Optics: measuring and testing – For light transmission or absorption – By comparison
Reexamination Certificate
1999-10-07
2001-05-08
Pham, Hoa Q. (Department: 2877)
Optics: measuring and testing
For light transmission or absorption
By comparison
C250S339100, C162S198000, C162S263000, C162SDIG006
Reexamination Certificate
active
06229612
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to determining the average paper fiber area density, particularly to an optical method for determining the average paper fiber area density, and more particularly to an optical method for determining the average paper fiber area density (weight per unit area) from the intensity of transmitted and scattered light at a non-absorbed wavelength using a two-wavelength measurement.
The average paper fiber area density (weight per unit area) is an important parameter in the determination of paper characteristics. It is often calculated from the ratio of its dry weight and its total area. This measuring process is quite laborious and can be done only in well equipped laboratories.
Also, the measurement of the water content of paper is of great concern. Recently, an optical measuring technique for measuring water content was developed. See U.S. Pat. No. 4,840,706, issued Jun. 20, 1989 to N. F. Campbell, which was designed for point detection, wherein a modified infra-red scanning gauge was used in measuring the moisture content of a paper-web during manufacturing, utilizing a measurement channel and a reference channel, with the measurement channel preferably being at a wavelength band having a center wavelength of 1.83 micron, and the reference channel preferably being at a wavelength band having a center wavelength of about 1.7 micron. The reference wavelength is remote from the measurement wavelength wherein radiation in the reference wavelength and is substantially unaffected by the moisture in the paper.
As an optical beam transmits through a sheet of paper, which is generally composed of about 20 to 30 multiple layers of cellulose fibers, more or less oriented in some preferred direction, the part of the optical beam which is not absorbed, scatters in all directions. In the forward direction, the intensity distribution of the scattered optical beam generally depends upon dielectric constant, shape, size, orientation of the paper fibers, and the scattered wavelength. In general, it is not evenly distributed. As the number of layers of cellulose fibers increases, scattering processes become more random. However, the forward transmitted optical beam becomes more uniform. Thus, in any sheet of paper, composed of 20 to 30 layers of cellulose fibers, forward scattered optical intensity may well be assumed to be uniform and isotropic. One defines this forward transmitted emission as a diffused emission and it satisfies Lambert's cosine law. See Max Born and Emil Wolf, “Principles of Optics”, Fourth Edition, page 181, Chapter 4.8, Photometry and Apertures.
The present invention involves an optical measuring technique or method wherein the average paper fiber area density (weight per unit area) can be directly calculated from the intensity of transmitted, scattered light at a non-absorbed wavelength coupled with the method of using a two-wavelength measurement approach for water content measurement. Also, it has been experimentally shown that it is possible to derive the water percentage per fiber area density from the two-wavelength measurement. Thus, using this invention, paper area density measurement can be obtained from forward transmitted scattered light. In this optical measuring method, the system can be calibrated initially by an optical method: an optical transmitted intensity at 2.1 microns cellulose absorption line is measured and compared with the same scattered, optical transmitted intensity reference used for density measurement in the nearby spectrum region (1.68 microns), where there is no absorption. From the ratio of these two intensities, one can calculate the scattering absorption coefficient at 2.1 microns and calibrate the system. This absorption coefficient for this wavelength is, then, experimentally correlated to the paper fiber area density.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for determining the average paper fiber area density.
A further object of the invention is to provide paper area density measurement utilizing an optical measuring technique.
A further object of the invention is to provide paper area density measurement from forward transmitted scattered light.
Another object of the invention is to provide a method that will show that the average paper fiber area density (weight per unit area) can be directly calculated from the intensity of transmitted, scattered light at a non-absorbed wavelength.
Another object of the invention is to provide paper area density measurement using a two-wavelength measurement technique.
Another object of the invention is to provide an optical measuring method wherein optical transmitted intensity at one wavelength, such as 2.1 microns, cellulose absorption line is measured and compared with another scattered, optical transmitted intensity reference in the nearby spectrum region, such as 1.68 micron, where this is no absorption, and from the ratio of these two intensities, one can calculate the scattering absorption efficiency at 2.1 microns, for example, and this absorption coefficient at this wavelength is then correlated to the paper fiber area density.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. Basically, the invention involves paper area density measurement from transmitted scattered light based upon experimental data, using the optical measuring method of this invention. It has been shown that the average paper fiber area density (weight per unit area) can be directly calculated from the intensity of transmitted, scattered light at a non-absorbed wavelength. The experimental data has also shown that it is possible to derive the water percentage per fiber area density from a two-wavelength measurement.
In the optical measuring technique, optical transmitted intensity at 2.1 microns cellulose absorption line is measured and compared with another scattered, optical transmitted intensity reference in the nearby spectrum region (1.68 microns), where there is no absorption. From the ratio of these two intensities, one can calculate the scattering absorption coefficient at 2.1 microns. This absorption coefficient at this wavelength is, then, correlated to the paper fiber area density. This method has been experimentally verified using a 0.8-2.2 &mgr;m linear scan camera, and involved monitoring the intensity of the forward scattered optical beam through increasing layers of three different kinds of papers at 1.68 micron and at 2.1 microns wavelengths.
REFERENCES:
patent: Re. 30884 (1982-03-01), Buchnea
patent: 3641349 (1972-02-01), Dahlin
patent: 3904876 (1975-09-01), Arendt
patent: 4577104 (1986-03-01), Sturm
patent: 4840706 (1989-06-01), Campbell
patent: 2044443 (1980-10-01), None
Carnahan L. E.
Pham Hoa Q.
The Regents of the University of California
Thompson Alan H.
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