GB1072139A - Improvements in or relating to spectrophotometers - Google Patents

Abstract

1,072,139. Photo-electric spectrometers. COMMUNICATIONS PATENTS Ltd. Oct. 19, 1965 [Dec. 18, 1964], No. 51659/64. Heading G1A. [Also in Division G2] A spectrophotometer for measuring optical characteristics of at least two samples (e.g. a reference sample of a given colour and a trial sample which is to match the given colour) as a function of a reference (e.g. a standard white reflecting surface) includes a device to direct sequentially to the photo-electric detector the light beam from the reference and from each of the samples, and separate storage means for each resultant output signal, and operates over a range of frequency bands by the use of a plurality of filters each determining a particular band. The stored signals are compared to give an output representing the difference in the light intensity from the two samples as a proportion of that from the reference. In one embodiment, (Fig. 1) a light source 11 is located at the centre of an integrating sphere 10 provided with three apertures 14-16 situated equidistant from an axis through the centre of the sphere. Standard reflecting surfaces or samples are located adjacent the apertures and reflect light from the source out of the sphere. A beam directing means comprising a rotatable plate 24 carrying a lens 27 allows light reflected at one aperture at a time only to reach a detector 35. In reflection measurements a white reference standard and the two samples are located at the apertures. In transmission investigation of transparent or translucent samples, these are located in mountings 38 between the plate 24, and the detector and the apertures 14-16 are provided with white standard reflectors, light from one of these at a time being selected by the beam directing lens to arrive direct at the detector or via either of the samples. The filters comprise a range of filters covering wavelength bands of 20 mÁ over the visible spectrum, and eighteen such filters are carried in a filter disc 29 disposed between sphere 10 and the detector. The disc also has two opaque stops, an open hole and three primary colour filters. Any of these can be selected by manual rotation of the disc 29 to the position required indicated by a scale 33. In operation, the plate 24 is rotated at a few revs/sec, and may be arranged to stop or slow down when the lens 27 is opposite an aperture 14-16. The times when it is in these positions are signalled to the indicator unit 37 by the closing of contacts 48-50 by cams mounted on a shaft coupled to the plate drive. The indicator unit Fig. 2 (not shown) also receives the output of the detector 35, which is proportional to the light obtained from either sample or the reference standard. The closing of a particular contact (48-50) causes the signal from the detector 35 to be fed to respective amplifiers (61-63) according to whether they result from the reference standard, the reference sample or the test sample respectively and the amplifier outputs are each stored by means of capacitors (64-66) respectively connected in parallel with each amplifier. By means of a two-position switch (90), the indicator unit can be arranged to compare the stored values of either the reference sample or the test sample with the standard. Thus, in the former comparison, the output of the amplifier (62) is fed to an amplifier (67) to operate a motor causing rotation of a shaft (74). An electromagnetic clutch, connecting this with the indication needle shaft (57) has been energized and thus this shaft (57) is caused to rotate carrying with it the wipers of potentiometers (70, 72). This allows the stored output from the reference amplifier (61) to be fed back to the input of amplifier (67) in opposition to the output from the sample amplifier (62). Rotation continues until the output of amplifier (67) is zero when the position of the indicator (54) shows the comparative intensity of the reference sample with respect to the standard. A similar indication for the test sample is obtained when the switch (90) is in position T. The difference value between the two samples is obtained on indicator 55. Another two-way switch (92) is provided so that alternative pairs of potentiometers (70 and 71) or (72 and 73) may be used in the measurements. The latter pair are specially contoured so as to produce indications which involve an inverse function of the reflectance or transmission. This is especially useful in computing dyeing mixtures for colour matching. To minimise zero errors in this computation, an alternative form of indicator unit may be used Fig. 3 (not shown) in which the contoured potentiometers are replaced by high precision linear potentiometers, the inverse function being introduced by a function generator. Another embodiment Figs. 4, 4a (not shown) utilizes two lamps (161, 162) at the centre of the integrating sphere, and a feedback-stabilized photomultiplier as detector. The sphere has only one sample aperture, this being in line with detector. Two samples and a reference standard are mounted in a holder which is reciprocated across the aperture once a second. The filter disc is similar to that of Fig. 1 but can also be rotated automatically. On automatic, an external computer controls the rotation of the filter disc to determine which filter is in the light path and also includes a comparator to compare the outputs of the storage amplifiers. When switched to manual, filters may be selected manually and reflectance values of samples compared with the standard may be obtained by a null method.