Improvements relating to radiation thermometers

Abstract: A radiation thermometer comprises an optical objective (43) through which radiation to be detected passes. A beam splitter (63) has a central region (64) through which a central portion of the radiation which has passed through the objective passes undeflected and an outer region which reflects radiation surrounding the central portion. An eye piece (44) receives radiation passing through the beam splitter; and a radiation detector (69) receives radiation reflected by the beam splitter.

Description: The invention relates to radiation thermometers, for example infrared thermometers. In radiation thermometers, visual sighting systems are regularly provided to facilitate accurate aiming of the instrument at the object to be measured. Optimally, these systems are through the lens, i.e. using the same objective lens as the radiation system, to remove parallax errors. Such systems require some form of beam splitting to separate the visible and non-visible radiation (usually infrared) beams within the instrument. Traditional methods for achieving this beam splitting have been either spectral, using spectral defining filters to transmit the visible spectrum whilst reflecting the infrared spectrum (or vice versa), or spatial in which a portion of the beam is sampled by the intrusion of a pick-off mirror. Whilst the spectral methods offer the highest theoretical efficiencies (lowest energy losses) they require expensive components which often need to be specifically tailored to the spectral response of the different instrument type being constructed. Further, the traditional spatial methods require the secure mounting of a small and accurately aligned mirror in the beam, and an additional beam steering mechanism in order to have the visible beam exit at the rear of the instrument. JP-A-57-84322 describes an optical measuring device in which radiation is incident on an objective lens which focuses the radiation on a focal plane coincident with an annular mirror, some of the radiation then being reflected to an eye piece for visual alignment with an object whose temperature is to be measured. The radiation which is used for measuring the temperature of the object is transmitted through the mirror to a radiation detector. However, this apparatus is such that the object whose temperature is to be measured appears as a black spot within the viewed image and is therefore not actually visible to the operator, thus preventing accurate alignment of the detector with the object whose temperature is to be measured. JP-A-42-44925 describes a radiation thermometer in which radiation is transmitted through a sub-mirror directly to an eye piece. Radiation which is used for measuring the temperature of the object passes around the edge of the sub-mirror and is then reflected from a main mirror, the sub-mirror and a further mirror to a radiation detector. In this case, optical radiation used for alignment and radiation used for temperature measurement take substantially different paths through the apparatus and accordingly, the radiation which is detected by the detector is not from the same source as the radiation which is detected by the eye piece. In accordance with the present invention, a radiation thermometer comprises an optical objective through which radiation to be detected passes and is focused; a beam splitter having a central region through which a central portion of the radiation which has passed through the objective passes undeflected and an outer region which reflects radiation surrounding the central portion, the beam splitter being spaced from the focal plane of the objective; an eye piece for receiving radiation passing through the beam splitter, the eye piece being positioned at the focal plane of the objective; and a radiation detector for receiving radiation reflected by the beam splitter. We have realised that in JP-A-57-84322, the eye piece does not see the information which is seen by the detector. This significantly reduces the usefulness of this approach. The reason for this is that the objective lens focuses the incoming radiation into a focal plane coincident with the beam splitter. In contrast, with the invention, by positioning the beam splitter spaced from the focal plane, the eye piece sees radiation from all parts of the target although with a reduced intensity. Typically, the central region will comprise a physical hole through the beam splitter but alternatively could be realised by masking the central region of a transparent substrate during a coating process with a reflective coating (e.g. aluminium) while leaving an outer region unmasked, and then removing the mask. Conveniently, the eye piece is physically aligned with the central region of the beam splitter and with the objective so that the visible radiation passes undeflected through the objective to the eye piece. However, the path of the radiation could be adjusted using conventional optical systems if necessary. The invention is particularly suited for use with infrared thermometers but could also be used with other types of radiation thermometer of known construction. An example of an infrared thermometer assembly according to the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section through the assembly; and, Figure 2 illustrates the optical arrangement in more detail. Figure 1 illustrates an infrared thermometer assembly 10 having a housing 40 which at its front end has a bore 41 in which is slidably mounted a housing 42 supporting an objective lens assembly 43. The rear end of the housing 40 supports an eye piece assembly 44 having a central bore 45 at opposite ends of which are mounted a pair of lenses 46,47 with a graticule 48 upstream of the lens 47. As seen in Figure 1, infrared radiation entering the objective lens assembly 43 passes to a beam splitter 63 to be described with reference to Figure 2. The beam splitter 63 has a central aperture 64 through which some of the light passes to the graticule 48 and from there to the eye piece assembly 44. Radiation surrounding the central portion is reflected by the beam splitter 63 onto an infrared detector 69. In order to move the objective lens housing 42 relative to the housing 40, a control mechanism is provided which includes a focusing knob 49 rotatably mounted about the eye piece assembly 44 and constituting a control member, the focusing knob having a gear 50 which intermeshes with a pinion 51 rotatably mounted in a side extension 51A of the eye piece assembly. The pinion 51 terminates in a screw threaded spigot 52 which is received in a correspondingly screw threaded socket 53 of a push rod 54 secured at its other end to the objective lens housing 42. As the knob 49 is rotated, this will rotate the pinion 51 which in turn rotates within the spigot 53 causing linear movement of the push rod and consequently linear, sliding movement of the objective lens housing 42. A compression spring 55 acts between the eye piece assembly and the push rod 54 to remove any free movement. Figure 2 illustrates the optical arrangement in more detail. Infrared radiation 60 is received by the objective lens assembly 43 and focused onto the graticule 48. The beam splitter 63 is positioned downstream of a field stop 61 defined by part of the housing 40. The beam splitter 63 has a central aperture 64 through which a central portion 65 of the beam passes undeflected to the graticule 48 and to the field lens 47 and the eye piece lens 46. Radiation in the region surrounding the central region 65 is reflected by the beam splitter 63 as shown at 68 on to the infrared detector 69. Conveniently, the beam splitter is a transparent member which is coated in the surrounding (or outer) region with a reflective material (e.g. aluminium) and is left uncoated - the central region. In use, the operator looks through the eye piece assembly 44 and focuses the radiation by rotating the knob 49. Once this has been achieved he then observes the image indicating the target from which infrared radiation will be received and if necessary adjusts the orientation of the housing 40 so that the thermometer is aimed at the correct part of the target. Thereafter, the operator will know that the infrared radiation received by the detector 69 has been received from the desired target.

Claims: A radiation thermometer comprising an optical objective (43) through which radiation to be detected passes and is focused; a beam splitter (63) having a central region (64) through which a central portion of the radiation which has passed through the objective passes undeflected and an outer region which reflects radiation surrounding the central portion, the beam splitter being spaced from the focal plane of the objective; an eye piece (44) for receiving radiation passing through the beam splitter, the eye piece being positioned at the focal plane of the objective; and a radiation detector (69) for receiving radiation reflected by the beam splitter. A thermometer according to claim 1, wherein the central region comprises a physical hole (64) through the beam splitter (63). A radiation thermometer according to claim 1, wherein the central region (64) comprises an uncoated area of a transparent substrate. A radiation thermometer according to claim 3, wherein the outer region is coated with a reflective material. A thermometer according to any of the preceding claims, wherein the eye piece (44) is physically aligned with the central region of the beam splitter and with the objective so that the visible radiation passes undeflected through the objective (43) to the eye piece (44). A thermometer according to any of the preceding claims, wherein the thermometer further comprises a graticule (48) onto which the image from the objective lens is focused. A thermometer according to any of the preceding claims, wherein the thermometer further comprises a field stop (61) positioned between the optical objective (43) and the beam splitter (63). A radiation thermometer according to any of the preceding claims, further comprising a housing (40) to a front end of which is movably mounted an objective lens housing (42) supporting the objective (43), and to a rear end of which the eye piece (44) is mounted; and control means (54) for causing relative movement between the objective lens housing (42) and the radiation thermometer housing (40), the control means being connected to a manual control member (49) for operating the control means, the control member being mounted to the radiation thermometer housing at a position spaced from the objective lens housing. A thermometer according to claim 8, wherein the control member (49) is positioned at the rear of the radiation thermometer housing. A thermometer according to claim 8 or claim 9, wherein the control member (49) comprises a knob which is rotatably mounted about the eye piece (44). A thermometer according to any of claims 8 to 10, wherein the control means includes a pair of members (52,53) connected together by a screw threaded spigot/socket connection, whereby rotation of one of the members (52) connected to the control member (49) causes linear movement of the other member (53) which is connected to the objective lens housing (42).

Inventors: FEARNEHOUGH PETER, RIDLEY IAN HAMILTON, FEARNEHOUGH,PETER, RIDLEY,IAN HAMILTON
Applicants: LAND INSTR INT LTD, LAND INSTRUMENTS INTERNATIONAL LIMITED
Application year: 1995
Main classification: G01J5
IPC codes: G01J5