Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1999-05-19
2001-08-28
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S339100, C250S339120, C250S339010
Reexamination Certificate
active
06281498
ABSTRACT:
The present invention relates to infrared absorption gauges and in particular to the analysis of signals produced from such gauges to measure a parameter of a sample under investigation.
Infrared absorption gauges are well-known and used for example for measuring constituents of samples (e.g. the moisture content of paper or tobacco, or the fat, protein and water contents of foodstuffs), the amounts of substances absorbed or adsorbed on a substrate, the thickness of coatings or films on a substrate or the degree of cure of resins in a printed circuit board. In this specification, the term “parameter” is used to denote the property (composition, coating thickness etc.) of the sample being measured.
Infrared absorption gauges operate by projecting infrared radiation at two or more wavelengths onto a sample or substrate and measuring the intensity of radiation reflected, transmitted or scattered by the sample. Signals proportional to the measured intensity are processed to provide a value of the parameter being measured. At least one of the two more wavelengths projected by the gauge is chosen to be absorbed by the parameter of interest while the other wavelength is chosen to be substantially unaffected by the parameter of interest. For example, when measuring the amount of water in a sample, one of the wavelengths (the “measuring wavelength”) can be chosen at an absorption wavelength of water (either 1.45 micrometre or 1.94 micrometre) and the other wavelength (known as the “reference wavelength”) is not significantly absorbed by water.
Generally, gauges include an infrared radiation source having a broad emission spectrum and a detector for receiving radiation reflected, scattered or transmitted by the sample; filters are placed between the source and the sample to expose the sample only to the desired measuring and reference wavelengths; in this case, the sample is successively exposed to radiation at the selected wavelengths, e.g. by placing appropriate filters on a rotating wheel in front of the radiation source. Alternatively (but less preferably), the filters can be placed between the sample and the detector and each filter is successively interposed between the sample and the detector. Naturally, if the source can produce radiation of the desired wavelength without the use of filters, then such filters can be dispensed with.
The detector measures the intensity of light after interaction with the sample and produces a signal according to the intensity of the radiation incident upon it. In the most simple case, by calculating the ratio between the signal from the detector when receiving light at the measuring wavelength to that when receiving light at the reference wavelength, a signal can be obtained that provides a measure of the parameter concerned, for example the amount of moisture in a sample. Often, several measuring wavelengths and/or several reference wavelengths are used and the signals of the measuring wavelengths and of the reference wavelengths are used to calculate the parameter concerned.
In fact, conventionally, the value of the parameter is calculated according to the following algorithm (I):
P
=
a
0
+
∑
i
=
1
n
a
i
log
S
i
where:
is the predicted value of the parameter concerned, for example film thickness or moisture content;
a
o
is a constant;
i is 1, 2, 3 . . . and denotes the different wavelengths used;
n is the number of wavelengths used in the gauge;
S
i
is the signal produced when the sample is exposed to a given wavelength i; and
a
s
is a constant to be applied to signal S
i.
The above formula can be derived from the Beer-Lambert law for non-scattering materials. The base of the logarithms in the above algorithm is immaterial because the value of the constants a
i
can be scaled according to the basis of the logarithm used.
The constants a
o
and a
i
can be calculated from a so-called “calibration” set of data, which is a set of infrared absorption data from a range of samples whose parameter of interest is known (so-called “reference samples”); thus by measuring the signals produced when each of the reference samples is exposed at each wavelength, the constants a
o
and a
i
can be calculated by solving a straightforward set of simultaneous equations. It is possible to simplify the calculation by applying a constraint that the sum of the constants a
i
should equal zero. In fact, it is exceedingly unlikely that a single set of constants a
0
and a
i
will produce an exact fit across the whole range of parameter values and therefore the constants a
0
and a
i
are calculated to produce the “best fit” to the data obtained in the calibration set, for example the constants are set to give the minimum residual standard deviation (rsd) for the data concerned. The calibration set of data should be obtained from samples having parameters across the whole range of parameters that will, in practice, be encountered when using the infrared gauge.
It will be appreciated that the accuracy of the infrared gauge can be improved by increasing the number of wavelengths that the gauge uses. However, this greatly adds to the expense, complexity and response time of the gauge and accordingly it is desirable to provide an alternative method for improving the accuracy of infrared gauges.
We have found that it is possible to greatly improve the accuracy of an infrared gauge by the use of a new algorithm (II) as follows:
P
=
a
0
+
∑
a
i
f
(
S
i
)
b
0
+
∑
b
i
f
(
S
i
)
+
c
0
where:
P is the predicted value of the parameter concerned, for example film thickness or moisture content;
a
0
b
0
and c
0
are constants;
i is 1, 2, 3 . . . and denotes the different wavelengths used;
n is the number of wavelengths used in the gauge;
S
i
is the signal produced when the sample is exposed to a given wavelength i;
a
i
and b
i
are constants; and
f(S
i
) stands for a transformation applied to the signal S
i
; this transformation could be a log function, log (1/S) or, indeed, the transformation could be an identity transformation, i.e. no transformation at all). In the latter case, the algorithm would be algorithm (III):
P
=
a
0
+
∑
a
i
S
i
b
0
+
∑
b
i
S
i
+
c
0
In the algorithm of the present invention, it is not necessary to use all the signals to calculate the numerator or the denominator, in which case the value of any constant a
i
or b
i
would be set at zero.
According to one aspect of the present invention, there is provided an infra red gauge for measuring a parameter of a sample, the gauge comprising:
a source of infrared radiation directed at the sample,
a detector for detecting the amount of infrared radiation transmitted, scattered or reflected from the sample at at least one measuring wavelength and at at least one reference wavelength, wherein the parameter absorbs infrared radiation at the said at least one measuring wavelength and absorbs a lesser amount of infrared radiation at the said at least one reference wavelength,
means for calculating the value of the parameter of interest from the intensity of radiation detected by the detector at the measuring and the reference wavelengths, the value of the parameter of interest being calculated according to the following equation:
P
=
a
0
+
∑
a
i
f
(
S
i
)
b
0
+
∑
b
i
f
(
S
i
)
+
c
0
where:
P is the predicted value of the parameter concerned, for example film thickness or moisture content;
a
0
, b
0
and c
0
are constants;
i is 1, 2, 3 . . . and denotes the different wavelengths used;
S
i
is the signal produced when the sample is exposed to a given wavelength i;
a
i
and b
i
are constants; and
f(S
i
) stands for a transformation applied to the signal S
i.
According to another aspect of the present invention there is provided a method of measuring the value of a parameter in a sample, the method comprising:
directing infrared radiation at the sample,
measuring the intensity of infrared radiation reflected, scattered or transmitted by the sample at at least a first wavelength (measurin
Gagliardi Albert
Hannaher Constantine
Infrared Engineering Limited
Kondracki Edward J.
Miles & Stockbridge P.C.
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