GB791477A - Improvements in or relating to the measurement of the lustre of textile fabrics - Google Patents

Abstract

791,477. Photo-electric measuring systems. BRITISH RAYON RESEARCH ASSOCIATION. April 11, 1956 [April 23, 1955], No. 11781/55. Class 40 (3). An arrangement for the comparative measurement of the lustre of fabrics comprises means for measuring the variation in the intensity of light reflected by a sample of fabric as the latter is rotated about an axis passing through itself and lying in the plane containing the incident and reflected light beams. In one embodiment (Fig. 3, Prov.) the fabric sample is carried on a table 3 which is rotated slowly (about 1 r.p.m.) by motor 17, light from source 8 reflected by the sample being received by a photo-electric cell 14 the output from which is supplied via a differential circuit (Fig. 5, Prov., not shown) to a penrecorder. Fig. 1 (Prov.) shows a typical trace produced by the latter as the sample (in this a square weave fabric woven from low-twist filament yarn) is rotated through 360 degrees, the peaks E (180 degrees apart) being produced by reflection from the warp yarn and the peaks P (also 180 degrees apart and separated from the E peaks by 90 degrees) by reflection from the weft yarn and the minimum value c being produced by diffusely reflected light which is independent of the orientation of the sample. When the lustre of the fabric is decreased (by standard delustring treatments) the heights a and b, of the peaks E and P respectively, become less and the value c increases. The ratio of maximum to minimum, i.e. (a+c)/c or (b+c)/c is a measure of the lustre and this value is employed as an index for comparative tests so that, by taking successive measurements between delustring treatments the latter may be varied until the fabric conforms to the required standard. When the index ratio is either very high (20 or above, e.g. in the case of bright fabrics) or very low (1.30 or lower) the accuracy of measurement is reduced and to improve the latter, the angle of the incident and reflected light beams are varied by rotating the carriages 6, 7 of the light source and photo-electric cell respectively about the graduated circular base-plate 5, a reduction in these angles reducing the effective ratio and vice versa. The effective ratios may also be varied by employing a polarizing element 13 in the incident light beam and an analysing element 16 in the reflected light beam, the ratio being increased when the planes of polarizations of these elements are " parallel " to one another and reduced when the elements are " crossed." In an alternative constructional embodiment (shown in the drawing accompanying the Complete Specification) the housings 24, 25 of the light source and photo-electric cell respectively are fixed in position with an angular separation of 30 degrees and the sample carrying table 23 together with its driving motor are carried by a swivel arrangement 27 to enable the plane of the table to be altered. The whole arrangement is contained in a light-tight case 22 which also houses the electrical circuit &c. equipment and through a slot in which an adjustable slide 26 projects the latter being provided with a polarizing element set " parallel " to a similar element contained in the photo-electric cell housing 25 and with a shutter for marking the cell during " zero setting or when changing samples. Instead of rotating the sample slowly and employing a pen recorder the sample may be rotated at high speed and the photoelectric cell output supplied to a cathode-ray tube from the trace on which the lustre index may be evaluated. The lustre index may also be obtained, employing high-speed rotation of the sample, by measuring (electronically) the maximum-minimum value of the photo-electric cell output; by separating (electronically) the A.C. and D.C. components (of the curve, Fig. 1), the ratio of which is the required index or; by employing a D.C. moving coil instrument to measure the photo-electric cell output the ratio of the readings of the instrument when the analyser and polarizer are " parallel " and when they are " crossed " giving the required index.