Radiant energy – Calibration or standardization methods
Patent
1998-03-04
2000-06-20
Hannaher, Constantine
Radiant energy
Calibration or standardization methods
G01D 1800
Patent
active
060780428
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a calibration standard for an infrared (IR) absorption gauge. The standard may be used to calibrate such a gauge during measurement of some quantity or parameter. For example, quantities such as moisture content, fat and protein content of foodstuffs, sorbed amounts of substances, thicknesses of coatings or films can be measured; or parameters, such as the degree of cure of resins, may be determined by the gauge. The gauge will need to be calibrated periodically to ensure that accurate measurements or determinations are made and this is where the standard is used.
IR absorption gauges operate by projecting infrared radiation, at two or more wavelengths, onto a sample of material and measuring the amount of radiation reflected, transmitted, or scattered by the sample. Signals, proportional to the measured amounts, are processed to determine the required quantities or parameters. For example, moisture content can be computed from signals which represent the ratio of the amounts by which two different wavelengths are absorbed by a sample. This technique is well known in the art and requires no further description.
A typical absorption gauge includes (a) a sensing head, which houses an infrared radiation source, optical components and an IR detector, (b) an electronic unit, which may be separate and which provides the necessary computation and display of measurements. Calibration standards according to the invention can be used to check the stability and precision of such a gauge at regular intervals.
U.S. Pat. No. 3,478,210 and U.S. Pat. No. 4,082,950, both describe calibration standards using moisture containing materials, which are viewed through windows, in sealed containers. However, with such standards, there is a problem of effectively sealing the moisture containing materials, in their containers, to prevent degradation with time. Hence, long term stability of these standards, over months or years, is rarely satisfactory.
EP-A-0098075, discloses a calibration standard which employs spectrally selective absorption means for transmitting incident infrared radiation, which is then reflected and diffused before being measured. In particular, anhydrous materials, made of glass containing a rare earth substance, such as didymium glass, provides accurate calibration with good long term stability. However, whilst the latter standard solves the problem of long term stability, the available rare earth substances are useful with only a limited number of IR wavelengths. Moreover, it is difficult to simulate accurately the prominent absorption bands of many substances which need to be measured by IR absorption gauges (for example, substances such as powdered or granulated foodstuffs). Hence, there are limitations on the practical usage of standards, in accordance with EP-0 098 075.
The invention seeks to overcome these limitations by providing a calibration standard including an assembly comprising: transmission and reflection characteristics, and absorb infrared radiation in a spectrally unselective manner; selected so as to provide, in the assembly, an infrared reflection spectrum having at least one prominent predetermined wavelength absorption band with a predetermined strength.
Compared with the calibration standard of EP-0 998 075, the calibration standard of the invention is an assembly of integers which are selected to produce a required effect. Consequently, the calibration standard of the invention is not limited to a few special wavelengths, because the integers of the assembly can be selected with regard to their type and individual performance characteristics in order to obtain the necessary spectral response. By way of example only, the interference filter could be either an edge filter, or a three cavity peak element with an appropriate narrow bandpass. The other "optical elements" may include a neutral density filter with an appropriate strength, and/or a front coated aluminium mirror. However, these are only examples of how the invention may be put into practice, because o
REFERENCES:
patent: 3478210 (1969-11-01), Janacek
patent: 4082950 (1978-04-01), Chen
patent: 4465929 (1984-08-01), Edgar
patent: 4832448 (1989-05-01), Jones
Feeney William L.
Hannaher Constantine
Infrared Engineering Limited
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