then subsequently determining the distribution of heat within the body.

In an existing method disclosed in EPO Patent 0 089 760, a body under test is illuminated with light from one or more high intensity flash lamps and the body is observed with an imaging infra-red camera which produces a TV compatible signal which is displayed on a monitor as well as recorded on video tape.

A limitation of the above technique is that imaging infra-red cameras generate their image by raster scanning the surface under observation and have a frame scan period of some 20 ms. Hence if the material from which the body under test is made has too high a thermal diffusivity, the initially uneven temperature distribution which results from the irradiation of the specimen disappears before it can be observed by the camera.

The present invention seeks to overcome this limitation, so broadening the area of application of the technique of transient thermography.

Another method of transient thermography scans a high-intensity spot of radiation from a laser over tghe surface of a specimen under test and then observes thermal radiation emitted by the specimen after a known time interval. The emitted radiation is observed by a thermal imaging camera the line scan of which is linked to the laser heating scan. The specimen under test is moved underthe scan lines of the laser and the thermal imaging camera to enable a thermal wave image of the specimen to be built up. Such a technique is described in a paper by P.K.

Kuo et al published in Rev. Quant. NE 7A 1988 pp 273-277.

However, the above technique is only suitable for relatively small specimens which can be mounted on a a stage and moved by a stepping motor. The present invention on the other hand does not suffer from this limitation and can be used with much larger specimens which may form part of another structure without the need for moving the specimen. This capability extends the applicability of the technique of transient thermography even further.

According to the present invention there is provided a transient thermography apparatus including means for producing a spot of high intensity radiation upon a region of a surface of a stationary body under test, means for raster scanning the spot of high intensity radiation over the region of the surface of the body under test, means for detecting thermal radiation emitted from the region of the surface of the body under test as a result of the irradiation of the region of the surface of the body under test by the spot of high-intensity radiation, means for raster scanning the thermal radiation detecting means over the region of the body under test with a predetermined delay relative to the scanning of the spot of high intensity radiation over the region of the body under test, and means for deriving from the output signal of the radiation detector a thermal image of the region of the body under test.

Preferably the means for producing the spot of high intensity radiation is a laser.

Momentary application to a point on the surface may be achieved by using a pulsed laser which has its beam focused onto the surface directly or by being transmitted to the surface by a fibre optic or other suitable form of optical system, or by using a continuously working laser which has its beam focused and then traversed across the surface of the body under test using moving mirrors or prisms.

Measurement of the surface temperature a short time after the application of the concentrated heat source is achieved by imaging the point on the surface onto a suitable detector, in the former case either directly or with the aid of a fibre optic or other suitable form of optical system, and in the latter by arranging a system of mirrors of prisms to scan the imaged spot along the same path taken by the focused laser beam a short time earlier. The system of mirrors or prisms could be those which are also used to scan the concentrated heat source over the surface of the sample.

The invention will now be described, by way of example, with reference to the accompanying drawings which represent, diagrammatically, embodiments of the invention.

Referring to the Fig. 1 of the drawings, a beam 1 of laser radiation is generated by a powerful laser 2. The beam 1 of laser radiation passes through a focussing lens 3 and falls upon a first galvanometer mounted mirror 4 from which it is reflected to a second galvanometer mounted mirror 5 and thence to a specimen 6. The specimen 6, for example, may be a composite panel which is to be examined for sub-surface defects or a thin film coated substrate which is to be examined for disbonds. The galvanometer mounted mirrors 4 and 5 are so driven by respective horizontal and vertical scan drive systems 7 and 8 that the beam 1 of laser radiation is raster scanned over the surface of the specimen 6 in a vertical sense as shown in the drawing.The laser raster scan is initiated by a frame scan start pulse 9 from a thermal imaging camera 10, which is applied to the horizontal scan galvanometer drive system 7 and also to the vertical scan galvanometer drive system 8 through a delay circuit 11.

The thermal imaging camera, shown generally by the numeral 10, includes a single element infra-red detector 12 upon which infra-red radiation 13 from the specimen 6 is focused by a large aperature objective lens 14. The lens 14 is opague to reflected laser radiation (alternatively, a separate filter can be used). Also in the thermal imaging camera 10 are two rotating prisms 15 and 16 which act to cause the infra-red detector 12 to raster scan the surface of the specimen 6 in a horizontal sense as shown in the drawing. The prism 16 produces the horizontal line scan component and the prism 15 produces the vertical frame scan component of the camera horizontal scan of the component 6.

The output signal 17 from the infra-red detector 12 is displayed on a monitor 18. The motion of the vertical laser beam scan mirror 5 is locked to that of the frame scan prism 15 by means of the frame synchronising pulse 9 generated by the camera frame scan prism 15. The motion of the frame scan prism 15 of the thermal imaging camera 10 is caused to precede that of the vertical laser beam scan mirror 5 by an amount determined by the delay circuit 11.

The net result of the actions of the mirrors 4 and 5 and the prisms 15 and 16 is that the laser beam 1 scans the surface of the specimen 6 in a vertical sense in relation to the drawing and the infra-red detector scans the surface of the specimen 6 in a horizontal sense in relation to the drawing. The relationship between the scans is such that the velocity of the laser beam 1 down the surface of the sample 6 is the same as the vertical frame scan speed of the thermal imaging camera 10 and the two are synchronised although there is a delay between the passage of the laser beam 1 over any given region of the surface of the specimen 6 and the observation of the infra-red radiation 13 from that region by the infra-red detector 12 during a camera line scan. The delay is set by the delay circuit 11.The combined motions of the vertical laser scan and the camera raster scan produce a vertical line of spots 19 on the screen of the monitor 18. These represent a temperature measurement scan down the surface of the sample 6 each measurement being taken at the same time interval 6t after the passage of the laser beam 1 over the corresponding region of the surface of the specimen 6. The total image on the screen of the monitor 18 represents the temperature distribution over the surface of the specimen 6 as it would be bt seconds after an overall irradiation of the surface of the specimen 6. Thus the thermal diffusivity of the material of the specimen 6 is of no consequence. Indeed, the higher it is, the less the possibility of any interaction between the previous vertical scans of the laser beam 1 and the infra-red detector 12.The detection of sub-surface flaws in the specimen 6 may be maximised by adjusting Et until the maximum contrast is shown in the image on the screen of the monitor 18.

A second means for achieving the co-ordinated scans of the irradiating laser and observing detector is illustrated in Figure 2. The laser beam 21 from a laser 22 is reflected from a rotating polygonal mirror 23 onto a second rotating polygonal mirror 24 and thence to the specimen 25 under test. The axes of rotation of the mirrors 23 and 24, 23' and 24', respectively, are orthogonal and the rotational speeds are arranged to be such that the resultant action is to cause the laser beam 21 to scan over the surface of the specimen 25. Radiation from the heated region of the specimen 25 is picked up by the mirrors 23 and 24 and returned to a detector 26 which is displaced from the laser 21.The separation between the laser 21 and the detector 26 causes the detector 26 to receive radiation from any given region of the surface of the specimen 25 a short interval after the laser beam 21 has traversed that region. This interval is a function both of the speeds of rotation of the mirrors and the separation of the detector 26 from the laser 21 and can be adjusted to be of any desired value. The output from the detector 27 is processed and displayed on a monitor 27.

In practice the irradiated spot and the corresponding observed spot will track across the specimen 25 at an acute angle to the longitudinal axis of the total scan. If this is considered undesirable, then the axis of rotation 24' of the rotating mirror 24 can be displaced from its orthogonal position by an angle dependent upon the rotational speeds of the mirrors 23 and 24. It is still necessary, however, for the axes of rotation 23' and 24' to lie in orthogonal planes 28 and 29 respectively, as shown in Figure 2.

Alternatively, the mirror 24 can be driven by a stepping motor so that it is stationary during each scan line. In this case, the axes of rotation 23' and 24' of the mirrors 23 and 24 can remain orthogonal to each other.

Claims

1. A transient thermography apparatus including means for producing a spot of high intensity radiation upon a region of a surface stationery body under test, means for raster scanning the spot of high intensity radiation over the region of the surface of the body under test, means for detecting thermal radiation emitted from the region of the surface of the body under test as a result of the irradiation of the region of the surface of the body under test by the spot of high-intensity radiation, means for raster scanning the thermal radiation detecting means over the region of the body under test with a predetermined delay relative to the scanning of the spot of over the region of the body under test high intensity radiation, and means for deriving from the output signal of the radiation detector a thermal image of the region of the body under test.

2. An apparatus according to claim 1 wherein the means for raster scanning the spot of high intensity radiation over the region of the surface of the body under test comprises two moveable reflecting elements and means for moving the reflecting elements about two orthogonal axes in such a manner as to create the desired raster scan.

3. An apparatus according to claim 2 wherein the means for raster scanning the thermal radiation detecting means over the region of the surface of the body under test comprises two moveable reflecting elements and means for moving them in a manner such as to cause the radiation detector to raster scan the region of the surface of the body under test.

4. An apparatus according to claim 3 and claim 4 wherein the motions of the reflecting elements in the means for raster scanning the spot of high intensity radiation over the region of the surface of the body under test are arranged to produce a raster scan in one sense and the means for moving the reflecting elements which cause the radiation detector to scan the region of the surface of the body under test are arranged to produce a raster scan in a sense orthogonal to that of the spot of high intensity radiation and the relationship between the two raster scans is such that the radiation detector scans any given position on the region of the surface of the body under test a pre-determined interval after the spot of high intensity radiation has irradiated that position on the region of the surface of the body under test.

5. Apparatus according to claim 4 wherein the radiation detecting means and associated reflecting elements comprise a thermal imaging camera and the scan of the spot of high intensity radiation over the region of the surface of the body under test is imiated via an adjustable delay circuit by the beginning of the frame scan of the thermal imaging camera.

6. Apparatus according to claim 2 wherein the means for scanning the spot of high intensity radiation over the region of the surface of the body under test also is utilised to scan the radiation detector over the region of the surface of the body under test.

7. Apparatus according to claim 6 wherein the moveable reflecting elements comprise two rotatable polygonal mirrors the axes of rotation of which are in orthogonal planes.

8. Apparatus according to claim 7 wherein the radiation detector is so displaced from the source of high intensity radiation that it effectively scans the region of the surface of the body under test a pre-determined interval after the spot of high intensity radiation.

9. Apparatus according to claim 7 wherein the axis of rotation of the mirror which produces the longitudinal component of the scan is adjustable in relation to that of the other rotating mirror so as to cause the spot of high intensity radiation and the detector to traverse the region of the surface of the body under test at right angles to the longitudinal axis of the scan.

10. Apparatus according to claim 7 wherein the polygonal mirror which produces the longitudinal component of the scans of the spot of high intensity radiation and the detector over the region of the surface of the body under test is driven by a stepping motor and is arranged to be stationary during each transverse scan line of the spot of high intensity radiation and the detector over the region of the surface of the body under test.

11. Apparatus according to any preceding claim wherein the spot of high intensity radiation is derived from a laser.

12. A transient thermography apparatus substantially as hereinbefore described and with reference to figures 1 or 2 of the accompanying drawings.