Colour measurement and its rating scales

How to assess the colour based on the measurement? What are the rating scales and how do they differ? We will answer these questions in this and the next article.

Operation and construction of the spectrophotometer

The colour measuring instrument became very popular when it became clear it enables determining the colour of the test substance in an accurate way, i.e. using numbers. The big advantage of the spectrophotometer is the speed of analysis and simplicity of sample preparation. The spectrophotometer measures how much light transmitted through the sample it has absorbed.

The mechanism of operation of this device is based on the Lambert-Beer law, which determines the relationship between the absorbed light and three factors, such as the concentration of the substance, the length of the path that light travels when passing through the analysed substance, and the extinction coefficient of the test substance. The path length is known during each measurement and equals the width of the measuring cuvette.

The spectrophotometer consists of several elements:

• light source, which is usually a deuterium or hydrogen lamp,
• monochromator, selecting from the entire spectrum only the radiation flux of the selected wavelength, and then transmitting this light through the cuvette containing the analysed sample,
• measuring cuvette – a container into which the test substance is poured,
• detector, which role is to convert electromagnetic radiation into an electrical signal. Most often these are photocells and photomultipliers.

The measurement may start after placing the test sample in the cuvette and then the cuvette, in the spectrophotometer. The light emitted from the source passes through a prism or diffraction grating (dispersion element in the monochrome). The appropriate wavelength selected by the monochromator is directed to the cuvette in which the sample is located. Next, the light passes through the analysed liquid and falls onto the detector, which displays the analysis result on the screen of the device.

The colour of the dry samples and solids for which the reflected light, not the passing light, is measured, is evaluated in a slightly different way. This method, commonly used, i.a. by paint and varnish manufacturers, will be discussed in the next article.

Currently, spectrophotometers are used in almost every industrial enterprise, where accurate monitoring and analysis of colour differences are needed. Spectrophotometers replace all previous colour measurement methods in quality control departments. Thanks to these devices it is possible to monitor differences occurring for individual samples from different production batches. It is worth emphasizing that these devices adapt to constantly changing regulations regarding the use of colours and technologies.

Colour rating scales

Liquids and covering materials differ significantly. For this reason, the colour rating scales of their samples will be different for transparent, clear liquids and different for coatings and covering substances. To rate transparent samples, Gardner and APHA – Hazen scales are used as well as others, such as the Saybolt scale. The colour of non-transparent materials can be rated using the Hunter scale and CIELab.

Colour measurement and rating using the above-mentioned scales enable determining the product quality expressed by numerical values. By measuring the colour of the light passing through the sample, it is possible to obtain details concerning each product, including paints, medicines, chemicals and even food. It is worth noting that thanks to the knowledge of the colour scale, it is possible to choose the right tools to work for measuring colour.

Colour rating of clear liquids

APHA-Hazen scale

The name of this scale comes from the first letters of the American Public Health Association, which is the organization that is responsible for implementing the visual colour scale as a method of assessing water quality.

The APHA scale, also known as the Hazen scale, is used to evaluate samples of oils, petroleum derivatives, as well as solvents, plastics and pharmaceuticals. This is a visual assessment method based on the colours of the liquid patterns of the platinum-cobalt solution. On this scale, the value of distilled water is 0, and the value of 500 ppm platinum-cobalt solution is 500. The calibration curve is made by from diluting a Pt-Co solution (concentration: 500-ppm). Depending on the sample, the colour varies from transparent and colourless to yellow.

Product colours rated on the APHA-Hazen scale can be determined using a spectrophotometer. In the quantitative indication, trace yellowness is determined. This method can also be used as a visual indicator of sample degradation after exposure to light, heat and the presence of impurities. Examples of PCC Group products for which this parameter is measured as part of quality control are: EXOplast OTE3, ROKAnol IT10, ROKAmer G5000E.

Gardner scale

The Gardner scale was created to evaluate the colours of transparent products of brown and yellow colour. Using this method, substances such as varnishes, oils, resins as well as fatty acids are tested. The Gardner scale enables rating the brightening of a sample’s colour due to processes causing a change in substance. This colour change is measured and then based on the measurement it is possible to assess the age, processing method of the substance or light exposure.

The Gardner scale is made of 18 standard solutions. Today, this rating method of the colour of oils and other brown-yellow, transparent substances is not popular. It has been replaced by spectrophotometer measurement, which is precise and carry much smaller error than the subjective assessment of the researcher.  Using the Gardner scale, the quality control is performed for products such as ROKAdis 900, EXOdis PC950, ROKAdis PC440.

Saybolt scale

The third scale for rating the colour of transparent samples is the Saybolt scale. It is used to determine the quality of pharmaceutical products as well as products of petrochemical origin, such as kerosene, diesel fuels, unstained gasoline, petroleum waxes. Using the Saybolt method, it is possible to rate the yellowness of a light substance on a scale of -16 (which means strong colouration) to +30 (which means a colourless substance).

Visual assessment on the Saybolt scale may carry a large error, resulting from differences in the interpretation of colours, light conditions and the environment. Due to the inaccuracy and ambiguity of the rating, this method has also been replaced by automatic spectrophotometer measurements.

Summary

The main purpose of colour analysis using a colour scale is to measure the impurities present in the sample. The analysed colour of the substance is not, however, sufficient information to identify contamination and assess product quality. The complementary parameter is the haze measurement result. Some spectrophotometers are equipped with elements enabling the measurement of haze and colour, which enables a constant control of the quality of the production. For this reason, spectrophotometers are one of the devices commonly found in the quality control laboratories.