The invention relates to a device for remote or non-contact temperature measurement. Such a devices, which is known in the art as a radiometer, performs non-contact temperature measurement and comprises a detector for receiving heat radiation emanating from an energy zone on an object surface of an object of measurement, an infrared (IR) optical system for imaging the heat radiation emanating from the energy zone onto the detector and a sighting arrangement for identifying the position and size of the energy zone on the object of measurement by means of visible light. A further processing arrangement which converts the detector signal into a temperature indication is also connected to the detector.
In this case the IR optical system is so designed that at a certain measurement distance for the most part only heat radiation from a certain area of the object of measurement, namely the energy zone, is focused onto the detector. In most cases the size of the energy zone is defined by the area from which 90% of the heat rays focused onto the detector are emitted. However, applications are also known in which there are reference to values between 50% and 100%.
The pattern of the dependence of the size of the energy zone upon the measurement distance depends upon the design of the IR optical system. A fundamental distinction is made between distant focusing and close focusing. In distant focusing the IR optical system images the detector into infinity and in close focusing it images it onto the focus plane at a finite distance. In the case of distant focusing it is necessary to deal with an energy zone size which grows linearly with the measurement distance, whereas in close focusing the energy zone size can decrease between the radiometer and the focus plane.
In non-contact temperature measurement it is indispensable in practical use that the energy zone on the object to be examined should be rendered visible in a suitable way. In the past, various attempts were made to render the position and size of the energy zone, which is invisible per se, visible by illumination. According to JP-A-47-22521 a plurality of rays which originate from several light sources or are obtained by reflection from a light source are directed along the marginal rays of a close-focused optical system onto the object of measurement. In this way the size and position of the energy zone for a close-focused system can be rendered visible by an annular arrangement of illuminated points around the energy zone.
U.S. Pat. No. 5,368,392 describes various methods of outlining energy zones by laser beams. These include the mechanical deflection of one or several laser beams as well as the splitting of a laser beam by a beam divider or a fiber optic system into several single beams which surround the energy zone. However, these sighting arrangements can only be used in an optical system which images into infinity. In an optical system which images into the finite an image of the detector is reduced and then enlarged by the optical system along an optical axis onto an energy zone on the object from the optical system to the so-called sharp point energy zone.
In U.S. Pat. No. 6,234,669, which is assigned to the assignee of the present application, a device for non-contact temperature measurement of an object is described with an IR optical system in which an image of the detector along an optical axis is imaged onto an energy zone on the object in such a way that the image of the detector decreases in size between the optical system and a sharp point focus zone and then enlarges. A sighting arrangement is also described which identifies the outer limit of the energy zone by means of visible sighting rays. Each sighting ray is aligned obliquely with respect to the optical axis in such a way that each sighting ray can be used both before and also after the sharp point energy zone to identify the energy zone.
Accordingly, improved systems for indicating the extent of the energy zone to a user are the subject of active investigation in the industry.
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