Instruments have been available for measuring the reflective properties of flat, uniform surfaces for many years.  The use of microprocessor chips and the developments in optics has meant that the ease of use and the performance of the instruments have improved dramatically and the cost and size has been reduced.  As a result, the use of instrument based methods for the assessment of colour and colour difference is becoming normal practice.

There are two types of instrument in common use for the assessment of colour, reflectance spectrophotometers and tristimulus colorimeters.  In both cases light is shone onto the object and part of the reflected light is collected, analysed in some way and the intensity recorded.  The instruments measure the reflection properties of the test surface compared to those of a reference “perfect” white surface under the same conditions.  Instruments are generally able to determine colour differences at least 10 to 20 times smaller than can be seen with the human eye

Reflectance spectrophotometer instruments are more versatile than filter-based colorimeters and the data obtained from a spectrophotometer can be used in the prediction of colour by computer-based systems for the formulation of inks and paints.  In general, the reflectance spectrophotometer is the more accurate and reliable of the two types of machine, but is also the most expensive costing between two to five times more than a tristimulus colorimeters.

Reflectance spectrophotometer

Figure 2 illustrates a reflectance spectrophotometer with an 8/T optical geometry.  The sample is illuminated with collimated beam of light at 8° to the sample normal.  The light trapped within the sphere is collected from part of the sphere wall and passed into the spectral analyser.  The total intensity emitted by the lamp is monitored at all times by a reference detector mounted in the lamp housing.

Tristimulus colorimeter

The tristimulus colorimeter analyses the collected light by measuring the intensity transmitted through each of three different optical filters.  Figure 3 illustrates a filter based tristimulus colorimeter with a T/6 optical geometry.  The sample is illuminated with diffuse illumination provided by a xenon flash unit and an integrating sphere.  The cone of rays reflected by the sample at 6° 2° to the normal is collected by a fibre optic light guide (sample channel).  A second fibre optic guide collects light reflected from part of the sphere wall (reference channel).  The sample and reference light is analysed by splitting into three equal portions and passing each portion through an optical filter.  A light sensor mounted behind each filter detects the intensity of light transmitted.  The signals from the photo-sensors are processed to give L*, a* and b* for the selected Standard Illuminant and Observer combination.

Advantages of a colorimeter

Drawbacks of a colorimeter

Standard CIE optical geometry

An instrument should be able to give repeatable measurements of the same sample.  It is also reasonable to expect that different instruments of the same type from the same manufacture will give similar results when measuring the same samples.  Ideally, instruments of the same type should give the same result, irrespective of the manufacturer.  In an attempt to achieve this aim the CIE have defined four standard optical geometries and in general instruments of the same optical geometry will give similar results.

In all four geometry’s, it is assumed the instrument is measuring the colour of a smooth, flat surface.  The definitions specify the angular distribution, relative to the normal to the surface, of the illuminating light and of the observed light.  The geometry of an instrument is denoted by two terms separated by a backslash, for example 0/45 and 45/0 are shown in Figure 4.  The first number refers to the illumination geometry and the second the observation geometry.

CIE 0/45 and 45/0 geometry

The observation angle can be either 45° or 0° relative to the surface normal, corresponding to illumination at 0° and 45° respectively.  In principle, since the photons in the light beams do not "know" which way they are travelling, both the 0/45 and the 45/0 arrangements should measure identical values for the same sample.

CIE 0/T and CIE T/0 geometry

In the third arrangement (0/T), shown in Figure 5, the sample is illuminated with a collimated beam of light normal to the surface and all of the reflected light is collected, irrespective of direction (diffuse observation).

The definition does not restrict the collimated beam to the direction normal to the surface, an angle of up to 10° with the surface normal is allowed.  As will be explained later, a small angle has some advantages over a 0° system.  The equivalent notation for a system with an angle of 8° for example is 8°/T and T/8° respectively.

Integrating sphere

The integrating sphere is the optical component used to generate the condition of diffuse illumination or diffuse observation.  It consists of a hollow metal container, not necessarily spherical in shape, whose inner surface is covered with a very high reflectivity, matt coating material.  The coating is commonly made from either barium sulphate, magnesium oxide or poly(tetra fluoroethylene) powder.  The coated sphere not only creates the diffuse condition but also "traps" light within the system to give a higher intensity of collected light than a 0/45 or a 45/0 arrangement.

The observation port is in the sphere wall, 90° around from the sample port.  A lens collects light coming from the opposite wall of the sphere.  This part of the wall will receive light scattered from all other parts of the sphere and, in turn, from all directions of the light reflected by the sample.

Highly reflecting baffles are mounted inside the sphere to prevent light being collected directly from the illumination port or directly from the sample itself.

SPIN and SPEX measurement

The integrating sphere geometries shown in Figure 6 and in Figure 7 include a gloss trap.  This is a section of the sphere wall that absorbs all of the radiation that falls on it.  The position and size of the trap acts to remove from the measurements the specular or mirror like portion of the light reflected from the sample.  Modern integrating sphere instruments have the option of automatically replacing the gloss trap with a highly reflecting section.  In this way, the operator can choose to make measurements either in the specular excluded (SPEX) or in the specular included mode (SPIN).  The instrument geometry notation T is changed to D when specular light excluded measurements are being made to indicate that only diffuse (D) light directions instead of the total (T) are being used.

Instrument light source

Most modern instruments measure the ratio of the light reflected from the sample to that reflected by a reference white material.  In a true doublebeam arrangement, the sample and reference panels are measured simultaneously.  In a pseudo doublebeam system, the sample and reference are measured sequentially and it is assumed that the power output and spectral distribution of the light source is stable over the short time between measurements.  In both systems, any variation in the illumination intensity with wavelength is automatically taken into account.  In other words, provided the light source gives sufficient radiation to obtain a reasonable response from the detection system, the spectral power distribution of the illumination should have no effect on the measured values.  In reality this is not the case for certain samples.

There are two types of light source in common use, a xenon flash lamp and a tungsten halogen bulb.

Xenon flash lamp

The xenon flash lamp provides a spectrum similar to that of daylight and by passing the light through a suitable filter, a reasonable simulation of CIE Illuminant D65 is possible, including the ultra-violet portion of the spectrum.  This has the advantage of stimulating any fluorescent dyes or pigments that may be present in the sample.  Such instruments, when correctly calibrated, can be used to assess whiteness.  When first developed, the xenon flash systems were expensive to buy and maintain; however developments in optical and electronic components means that this is no longer the case.

Tungsten halogen lamp

In systems that use a tungsten halogen lamp for illumination, the lamp is operated continuously during the measurement.  Therefore the instruments are simpler in construction, both electronically and optically, less expensive to buy and easier to maintain than flash based systems.  The spectral power distribution is similar to that of CIE Illuminant A with the majority of the intensity being in the orange, red and infrared part of the spectrum.  There is comparatively little ultra-violet light present and such instruments are not recommended for measurements of the “whiteness” of samples containing fluorescent dyes or pigments.

Thermochromic materials

The accurate measurement of the colour characteristics of samples that show thermochromism be a problem, the colour of a thermochromic sample changes with temperature.

A xenon flash based system exposes the sample to a short pulse of light, a few milliseconds, and there is unlikely to be any change in the temperature of the sample.  A tungsten halogen based system illuminates the sample for a longer time, and there is a greater proportion of infrared light present in the illumination.  Therefore, it is more likely for the sample to warm up during measurement than with a xenon flash system.  This problem has been partly overcome by the use of low power tungsten halogen lamps and an optical filter to remove the infra-red component of the radiation.

Instrument calibration

A white standard, usually a ceramic tile, and black standard, either a tile or a light trap, is provided with each instrument.  The reflectance ratio spectra of the white and the black standards are known and stored as data in the computer memory.  During the calibration process each tile is measured and, in simple terms, the black tile or trap is used to set a zero reflectance point for each wavelength and the white tile is used to determine a scale factor for each wavelength.

The calibration standards must be treated with care, kept clean and replaced in the protective packaging immediately after use.  Never use the white tile as a backing material during the measurement of semi-transparent materials.

Calibration check

Instrument maintenance

Modern colour measuring instruments are reliable and require little maintenance, however this does not mean that checks on performance are unnecessary.  The properties of the optical components will gradually change with time.  The double beam principle combined with the calibration process means that such changes are, to a certain extent, automatically taken into account.