J. Soc. Cosmetic Chemists 9,0 265-281 (1969) ¸ 1969 Society qf Cosmetic Chemists of Great Britain Instrumental and control colour measurement P. R. BUNKALL and M. QUINN* Presented at the symposium on "Technical developments in cos- metics packaging", organised by the Society of Cosmetic Chemists of Great Britain, at Harrogate, Yorks., on 27th March 1968. Synopsis--The introduction of instrumentally defined tolerances for colour control is a problem which is dependent on the availability of reliable colour measuring equipment, techniques to express instrumentally measured colour differences in visual terms and common agreement between manufacturer and customer on the size of acceptable tolerance limits. Some of the more reliable colorimeters are mentioned and their main features compared. Indication is given of some of the preferred colour difference oeormulae and particular reference is made to the use of the graphical method of Simon and Goodwin. Practical comparisons of this method with visual assessment are made. Sophisticated equipment for the automatic determination of colour differences is suggested and an automatic on-line colour monitor, which has been used in continuous colour control on a paper machine, is described. INTRODUCTION The aspect of manufacture termed quality control is of great im- portance to the packaging industry since it represents an activity in which significant productivity losses can result either through the use of in- adequately described colour tolerances or through inaccurate assessment of whether material is within tolerance. Traditional methods of colour control rely entirely on human response to shade variations of the manufactured article. A sample from a previous manufacture is frequently retained as a standard and the present manu- facture compared with it. The colourist has then to decide on a tolerance and, if this is exceeded, take the necessary corrective action to bring the *Imperial Chemical Industries Ltd., Dyestuffs Division, l•ianchester 9. 265

2•3•3 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS colour within tolerance. Modern high speed methods of manufacture, where material is coloured continuously as it passes through the machine, serve only to highlight the shortcomings of antiquated colour control methods, and off-machine subjective quality control is no longer sufficient for today's packaging materials. Colorimetry presents an opportunity to define toler- ances in precise mathematical terms. Provided all parties accept the instrumental decision as to whether material is acceptable or otherwise, problems of variable visual judgment, and disagreement between tolerance sizes set by different observers are removed. The multiplicity of colour measuring instruments and methods for colour difference determination presents advantages and disadvantages. On the one hand, the user is able to select equipment which is most suited to him with regard to cost and convenience of use. On the other hand, progress toward standardisation of equipment and application techniques for colour tolerance setting has been impeded. Enough information has been collected, however, to permit some recommendations to be made and to indicate the reliability of certain techniques. There is, nevertheless, a considerable amount of work still to be done before a clear picture emerges, and before the C.I.E. will be in a position to make official recommendation on the subject. THE ROLE OF INSTRUMENTATION IN COLOUR CONTROL There are three distinct types of application of instruments in colour control work: Colour control in a batchwise colouring process A sample of material is removed from the machine at the end of the colouring process, its colour measured and the colour difference of the sample from the control determined. An additional colouring process is carried out if the sample is outside tolerance. Colour sorting Batches of material which have been coloured in separate processes and accepted as being within a given colour tolerance may require sorting into lots which fall within a smaller colour tolerance. This problem occurs mainly in the textile industry where cloth from different dye lots may be