Evaluation of the Hunterlab Colour Measuring Instrument

Preliminary work done by the Sugar Milling Research Institute (SMRI) showed that the HunterLab colour measuring system could possibly be used to estimate the colour of raw sugar. With this system it was possible to measure the colour within a short time (less than one minute). The instrument (ColorQuest LAV 4S0/0") has been used for several seasons at the Transvaal Sugar Limited Malelane (ML) factory for measuring the colour of brown sugar. Work was also done on refined and C-sugars. This paper summarises the evaluation of this instrument by both the SMRI and ML. Introduction Due to market requirements, ML produces two types of raw sugar, namely light (1000 ICUMSA 420 colour) and dark ( 1 500 ICUMSA 420 colour) and the same equipment (pans, crystallisers, etc.) in the raw sugar factory is used to make the two types of sugar. In order to assist the packaging department personnel to decide which type of brown sugar has to be bagged, a rapid indication of raw sugar colour is required. The thirty minutes required to determine colour by Figure I. the ICUMSA method is too long and it was therefore decided to evaluate instruments that could measure the colour of The spectrally based system is considered to measure 'what sugar "on-line". the eye sccs' and utilises the measurement of three parameThere are a number of instruments available which are suitable for this task (Nielson, 1996 ; Anon, 1996). The SMRI evaluated two HunterLab instruments during 1995 and 1996, namely the Miniscan which is a portable instrument (Dunsmore, 1995) and the ColorQuest LAV 45"/0°, which is a laboratory model, with a view to their suitability for use in the South African sugar industry. The laboratory model was subsequently evaluated by the Malelane factory and used i n the factory during the past three seasons. Description and basis of operation of the ColorQ~iest system The ColorQuest spectrophotometer (Figure 1) is a colour measurement system with a choice of optical sensors depending on the application. Each sensor has its own illumination and viewing geometrics. The system evaluated was the ColorQuest LAV 45"/0°. ters, the tristimulus values. These parameters are represented by three axes, which are illustrated in Figure 2. The vertical axis depicts the lightness "L" of the sample expressed on a scale of 0 to l00 where black = 0 and white = 100, the horizontal axis runs left to right and depicts "a" the redness or greenness, where the redness is positive and the greenness is negative, and the third axis runs from front to back and represents "b", the yellowness (positive), or blueness (negative). In addition, the instrument's software allows these three parameters to be combined and expressed in one of three indices. These are "Z", the brightness index, "WI", the whiteness index and "YI", the yellowness index. The system consists of a sensor and a detector. The sensor uses circumferential illumination, which is provided by a fibre optic ring, to accommodate a variety of samples. Light from the fibres illuminates the sugar sample (400 grams) to be analysed at an angle of 45". The diffused light reflected from the sample is collected at 0" and then directed to a difProc S Afr Sug Teclznol Ass (1999) 73 Evaluation of the HunterLab colour measuring instrument M Moodley, A Dunsmore & L De Jager