Patent GB-506282-A (GB)
Title: Improvements in or relating to colorimetric apparatus
IPC: G01J3/10
Filed: 1937-11-24
Abstract: 506,282. Colorimeters. SCHOFIELD, R. K., and RUSSELL, Sm E. J. Nov. 24, 1937, No. 32422. [Class 97 (i)] In colour-matching apparatus in which the colour exhibited by a test sample is equated to that of a control sample whilst both sample, are illuminated by liglit of fixed spectra! distribution, the control sample is so selected that its colour quality is Visually equivalent. to the colour quality of the test sample, and the visual brightness of the two colours is adjusted to equality without modifying substantially, by reason of this adjustment, their spectral distributions. The quality of a colour is determined from its co-ordinates X, Y, Z with reference to the axes defined in the proceedings of the Cambridge meeting, 1931 of the Commission Internationale de l'Eclairage (C.I.E.). These proceedings also define three standard sources of illumination. referred to as C.I.E. standard illuminants A, B, and C. The apparatus shown consists of a rectangular casing 11 having hinged covers 16, 17, 18 by which the interior can be rendered light-tight, the cover 18 being in two parts hinged together. The interior is provided with partitions 22 .. 25 defining two triangular compartments containing identical light sources 50, 51. Partitions 29. 30 extend between the front wall and the partitions 24. 25 to define three compartments of which the central one contains a filter system 31. Partitions 32, 33 extend eonvergently from the partitions 22, 23 and are formed at their rear ends with grooves 34 to receive a ridge 36 on a double-compart mented glass filter-cell 35. The space bounded by the rear wall 12; partitions 22, 23, cell 35. and partitions 32. 33 is covered by a panel 38. Behind the cell 35 the wall 12 is provided with an opening 40, behind which is attached an apertured packing-piece 41 and an iron plate 42 formed with windows 44, 45 which are offset as regards the vertical medial line of the plate, Fig. 1. Each of the partitions 22, 23 is provided with a circular aperture covered by a diffusing glass 49 through which the lamps 50, 51 illuminate the windows 44, 45, the angle subtended between the two lamps at the medial position between the windows being approximately 90‹. Within the compartment beneath the panel 38 is an obturator vane 52 which is attached by hinges 53. 54 to the wall 12 so as to lie mid-way between the windows 44. 45. The vane can be tilted about its hinges by a spindle 56 having a pinion 58 engaging a rack segment 61 on the vane, the spindle also carrying a scale 59 which can be read against an index 63 on a window 62 in the lid section 17. When the vane is in its mid-position the light from cach lamp is equally divided between the windows 44, 45, and as the vane is raised the light reaching the window 44 progressively decreases and that reaching the window 45 increases. The inner walls of the lamp housings, both sides of the vane 52, and the wtills of the partitions 22. 23, 32, 33 adjacent thereto, as well as the inner surface of the wall 12, all consist of white diffusing surfaces such that the spectral quality of light diffused therefrom. is substantially the same as that of the incident 'light from the lamps 50, 51. The diffusing surface of the test sample is held against the window 45 so as to be displayed to the illumination and the observer and a control sample which may be a standard white diffusing substance, is held against the window 44. The viewing device by which the two samples are observed comprises an eyepiece 110 at the end of a tube 90 containing mirrors 112, 113 so that the images of the two surfaces are juxtaposed. The filter carrier is provided with apertures 101 .. 104 by which the rays reach the mirrors 112. 113. The filter system consists of a housing 91 provided at its top with nine parallel' slots 93 and in its base with nine parallel grooves 95. Slidably mounted in the housing between the slots and the grooves are three racks 96 of red filter glasses, three racks 97 of yellow filters, and three racks 98 of blue filters. The filter in each rack consists of nine strips of graded density and a strip having no colour and the rack can be sudabty adjusted to bring any strip into alignment with the aperture 101, au opening 107 being provided at the end of each slot 93 through which can be seen an indication of the serial number of the filter so positioned. Each rack is also provided with an untinted glass 106 which compensates the absorption of light by the glass of the corresponding filter, i.e. the absorption which is independent of the darkening factor due to the colour. In making a measurement, for example of a test sample consisting of block of opaque material of violet colour, the observer manipulates the red and blue filters until an approximate visual colour match of the two halves of the field seen through the eyepiece 110 is obtained. The vane 52 is then tilted in the appropriate direction to equate the visual brightnesses of the two parts of the field. Further adjustment of the filters is then made to restore the colour match and the vane is then again adjusted if necessary, and so on alternately until a perfect match between the two halves of the tield is finally obtained. The reading of the scale 59 and the serial numbers of the filter passes in position then define the colour of the test sample in terms of the instrument and in the illumination employed. In order that the results may be capable of general interpretation it is necessary, inter alia, that the illuminant should have a standard spectral distribution. The lamps 50, 51 may be arranged to produce an illumination equal to that of C.I.E. standard A, and the filter 35 may contain the solutions specified in the above-mentioned proceedings to convert the illumination to C.I.E. standard B. The sorial numbers of the filters are those of the Lovibond Colour Sca,le. The results are interpreted by means of graphs, one of which correlates the colour qualities of Lovibond filters to the C.I.E. quality co-ordinates with references to standard illuminant B; the other graph shows the brilliance values of the filters with reference to the said illuminant. The apparatus may be modified by arranging for the relative intensity of illumination of the two samples to be varied continuously over a limited range only and using as the control sample a series of standard sur, faces which can be displayed in turn in the controllable part of the field. Specification 299,194; [Class 97 (i)], is referred to.