Opinion Pieces

The articles contained in this section are meant to provide relevant information on a wide range of subjects, and to be entertaining to the reader.

They are, as the title suggests, the opinion of the Editor and intended to promote discussion.

We would value your responses to these articles - Your response will determine whether pieces on similar subjects appear in the future.

Contributions from readers may be published at the discretion of the Editor.

Colour QC. Which Instrument? Too Many Still Use the "Wrong" One!  
Colorimetry by NIR - Who's Confused?
 
 

It is surprising how many companies use an instrument geometry that is almost guaranteed to be the cause of disputes between buyers and sellers when visual and instrumental assessments are compared.

There are two colour instrument geometries: 45/0 (0/45) and diffuse (d/8).

The 45/0 instruments always measure with the specular or mirror-like component of reflection excluded. The diffuse instruments usually measure with the specular component included, although some, but not all, of the specular component can be excluded.

The 45/0 instruments measure appearance, i.e. the surface effects of the sample are taken into account by the instrument as well as the colour, in the same way that the human observer does. These surface effects include texture, grain and gloss. The result is that the visual and instrumental assessments correlate very well.

The diffuse instruments in specular included mode measure colour only, i.e. the surface effects are ignored by the instrument, something the human observer cannot do! If operated in specular excluded mode, visual and instrumental assessments will correlate for glossy surfaces ONLY! Poor correlation occurs for low gloss samples because not all of the specular component is excluded.

Diffuse instruments are valuable for colorant formulation, such as for textile dye recipes, coatings formulation and plastics formulation where surface effects would introduce errors. On the other hand diffuse instruments are quite inappropriate for Quality Assurance and Control applications where objective correlation with visual assessments ought to be essential.

Why then, are diffuse instruments still being acquired for applications such as acceptance of grained, low gloss automotive plastics?

One possibility is that the purchaser, such as an automotive company, made a mistaken purchase in the first place, as a result of being poorly advised by the instrument salesperson.

Mistakes can be made, but why is this one perpetuated? It is now widely accepted by most industrial colourists and all instrument manufacturers that 45/0 geometry is more appropriate for QC purposes and diffuse instruments for formulation. Compromises are made for purely financial, not technical, reasons where both applications occur at the one site, but there is little excuse for choosing the diffuse option where QC only applications are involved.

Perhaps one day, one of the major specifiers will admit their original mistake and insist on 45/0 geometry in future for good technical reasons. How much time and resources are needlessly wasted because human and instrument appear to disagree?

The use of inappropriate instrumentation would not be allowed to continue in any other field of quality engineering. Why should colour be treated differently?

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The April 2005 issue of "The Australian and New Zealand Grapegrower and Winemaker" contains an article from the CRC for Viticulture entitled, "Practical and affordable colour measure (sic) for industry".

As long time workers in the field of colour measurement, we were naturally attracted to the article, but we were amazed to read in the very first paragraph, "The rapid measurement of red grape colour and other quality measurements with Near Infrared Spectroscopy throughout the wine industry...." and went on to discuss how wineries could measure colour (total anthocyanins) with a portable NIR instrument that could also measure pH and total solids in red wine grapes.

We have no arguments with NIR as an analytical technique that is probably a valuable tool for measurement of the concentration of anthocyanins and lots of other stuff in red wine grapes, but how this relates to colour leaves us mystified and wondering whether it is us or the CRC that is confused about colour.

In the field of colorimetry and human psychology it is generally accepted that perception of colour is a psychophysical process involving, amongst other things, the modification of light by some means, usually by interaction with a physical object, that is detected by sensors in the eye and interpreted in the brain as "colour". "Light" includes the wavelengths of electromagnetic radiation in the visible range of 400nm to 700nm..

The Near Infra Red Region extends from above 700nm up to around 1200nm and these wavelengths cannot neither be detected by the human eye nor interpreted as colour by humans. NIR instruments detect the interaction of electromagnetic radiation above 700nm with physical objects and these results allow quantification of characteristics of the object, BUT THIS CANNOT BY ANY STRETCH OF THE IMAGINATION BE INTERPRETED AS COLOUR AS WE HUMANS KNOW IT.

The wine industry has some peculiar practices when it attempts to objectively quantify the colour of wines and grapes. "Colour density" is a term that has meaning only within the wine industry. The measurement of red wines at around 520nm (the wavelength of maximum absorbance of anthocyanins in the visible spectrum) is also referred to as "colour measurement". As a means of determining concentration of a colorant, this method has been well known and used in the textile dyeing industry ever since appropriate instrumentation became available decades ago.

The food and beverage industry in general uses colorimetry is a way that brooks no confusion. Objective measurements are made across the visible spectrum in such a way that instrumental measurements can be compared and correlated with visual observations made under standardised conditions. Raw material suppliers, processors and consumers are all talking the same language. Not so with the wine industry where some are visually assessing wine and grape colour, red and white, and others are using analytical techniques and confusing the situation by referring to these as "colour measurement".

We have attempted to communicate our opinions to those in the wine industry who continue to use the term, "colour measurement" in what we believe is an inappropriate way, but they don't want to talk with us!

This is a pity, because colour measurement, "as we know it",  has proven to be a valuable tool for the food and beverage industry and there is no reason why the wine industry should not enjoy similar benefits. Colour measurement instrumentation exists already and is affordable. We don't doubt that inexpensive, portable NIR instrumentation would also benefit the wine maker, but please don't confuse what it does with colour measurement.

One more thing for the person who wrote the CRC article. Instruments measure... machines do work! There is even confusion on this definition within the article.

YOUR comments welcomed - especially if you are working in the wine industry?

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