GB707651A - Apparatus for use in estimating the total area and/or size-distribution and/or totalnumber of discrete particles situated in or projected on a field of view - Google Patents

Abstract

707,651. Photo-electric particle counting apparatus. NATIONAL RESEARCH DEVELOPMENT CORPORATION. July 25, 1952 [May 3, 1951], No. 10355/51. Class 40 (3) An estimate is made of the total number of particles in a collection and of the number in each of a number of size groups by traversing a mesh of alternate opaque and clear spaces relatively to the collection and measuring the mean square current of a photo-electric cell receiving light through the mesh. The invention is based on that fact that if the fluctuating output current of the photo-cell is analysed into a number of frequencies by the use of filters in the photo-cell circuit then the mean square current I<SP>2</SP> r in a frequency band of width df and with a centre frequency f where f is small compared with <SP>1</SP> t (t being the time for the boundary of light area on the grid to traverse the particle) is given by the equation I<SP>2</SP> f = C d f # N k A k <SP>2</SP> where N is the number of particles of area A k . Thus the value of # Nk Ak<SP>2</SP> may be determined by measuring the mean square current at a low value of f or by observing this mean square current when the pattern is moved at a high speed so that :all frequencies are small compared with <SP>1</SP> t . If a particle is of comparable size to the mesh its effect on the means square current will be negligible, thus by varying the size of the mesh a measure may be made of the number of particles in particular size groups. Alternatively if a large mesh is used a curve may be plotted of mean square intensities at different frequencies this curve then being compared with a set of curves appropriate to various size distribution. As shown, Fig. 3, the specimen is mounted on the stage 4 of microscope 5. A parallel beam of light illuminates a grid 7 and a polygonal rotating mirror 10 traverses a reduced image of the mesh, produced by a lens system 8 across the specimen. The light passing through the specimen is concentrated by the microscope on to a photo-cell 11 which is connected through one or more filter networks to root-mean square measuring devices. Instead of reducing a large mesh optically a small mesh, such as a powder sieve may be used. In an alternative arrangement the image of the grid may be projected through the eyepiece, the photo-cell being placed below the stage. In the arrangement of Fig. 4 (not shown) the grid is fixed in the place of the image of the specimen produced by the objective. Relative movement between the field of view and the grid is achieved by means of a transparent rectangular block rotating between the grid and the specimen.