METHOD AND APPARATUS FOR IDENTIFYING PARTICULATE MATTER

Abstract: A witness plate (12) collects dust or other particulate matter in a controlled environment. The plate is illuminated by a light source (14) so that only the dust appears bright, and causes bright spots to be focussed on a focal plane (25), defined by a CCD array (24). The focal plane is read out on a computer (32) which is programmed to provide sizes and count of the dust particles.

Description: METHOD AND APPARATUS FOR IDENTIFYING PARTICULATE MATTER BACKGROUND OF THE INVENTION The present invention relates to a method and apparatus for identifying surface particulate contamination and, in particular, to contemporaneous witnessing and identification of such surface particulate contamination. Cleanliness or the lack of contamination in opti¬ cal systems is proportional to the quality of sensing and detection obtained. Prior to assembly, structural components and piece parts are normally washed in special spraying facilities equipped for the collection of the solvent run-off. The solvent is then filtered for collection and optically counting of the residue of particles. Because a small device can be more easily cleaned than a complex assembly, for such complex assemblies as knife-edged structures, an alternate approach is to utilize a witness plate, for example, a one-square foot sheet of Kapton to accompany the piece. The wash technique is subsequently applied to this witness sample. Witness plate techniques using smaller optical mirrors or black glass, one inch or greater, lend them¬ selves to direct optical counting of the dust, such as is obtained by a Saab photometer, which provides an integrated scatter roughly proportional to the commu- nitive area covered by dust. Experimental apparatus utilized a high resolution photo of a witness plate taken with a dedicated camera, with special grazing incidence lighting. With the Saab photometer, a single electrical reading results, with no further information regarding the size distribution of the dust particles. Primary disadvantages of the other technique is one of delay. A complete readout of the dust on a witness mirror or sheet, whether by optical devices directly or by washing and analyzing the wash takes considerable time. The final counting is performed on a photographic negative or a special filter paper and is manually done. No technique, therefore, lends itself to instan¬ taneous, on-site monitoring, with full detection of the several species of contaminants according to size. In addition, such known devices utilize moving parts and are intricate and troublesome to design, build and operate. One device translates a witness plate in one direction while a laser beam is swept across the sample in a back and forth raster along the other direction. The light is then picked up by an optical system and an integrating sphere, where it is detected. Another device utilizes circular witness plates similar to a phonograph record. The plate is spun in the device while a focussed beam illuminates the small spot. The beam thus tracks the plate much like the stylus follows a record groove. Such devices are formed from several moving parts, are intricate, and are troublesome to design, build and operate. Because of their detail and intricacy, they are some¬ what difficult to use, are subject to error, and are relatively expensive. SUMMARY OF THE INVENTION The present invention overcomes and avoids these and other problems by providing a witness plate used to collect samples of contaminants within a chosen environment. An optical imager focuses the illuminated contaminants onto a detector focal plane, such as a charge-coupled detector array. The information detected is then processed by a computer, which is programmed to provide such identification as sizes and count of the contaminants. Various advantages are obtained from this arrange¬ ment. The very few moving parts enable the present invention to be simply constructed and used, less prone to error, and less expensive. Identification of parti- culate contaminants is direct, as being immediately identifiable as to size and count, and provided auto¬ matically without delay. Other aims and advantages, as well as a more complete understanding of the present invention, will appear from the following explanation of an exemplary embodiment and the accompanying drawing thereof. BRIEF DESCRIPTION OF THE DRAWING The sole figure is a view of apparatus for automatic counting of particulate contaminants on witness mirrors. DESCRIPTION OF THE PREFERRED EMBODIMENT Accordingly, an apparatus 10 for viewing particu- late contaminants, such as dust, is positioned to view a witness plate 12 onto which the contaminants are to be deposited. Witness plate 12 is placed in and adja¬ cent to optical equipment where contamination is desired to be monitored. To illuminate the particulate contami¬ nation which alights onto the witness plate, a light source 14 is positioned to direct light 16 at a grazing angle with respect to the witness plate, so that the particles only are well illuminated. Apparatus 10 includes a high resolution lens system 18 which is secured to a housing 20 by an adjustable connection 22, such as a bellows. Positioned at the far end of housing 20 is a charge-coupled device (CCD) array 24 of conventional construction which defines a focal plane 25. Bellows 22 is adjusted in order to sharply focus the images of the particulate contamination onto the focal plane of array 24. Such adjustment may be obtained by utilizing a flip mirror 26 which is pivotable about point 28 so that the contaminants may be focussed on a viewing screen 30. Such focusing is conventional, similar to that of a single lens reflex camera. Once the focussing is sharply defined on screen 30, mirror 26 is pivoted so that the image will then be focussed on the focal plane at charge coupled device array 24. The images sensed on array 24 are electrically conducted to a computer 32 of conventional design, e.g. , Hewlett Packard 9836. The computer quickly calculates the sizes and the count of the particulate contaminants, which calculation and identification are then displayed or otherwise placed in graphic form. The focal plane preferably is a CCD array, which may be either a stationary array or a commercially available line array which sweeps through the image from plate 12. Such commercially available units currently provide about 1,600 image elements on a line, swept through 2,000 positions automatically. Although the invention has been described with reference to a particular embodiment thereof, it should be realized that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

Claims: CLAIMS What is Claimed is; 1. Apparatus for identifying particulate matter on a witness plate comprising: means for providing an illumination of only the particulate matter; a focal plane array of charge-coupled devices; means for focussing the illumination on said focal plane array; and means coupled to said focal plane array for providing descriptions of the particulate matter. 2. Apparatus according to Claim 1 in which said focal plane array comprises a stationary array of charge-coupled devices. 3. Apparatus according to Claim 1 in which said focal plane array comprises a line array of charge- coupled devices sweeping through the illumination. 4. Apparatus according to Claim 1 in which said focussing means comprises a high resolution lens magnifier. 5. Apparatus according to Claim 1 in which said description providing means comprises a computer for providing sizes and count of the particulate matter. 6. Apparatus according to Claim 1 further including a viewing screen and a pivotable mirror positionable betweeen the witness plate and said viewing screen for enabling said focussing means to be finely focussed. 7. Apparatus according to claim 1 in which said illumination means, said focal plane, said focussing means and said description providing means are placed in and adjacent to the contaminants' environment. 8. A method for instantaneous sizing of particulate matter comprising the steps of: illuminating the particulate matter; focussing the illuminated particulate matter onto a focal plane array; and obtaining the instantaneous sizing from the focal plane array.

Inventors: CHAMPETIER ROBERT J, CHAMPETIER, ROBERT, J.
Applicants: HUGHES AIRCRAFT CO, HUGHES AIRCRAFT COMPANY
Application year: 1987
Main classification: G01N21
IPC codes: G01N15, G01N21