Colour measurement instruments make the subjective objective
Colour is a subjective measure. Person to person we cannot be sure that what one person sees as green is in fact the exact same hue that someone else sees as green. This lack of consistency can lead to problems if relying on the description of one person to determine a colour. The field of colour measurement arose in response to the need for a standardised method of measuring the colour of the world and objects around us, without turning to faulty human perception. The International Commission on Illumination (known as CIE from its French title) organisation developed a series of mathematical functions that represent the light source (illuminant), the observer (the colour matching functions x,y,z), and the object. The colour measurement instruments include colourimeters, condition-simulating light booths and the spectrophotometer.
In this article we look at the colour measurement method and how it works, the instruments that measure colour including the spectrophotometer and light booth, and where to find these colour measurement instruments from suppliers in the UK.
Understanding the science of colorimetry in colour measurement
To see colour, you need a light source, an object, and an observer. To measure colour in a quantifiable way, you need a way to turn these variables into data. The human eye perceives colour through the action of cone cells, concentrated on the retina. Cones come in three types based on their light sensitivity: long-wavelength (red), medium-wavelength (green), and short-wavelength (blue). To quantify the human perception of colour (the science of colorimetry), scientists at CIE conducted some vision experiments which led to the development of the x (red), y (green), and z (blue) colour matching functions.
The X, Y, Z colour space
The colour matching functions work like coordinates for locating a colour in a ‘colour space’, and by describing all three you can define any human colour sensation. The CIE functions are collectively used as the Standard Observer. For the Tristimulus method of colour measurement, an instrument (such as a colorimeter) has three sensors, one each for red, green and blue, which are filtered to have the same sensitivity as the colour sensitive cones in our eyes – through the x, y, and z functions. When the light hits the sensors, they can directly measure the x, y, and z tristimulus values which provide a quantitative description of the colour, based on what the eye would see.
Essentially, colour measurement is the process of breaking down the effect of wavelengths of light on the human eye into quantitative and replicable data – an instrument takes light and “sees” it as numerical data. The CIE XYZ colour space is one of the many colour spaces used to quantify human colour vision. By doing this we can communicate colour in a meaningful way.
Light booth – the colour measurement instrument for the right light
Anyone who has ever left the house thinking they looked respectable only to see their reflection in the fluorescent light of the supermarket and realized they look like death warmed up has experienced the difference the right light can make. This is why the colour measurement standards and instruments are so important for ensuring accurate and replicable results. What we see as ‘white’ light in everyday life is actually almost always slightly biased towards the red, yellow, green, or blue areas of the spectrum. When these differences interact with other colours, the results can be immediately noticeable.
A light booth allows for an objective visual colour assessment by testing a sample or product in a range of lighting sources. These might include daylight (illuminant standard D65), incandescent light, department store light, and ultra-violet light. A light booth is a booth in which these various lighting conditions can be simulated. When it comes to coatings it is mostly used to test the finish of products under controlled conditions. It can also reveal how coatings on different substrates will perform in different lights, and how special effect coatings will perform with different angles.
Measuring colour with spectrophotometry
Unlike colourimetry, spectrophotometry deals with colour not as perceived by the human eye but as a function of wavelength. It is the quantitative measurement of the reflection or transmission properties of a material as a function of wavelength. The specimen being measured is illuminated by a light source from the CIE standard illuminant space. The light reflected by the specimen then passes through an optical grating which breaks the light into the spectrum and hits a detector array. The array measures the intensity of the light at each wavelength, and so each colour.
The CIE L*a*b colour space
This form of colour measurement uses the CIE L*a*b* colour space. Here L* is the lightness value, and a* and b* are the colour values, representing green-red and blue-yellow colour components respectively. This colour scale is based on the opponent-colours theory, stating that two colours cannot be both red and green at the same time, nor blue and yellow at the same time. The other measure used by a spectrophotometer is difference (delta). By calculating the change in L*, a* and b* values and determining the overall difference (delta E*), it can be shown how far removed from a colour standard a sample is.
3 types of spectrophotometers – Multi-angle, 45°/0° and spherical
Spectrophotometers are the most commonly used instruments for measuring colour. There are three main types:
- Traditional 45°/0° spectrophotmeters – In these instruments the angle of illumination is 45°, and 0° is the viewing angle. The detector receives the reflected light at a 0° angle, perpendicular to the sample. These single-angle instruments work well for solid colours.
- Spherical/sphere spectrophotometers – The sample being measured is illuminated from all directions, and the detector receives the light at an 8° angle from the sample’s surface. The sphere is internal to the machine, providing the reflection and source of the light. These are best for specular or highly reflective surfaces.
- Multi-angle (MA) spectrophotometers – Primary illumination is at a 45° angle, through it can also be provided at a 15° angle. They can provide reliable colour data on special-effect coatings for example, in the automotive industry.
Colour measurement instruments availability in the UK – suppliers and products
Appearance is an important part of any coating application, and colour is one of the first things that attracts the eye. Colour matching across products and colour harmony across surfaces is vital for the first impression of quality. The UK has several companies providing colour measurement instruments and tools for the coating industry. Below we have made a sample list of just a few of these suppliers and their products including Elcometer, and PCE Instruments.
If you need colour measurement instruments for your business or project, get in touch! Our experts are here to help. We will collaborate with our coating partners to find the right tool for the job. Just use the “Request a Quote” link and fill in the contact form with your requirements to get started.
|Company Name||Colour Measurement Instrument||Instrument Type||Description|
|PCE Instruments||Color Meter PCE-TCR 200||Colourimeter||Determines colours using CIE L*a*b colour space and the RGB colour range. Made for use in plating and painting industries. It has three inspection conditions: D65 (daylight), D50, and F11.|
|PCE Instruments||Color Viewing Box PCE-CIC 10||Light booth||Supports six possible lighting types supplied as standard: “A/F”, “D65”, “TL84/F11”, “UV”, “CWF” and “TL83/U30”.|
|Elcometer||Elcometer 6085 Portable Sphere Spectrophotometer||Spectrophotometer||Measurements can be obtained from any of the nine illuminants with 2° or 10° observer angle: L*a*b*, DL*Da*Db*, L *C*h°, DL*DC*DH*, DE*ab, DECMC, DE CIE94 and XYZ. Whiteness and Yellowness per ASTM E 313-98.|
|PCE Instruments||Spectrophotometer PCE-CSM 10||Spectrophotometer||Spherical spectrophotometer with 8mm measuring aperture and a range of colour spaces (CIE L*A*B, XYZ, Yxy, LCh, LUV, LAB&WI&YI)|