SOP: Measurement of Colour with the Lovibond Tintometer

GIFSQ — Global Institute for Food Safety & Quality

Principle

The Lovibond tintometer is a subtractive colorimeter based on direct observation of an illuminated sample through a viewing tube. The instrument is provided with sets of red, yellow and blue glass slides to be used as permanent standards. The slides form an evenly graded series from very light tints (0) to deep colours (20), numbered according to their depth of colour. The colour intensities in each series of slides are additive, so that a slide of value 10 is equivalent to two or more slides having a total value of 10. The three series are so related that when three slides of equal value are combined and viewed against white, they appear grey or neutral in colour. By a system of prism and mirrors, the sample is made to occupy half the field of view, while the other half receives reflected light from a standard white surface which passes through coloured glass slides mounted in racks. When the colour of the sample is matched, it is specified by the values of the red, yellow and blue slides required for comparison, as for example: 14.0 R + 6.0 Y + 1.0 B. The Lovibond unit of colour is an arbitrary one, but it has become generally accepted throughout the world and has the advantage of extreme simplicity.

Apparatus required

• Lovibond tintometer with sets of red, yellow and blue glass slides (graded 0 to 20) and neutral tint (grey) slides for brightness measurement
• Standard glass cells for clear liquid samples (examination by transmitted light)
• Porcelain cups for solid samples or opaque liquids (examination by reflected light)
• Spinning cup for samples that are not homogeneous

Procedure

Take the sample in a standard glass cell (clear liquids, examined by transmitted light) or in a porcelain cup (solid samples or opaque liquids, examined by reflected light); use the spinning cup for samples that are not homogeneous

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By the system of prism and mirrors, view the sample occupying half the field of view, the other half receiving reflected light from the standard white surface through the coloured glass slides mounted in racks

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Adjust the red, yellow and blue slides until the colour of the sample is matched

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Note the values of the red, yellow and blue slides required for the match (for example 14.0 R + 6.0 Y + 1.0 B)

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For a bright sample — one in which the colour slides that most nearly match the sample appear dull by comparison — insert the neutral tint (grey) slides in front of the sample; the value of these slides is the measure of brightness

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For a dull sample — one in which all three of the colour slides, red, blue and yellow, are needed to make a match — take the lowest of the three values as the measure of dullness

Interpretation of the reading

Equal values of red, yellow and blue produce a neutral grey. The lowest value is combined with equal values of the other two colours to form a like value of neutral tint or “dullness”. This value is deducted from the respective values of the other two colours. The remaining two colours are combined in equal values of the lower: red and yellow form equal units of orange, yellow and blue form green, and red and blue form violet. The balance, in units of the remaining colour, stays unchanged.

Example: A tomato ketchup whose colour is matched on the tintometer has been found to contain 16, 6 and 1 units of red, yellow and blue respectively. Since all three slides have been used, the value of the lowest is a measure of the dullness. Thus 1 unit of blue combines with 1 of red and 1 of yellow to form 1 unit of neutral tint. The balance of 5 units of yellow combines with 5 of red to form 5 units of orange. The remaining 10 units of red stay unchanged, and the observed colour may be broken up into 1 unit of neutral tint (dullness), 5 units of orange, and 10 units of red.

Precautions

• The drawback of the tintometer is the extremely small area which appears in the “half-field” under the viewing eyepiece, and the eye-fatigue which occurs relatively quickly when making colour matches. A good reproducibility occurs between operators who do not suffer obvious vision defects.
• With the Lovibond instruments, it is more difficult to match dark coloured samples than light coloured samples; this difficulty is due in part to the inherent properties of the human retina.
• Modified instruments: the Lovibond–Schofield tintometer is similar to the basic Lovibond tintometer but is provided with renotated colour filters; the stabilized power pack controls the illumination of the tungsten lamp (Illuminant A), which is corrected by colour filters to Illuminants B and C, and together with the better sample viewing enables the instrument to conform to the CIE observer requirements. The Lovibond flexible-optic tintometer uses glass-fibre, flexible-optic leads, especially suitable for very large, very small or irregularly shaped samples, or for samples which require examination under controlled conditions of temperature and humidity; recordings can be made in both the Lovibond and the CIE systems.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Chamberlin, G.J., The CIE International System Explained, Tintometer Sales Ltd., Salisbury, England — cited in the book.

Kefford, J.F., Food Pres. Quart., 23 (1), 8 (1963) — cited in the book.