An Overview of Colorimetry and Appearance Measurements

Shakespeare said: "Beauty is bought by the judgement of the eye." It would also be true to say, however, that the "judgement of the eye" buys such things as quality, freshness and cleanliness.

How a product looks can be one of the most critical parameters affecting customer choice. Consumers will pay more for a product they see as being particularly 'attractive' - cars with metallic paint command much higher prices than those with standard finishes.

Words such as freshness, texture, gloss, pearlescence, translucency, transparency, opacity, bloom and finish describe aspects of appearance which are often key to product success. To ensure uniformity and reproducibility of desirable features, the appearance of a product needs to be quantifiable. Defining the object's colour is just the start - other attributes of the material from which it is fabricated contribute to the overall appearance. Surface texture, for example, will cause changes in colour depending on the lighting direction; the freshness of food is judged by its overall appearance, but in a way that is much more subtle than by just its colour. Novel effects such as pearlescence are added to products to enhance their attractiveness. For some products, such as cosmetics, it is not only their own appearance characteristics that are important, but also the visual effect after they have been applied to the skin, nails, hair, etc.

How Sight Becomes Insight

The act of seeing needs a lot of processing. To demonstrate this we might think about constructing a camera with the same functionality as the eye.

We would need a 6 megapixel detector to match the eye's resolution. To encode the eye's working range we would need 10 bits, to be processed at rate of a frame every 1/50th second. This totals 20 thousand million bits of information handled per second. In just one day, we deal with about 10¹⁵ bits of visual information.

Of course this information is compressed considerably to fit down the optic nerve by data-reduction in the retina. Visual data on variations in lightness and colour over space is processed in the brain to provide information in terms of shape, movement colour and texture, and ultimately identification and aesthetics.

Although there are huge bandwidths of data involved, nature uses a lot of information-reduction, so with advances in electronic imaging and computer processing, it is increasingly possible to be able to train machines to perform visual tasks in the same way people do. Colour and lighting levels are arguably the best-understood and competently measured quantities. Only recently have attempts been made to measure other aspects of vision qualitatively.

Work at NPL on Appearance

The NPL appearance programme aims to quantify various visual appearance parameters; specifically, colour (gonio-apparent), gloss, texture and translucently. Effectively, the aim is to develop new measurement techniques for industry. We have assembled a suite of optical instruments with which to determine the specific optical properties of a material that give it a particular appearance and hence determine the measurements necessary to characterise each aspect of its appearance.

NPL Appearance Instrumentation

Our team uses many instruments to evaluate a variety of appearance attributes. At the peak of the UK traceability chain lie the primary instruments, which realise national scales. For day-to-day measurements, including fast-track accredited measurement services we use instruments traceable to these primary instruments. These can be further subdivided into those measuring spectral reflectance and transmittance; and other visual aspects such as gloss and translucency. NPL has a long history in active involvement in the development of models and instrumentation for appearance including the popular ceramic colour standards

For more information, please contact: Teresa Goodman