Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1985-02-28
1987-09-01
Rosenberger, R. A.
Optics: measuring and testing
By particle light scattering
With photocell detection
356236, 356246, G01N 2149
Patent
active
046905600
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the optically based measurement of fluid parameters and as such has application to any context in which a fluid, which may be a sample, emits light in random directions in response to a defined incident beam. The emitted light may indicate, e.g., a scattering or fluorescence effect.
A particular class of measurement of interest is nephelometry, in particular turbidimetry of liquids. In the classical nephelometer, a beam of incident light is passed through the test liquid so that some light is scattered by particles suspended in the liquid. The light which is not scattered, or is scattered through a very small angle only, continues on to a transmission photocell detector. The ratio of the detected to the incident intensity, taking into account the path length through the liquid, is considered to be a reliable measure of the turbidity of the liquid, especially for medium scattering power.
Where the test liquid is contained in a sample cell, it is known to employ an integration sphere to collect and measure the scattered light. This sphere was first used to collect a conical portion of the scattered light. In order to improve sensitivity at very low degrees of turbidity, it has been proposed, in U.S. Pat. No. 4,320,978 to Sato, to mount the sample cell as a straight transparent tube diametrically of, and substantially wholly within, the integration sphere. The incident light is a collimated beam of width matching the interior cross-section of the tubular cell and is fully absorbed after one traversal to ensure that the transmission light is not variably reflected or diffused back into the sample liquid.
The present invention stems from the realization that an especially simple and sensitive measuring instrument, having application as a nephelometer or fluorimeter, may be obtained by appreciating the utility of total internal reflection within a sample cell.
The invention accordingly affords optical measurement apparatus comprising:
a cell adapted to receive fluid for testing;
a light source arranged to direct an incident light beam into the cell; and
means to detect at least a pre-determined portion of light emitted by a segment of the cell transversely of the direction of said incident light beam;
wherein the light source and the cell are arranged so that the light incident on the cell is not collimated but, as it traverses said segment of the cell, is confined to the cell by total internal reflection unless otherwise scattered by particles in the cell.
The segment of the cell may of course comprise the whole cell.
The apparatus may further include an aperture through which the incident light beam diverges into or towards the cell at an angle below the critical angle for total internal reflection in the cell.
Preferably, the incident light is arranged to traverse said segment of the cell more than once. This may be effected by configuring the sample cell as a helical tube or by placing reflectors at opposite ends of a straight elongate cell.
The detection means is advantageously a hollow integration sphere or ellipsoid enclosing said segment of the cell, and a light sensitive detector on or adjacent the inside surface of the sphere or ellipsoid.
The invention also provides a method for carrying out optically based measurement of a fluid parameter, comprising directing uncollimated incident light into a cell containing the fluid so that, as it traverses a segment of the cell, the light is confined to the cell by total internal reflection, unless otherwise scattered by particles in the cell, and detecting at least a pre-determined portion of light emitted by said segment of the cell.
The invention will be further described, by way of example only, with reference to the accompanying drawings, in which FIGS. 1, 2 and 3 schematically depict, in partial cross-section, three alternative embodiments of apparatus in accordance with the invention, intended primarily for use as nephelometers. Like parts in the drawings are indicated by like reference numerals wherever appropriate.
The measurem
REFERENCES:
patent: 4212539 (1980-07-01), Berber et al.
patent: 4320978 (1982-03-01), Sato
patent: 4488814 (1984-12-01), Johnson
Fujiwara et al, Liquid Core Optical Fiber Total Reflection Cell as a Colorimetric Detector for Flow Injection Analysis, Analytical Chemistry, May 1985, pp. 1012-1016.
Lei et al, Determination of Phosphorus in Natural Waters by Long Capillary Cell Absorption Spectrometry, Analytical Chemistry, May 1983, pp. 951-955.
Commonwealth Scientific and Industrial Research Organization
Rosenberger R. A.
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