Imaging pyrometer

Details Imaging pyrometer Imaging pyrometer

2002-09-13

2004-07-06

Fulton, Christopher W. (Department: 2859)

Thermal measuring and testing

Temperature measurement

Temperature distribution or profile

C374S121000, C374S141000

Reexamination Certificate

active

06758595

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an imagine pyrometer and a method for simultaneously determining surface temperature distributions and images of remote objects.
2. Description of Related Art
In machine vision and automatic manufacturing, high-resolution temperature maps and visible imagery must simultaneously be acquired that need to be in perfect geometric registration. In such applications, the readout speed of a complete frame or a sub-game must typically be 10 Hz or faster and the temperature range should be between 350° C. and several 1000° C.
U.S. Pat. No. 4,413,324 (Tatsuwaki et al.) describes an imaging pyrometer that makes use of an image pickup device whose pixels are covered with a mosaic of two types of infrared transmissive filters. The signals of two neighboring pixels are converted into a temperature value as in conventional two-wavelength pyrometers. This imaging pyrometer is capable of acquiring a two-dimensional temperature map of a scene. However, no provisions are foreseen to accommodate a large range of temperatures, requiring an unusually large dynamic range of the filtered pixels. Additionally, the temperature map is the only pictorial information acquired, and no visual image of the scene can be taken, for example in the visible spectral range, as is often required in machine vision for high-resolution optical inspection.
International publication No. WO-99/27336 (Koltunov et al.) describes an extension of U.S. Pat. No. 4,413,324 (Tatsuwaki et al.). The number of different types of infrared transmissive filter in a pixel is increased from 2 to N, where N is a natural number. This makes it possible to determine two-dimensional maps not only of the temperature but also the emissivity. As in U.S. Pat. No. 4,413,324 (Tatsuwaki et al.), no provision is foreseen to measure anything other than the temperature and emissivity map, or to accommodate a large range of temperatures requiring a usually large dynamic range of the filtered pixel in the filter mosaic.
U.S. Pat. No. 4,687,344 (Liliquist et al.) describes an imaging pyrometer that partially overcomes the limitations of Tatsuwaki et al. This device also consists of a single image pickup device, but it is completely covered with a single infrared transmission filter. Depending on the temperature range of interest, additional neutral density filters can be inserted to increase the effective dynamic range and therefore the temperature measurement range of the imaging pyrometer. However, since only one type of infrared filter is used, no two-wavelength correction can be made for surface properties of the emitting objects such as varying emissivity or surface finish, potentially leading to incorrect temperature readings. As in Tatsuwaki et al., only a temperature map is produced without any visible image.
U.S. Pat. No. 5,337,081 (Kamiya et al.) describes a triple-view imaging pyrometer, making use of a single image pickup device. The radiation incident from a scene is separated in two or more wavelength bands, and the resulting images of different wavelength bands are imaged onto different areas of the same single image pickup device. In this way, the registration problem is solved that exists when several different image pickup devices are used as described in previous patents. In this way, one can for example simultaneously acquire two-wavelength data for the calculation of the temperature map, as well as a visible image of the scene. However, the necessary optics to achieve the wavelength band separation and image combination for a single image pickup device is not trivial.
U.S. Pat. No. 5,963,311 (Craig et al.) describes an imaging pyrometer that makes use of a single image pickup device. In similar fashion as in Kamiya et al., beam splitting arid image combination optics is used to separate two wavelength ranges in the incoming radiation and to combine the two images onto one single image pickup device. The disclosed optical arrangement assures that the two images are in good geometrical registration on the image sensor. The two images are then used for conventional two-wavelength pyrometric determination of the temperature trap. However, the required optics is not trivial, and only a temperature map is acquired in this method.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an imaging pyrometer and a method for measuring surface temperature distributions of remote objects that overcome the above limitations of the prior art. More particularly, the invention shall solve the following two major problems:
(a) Two images of a scene shall be measured simultaneously and in perfect geometric registration: a reliable temperature map, based on the two-wavelength pyrometric measurement technique, and a high-resolution picture of the scene, for example in the visible spectral range.
(b) The dynamic range for the temperature map measurement and the simultaneous picture acquisition shall be increased compared to the prior art, so that both images are acquired tinder favorable signal-to-noise conditions, and the measurable temperature range between about 350° C. and several 1000° C. is accessible without additional neutral density filters or aperture stops.
The basic idea of the invention is to arrange at least three types of pixels for sensing electromagnetic radiation in at least three different spectral ranges in a mosaic pattern. This pattern has the following properties. In a neighborhood, there are two types of pixels with relatively narrow spectral sensitivity ranges in the infrared (IR), a first one (L) for sensing longer IR wavelengths and the other one (S) for shorter IR wavelengths. Additionally, there is a third pixel type (V) present for receiving electromagnetic radiation in a spectral range which is different from the sensitivity ranges of the first (L) and second (S) pixel types. (“Different” means in this connection that there are wavelengths in one spectral range that are not contained in the other spectral range.) This third pixel type V has the property of being densely arranged and regularly spaced, so that high-resolution, finely sampled images of the scene can be measured through these pixels, without being influenced by the measurements of the IR-sensitive pixels L, S. The sensitivity range of the third pixel type (V) is preferably adapted to the illumination of the scene to be imaged. Preferably, it is broader than the first (L) and second (S) sensitivity range, e.g., at least three times broader, and typically lies within or covers the visible part of the electromagnetic spectrum. Alternatively, it also may be relatively narrow, e.g., for cases where the scene is illuminated by a narrow-band light source such as a light emitting diode (LED).
In a preferred embodiment of the pyrometer according to the invention, a mosaic filter pattern is placed directly on pixels of an appropriate optoelectronic image sensor, for example by evaporation and photolithographic definition. Such color filters are well known in the art and may be, e.g., dielectric layer stacks, dye filters and/or diffractive filters (cf. K. Knop. “Color Pictures Using the Zero Diffraction Order of Phase Grating Structures”, Optics Communications. Vol. 18, No. 3, 298-303, 1976). The first and the second type of IR-sensitive pixels L, S are related to two different types of IR transmission filters, a first one transmitting longer IR wavelengths, with a maximum transmission towards the longest wavelength where the image sensor is still sensitive, the other one at shorter IR wavelengths. The third type of pixels V can be related to a third type of filters transmitting in the visible part of the electromagnetic spectrum, or it could yield maximum sensitivity by avoiding a filter deposition on the corresponding pixels.
As an alternative, one can also use more than one imaging, filter type V
1
. . . V
n
, each of which has a different central wavelength and possibly a different spectral width.
To increase the dynamic radiometric and temperature range, and to ada

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