Colorimeter

The noun color in Latin stands for colour and has been adopted in many other languages with this meaning. A colorimeter is therefore a measuring device for determining colours. These digital devices are usually easy to use and provide fast results regardless of the user's colour vision. Many colorimeters are compact handheld devices that can simply be placed on the sample to be tested. Larger table-top devices are usually equipped with fixtures for positioning the sample.

To measure the colourfulness of objects, light from an internal light source is shone through the measuring aperture onto the surface and the reflected light is detected filtered by wavelength. The ambient lighting and the colourfulness of the neighbouring objects therefore have no effect on the result when determining colour with a colorimeter. Among experts, only those colour measuring devices are referred to as colorimeters that work with only one type of standard light and one standard angle and that only record the proportions of red, green and blue in the light reflected from the sample. The measured values are displayed as standard values X, Y and Z. This three-range measurement is sufficient for many applications. However, it cannot provide clear results for structured surfaces, shiny parts and for colours that have different effects under different light sources.

Colour measuring instruments that offer more filters and functions than these tristimulus colorimeters are called spectrometers or spectrophotometers by professionals. We deviate from this here and do not draw this line. In the following text, we also refer to the spectrometers that are used for colour determination as colorimeters. In the following section, we will discuss important features that should be considered when selecting a device.

Selection criteria for the colorimeter
Before comparing different colorimeters, it should be determined which functions and features are necessary for the intended application and which features offer additional advantages. The following questions can help to determine this.

Which colour filters should the colorimeter have?
Is red, green and blue filtering sufficient for colour determination or should more detailed or other criteria be used for filtering?

Which light source should be used?
Many colorimeters work with a standard light source, some offer the possibility to determine the colour of objects under different light sources.

How large should the measuring aperture be?
If the samples are very small or curved, the measuring aperture should be as small as possible so that only the light reflected from the sample hits the light sensor.

In which format should the result be output?
In order to be able to reliably determine colours for objects in orders, the supplier and the customer must agree on the same colour determination system. There are different definitions for colour tones, the degree of whiteness and for light colours. Since these cannot simply be converted, it is very important to pay attention to the colour spaces for which the colorimeter can output the measurement results.

Should the data storage and interfaces be used?
Especially for frequent measurements and reliable documentation of the measurement results in the context of quality assurance, it is advantageous if the measurement data can be transferred directly into computer programs.

Is special software desirable?
Some colorimeters are offered with special software to facilitate the evaluation and documentation of the colour measurements.

Can the colorimeter be calibrated?
To ensure that the measurements are correct and comparable, colorimeters should be calibratable. There are models that can also be calibrated by the user in accordance with the colour standards and devices that are automatically calibrated on a regular basis.

Should direct colour comparisons be made?
If it is intended to compare the colours with the colour sample, it is advisable to use the colorimeter that has the functions of reference value determination and colour difference measurement. The colour sample then provides the reference value to which the colour differences of the samples are indicated.

Colorimeter for measuring light colours
The colour of light not only influences how the colours of the illuminated objects affect us. Together with the light intensity and the illuminated environment, the colour of light can also have a stimulating or calming effect on us. Whether at home, at work, in schools, hospitals or nursing homes, – the improvements in the quality of stay can often be achieved by means of the targeted planning of the artificial lighting. Plants and animals are also influenced by the type of artificial lighting. For some plants, the light colour can even be used to specifically stimulate either leaf growth or flower formation. A colorimeter for light colours can be used to measure not only the tristimulus values X, Y and Z but often also the illuminance, colour temperature, luminance and wavelength of the light.

Colorimeters for determining whiteness
There are colorimeters that can measure whiteness either exclusively or in addition to chromaticity. The degree of whiteness plays a major role for the materials that are to be coated, especially in the printing, packaging and paper industries. Also in the case of the consumer goods and tableware, sanitary ceramics, switches and socket covers, the whiteness of a product line should match to avoid the complaints. The whiteness levels determined according to the ISO standard and those determined in accordance with the CIE standard cannot be directly compared or converted into each other. Therefore, when selecting a device, make sure that the colorimeter can display the degree of whiteness according to the required standard.

Colours can be determined by comparison colours in a colour fan or with the help of the colour space system. There are different systems for both colour fans and colour spaces. In order to be able to communicate about a certain colour, both sides must refer to the same colour measurement system. Colour guides are often used by architects and painters to coordinate the colour of faсade elements or for interior decoration. Colour fans are naturally limited in the selection of colour shades, but help to quickly find the available matching varnishes and emulsion paints. Deutsche Bahn has its own DB colour chart for steel components. Their paints contain micaceous iron ore as a special gloss effect.

In contrast to colour charts, the colour space systems developed for digital measurement can represent an almost infinite number of colour tones. Each of these colour tones is characterised by its position in the respective colour space system. After measuring the colour components, the colorimeters indicate the hue as a position on the axes of the colour space. This measuring principle makes it possible for the colorimeters to also indicate the size of the deviations numerically when comparing with a colour sample. This makes it possible to influence the colouring process in a much more targeted way when corrections are necessary.

ColorSpace / Color Measurement Problems
The different colors of a color space are classified by a color space system. A color space system is a coordinate system in which each color is sorted by coordinates on different axes. To mirror the structure of the human eye, a color space works with three axes. In total, there are about thirty color space systems.

One of the main color spaces used is CIELab (CIE L*a*b*). The CIELab system is a color space established by the International Commission on Illumination (CIE) in the year 1976. In CIELab, color differences are determined numerically using a device-independent color 3D model. The model is objective and designed to replicate what is seen by the human eye.

How is CIELab structured?
The measured spectral curves are reduced to three coordinates. The coordinate axes are perpendicular to each other.

L * represents the brightness (from 0 = pure black to 100 = pure white)
a * represents the red - green axis (negative values are green, while the positive values are red)
b * represents the yellow - blue axis (negative values are blue, while the positive values are yellow)

Colorimeters strive to deliver measuremement results that match the way the human eye perceives color For example, for ink mixtures, yellows can be deceiving. The human eye can be cheated into seeing more whiteness or brightness by adding a higher proportion of blue. This relates to the phenomenon of optical whitening. To eliminate perception problems, often colorimeters are used. However, even different colorimeters can return different results.

Comparison problem:

Semi-finished products are measured with colorimeter A and sent out to a processing company that produces the end product. At the processing company, the incoming products are measured by colorimeter B. The processing company believes there's an issue, because colorimeter B returns a different result than that of colorimeter A. However, because the colorimeters used in the production chain aren't the same type and haven't been adjusted the same or calibrated in the same laboratory, the colorimeters won't reach identical measurement results. To avoid these kinds of issues, both parties must agree upon chromaticity, use the same color spaces and use the same type of colorimeter.

About the CIE L*C*h Color Scale
The CIE L*C*h or CIELCh color scale is an approximately uniform scale with a polar color space. The CIELCh scale values are calculated from the CIELAB scale values. The L* value (lightness) is the same in each scale. The C* value (chroma) and the h value (hue angle) are calculated from the a* and b* values of the CIELAB scale. At right is a diagram of the CIELCh color space.