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INSTRUMENTAL COLOUR MEASUREMENT AND CONTROL 277 an accurate tristimulus colorimeter. The repeatability of colour measure- ment of the Colorede has been obtained using the same pattern, following instrument recalibration (Table VI). Value X ¾ Z Table VI Average difference between repeat readings --0.334 --0.3O6 --0.0717 95 % Confidence interval --0.547 to --0.121 --0.544 to --0.068 --0.329 to --0.185 The repeatability of colour measurement is high, provided attention is given to positioning the sample pattern in the plane of measurement of the optical head. When the Colorede is used on-line in the paper machine, just before the reel, with the moving paper substrate passing below the white tile holder and above an opaque backing of the same reflectance as the substrate (maximum pass line variation + 6.35mm) it is sensitive to colour differences of the order of 0.7 MacAdam units. The stability of the instrument, which was unaffected by the paper machine operating environment, makes it particularly suited to the control of colour in continuous production units. The time between recalibration in continuous operation is as long as 8 hr (Table VII). Table VII Drift on colour difference meters with time 8hr 24 hr Change in AX, AY and /XZ at 100 % reflectance level ___0.3 _+0.5 PRACTICAL RESULTS OF QUALITY CONTROL ON THE PAPER MACHINE The requirement for a commercial match for "fine" papers has been found to be: patterns exhibiting a combined chromaticity and luminosity difference of not greater than 3 MacAdam units (approximately 3 traces in colour or 15ø//o in strength). By passing the Colorede output to an analogue