GB755725A - Improvements in or relating to light reflectivity and transmissivity comparators - Google Patents

Abstract

755,725. Photo-electric transmissivity and reflectivity measurements. STEELE, J. A. P., and BURDEN, A. G. April 15, 1954 [Jan. 16, 1953], No. 1316/53. Class 40 (3). Apparatus for measuring the transmissivity or reflectivity of a sample comprises a photocell arranged to receive light reflected from the surface or transmitted by the sample. The invention is described with reference to determining the whiteness and matting of laundry although other applications are mentioned. As shown, the apparatus consists of a control box 1 carrying a top panel 2 on which are mounted controls and a microammeter 3. The control box is joined by two tubes 6, 7 used for carrying connecting leads to two compartments 4, 5 open at the bottom and top respectively. The cross-section of these compartments is shown in Fig. 2. The top compartment contains two 10-watt lamps 16, 17 and a photo-cell 15 screened from direct light from the lamps, the compartment being closed by a glass plate 11. The bottom compartment contains a 40-watt lamp 20 and a flashed opal glass plate 12. The edge of each box is provided with rubber beading 9 by which, during reflectivity tests, the sample is gripped between the two compartments. Gloss measurements may be made by switching out one of lights 16, 17. The opal glass, plate serves as a standard of whiteness when no sample is in position. Transmissivity of the sample is measured by using lamp 20 only. Compartment 4 may be detached and used separately, connecting leads to the control box being provided. Circuits and operation.-If reflectivity is being compared with that of a standard, switch 21 is moved to its upper position, thus connecting series lamps 16, 17 and disconnecting lamps 20 in circuit. Closure of switch 24, or, if the unit 4 is being used separately, switch 18 carried by the unit, energizes lamps 16, 17 and supplies a backing-off potential to micro-ammeter 3 in opposition to the output voltage of photo-cell 15. With the standard specimen in position the backing-off voltage is adjusted by potentiometer 26 until the meter needle is brought to desired position. The standard is then replaced by the sample when the micro-ammeter indicates directly the difference of reflectivity. When whiteness is being tested the opal plate is used as a standard and a calibrating resistor 28 is put in series with the cell to reduce its output. The sensitivity of the meter may be varied by a starting variable resistor 30. Acting germanium diodes 31, 32 are provided to protect the meter.