CHAPTER-4

COLOUR MEASURING INSTRUMENTS

Granch Berhe

There are three major classes of measuring instruments available to the colouring of textile industry. They are:

• Colourimeters

• 0/45° and 45/0° geometry spectrophotometers

• Integrating sphere geometry spectrophotometers

Colourimeters• colourimeters were the pioneers of colour measuring

instruments. colourimeters view a sample through at least three filters measuring the quantity of light reflected from the sample and passed through each of the filters. The filters were originally designed to mimic the response of the red, blue and green cones of the eye as closely as possible. This, at first glance, would seem to have merit since it attempted to imitate the eye of the standard observer. An important deficiency of colourimeters is they cannot separate the pure colour from the appearance of an object. Additionally, a colourimeter is unable to detect if an object’s colour will appear differently under different light sources. Therefore, colourimeters have limited capability when compared with current technology.

Meter

Detector (X,Y,Z)

Reflectance Spectrophotometer • Reflectance spectrophotometers measure the

amount of light reflected by a surface as a function of wavelength to produce a reflectance spectrum. The reflectance spectrum of a sample can be used, in conjunction with the CIE standard observer function and the relative spectral energy distribution of an illuminant, to calculate the CIE XYZ tristimulus values for that sample under that illuminant.

The operation of a spectrophotometer is basically to illuminate the sample with white light and to calculate the amount of light that is reflected by the sample at each wavelength interval. Typically data are measured for 31 wavelength intervals centred at 400nm, 410nm, 420nm, ..., 700nm. This is done by passing the reflected light though a monochromating device that splits the light up into separate wavelength intervals. The instrument is calibrated using a white tile whose reflectance at each wavelength is known compared to a perfect diffuse reflecting surface.

The reflectance of a sample is expressed between 0 and 1 (as a fraction) or between 0 and 100 (as a %). It is important to realize that the reflectance values obtained are relative values and, for non-fluorescent samples, are independent of the quality and quantity of the light used to illuminate the sample.

Reflectance Spectrophotometer • Main component of a spectrophotometer are:

– Source of optical radiation

– An optical system defining the geometric conditions for measurement

– Light dispersing system

– Detector

– Signal processing system that converts light inti signals suitable for analysis

Reflectance Spectrophotometer

Reflectance Spectrophotometer

Reflectance Spectrophotometer