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3 I nstrumentation

2.4.4.3 SPECTRORADIOMETRY
Many different spectral power distribution curves can yield the same visual effect which we call colour. Itmeans that the colour of a light source does not tell us the nature of its spectral power distribution. In otherwords, two different light sources which have the same colour in x,y or colour temperature might not exhibitthe same spectral power distribution. The reverse, however, is true: knowledge of spectral power distributionof light will enable us to describe the colour (refer to Fig. 2.4.4.3 for the types of spectral power distributioncurve of some common CIE illuminants).Hence, the spectroradiometric method is the most accurate and complete method of specifying colour. Thespectral data can be analysed visually and/or compared to data from another light source. However, thebest use of spectral data is to calculate the CIE tristimulus values by mathematically integrating the data withthe CIE colour-matching function. The tristimulus values are then used to compute CIE chromaticity coordinatesand luminosity, which provide complete description of the colour.

3.1 R ADIOMETER

Radiometer is a device used to measure the intensity of radiant energy. A majority of radiometers useonly single photocell sensors. In order to measure radiation emitted from a specific spectrum or toincorporate the radiometer within a certain spectral response, an optical filter is normally used. Such optical filtering offers a simpler and more cost effective solution.The industrial applications of radiometer mainly involve irradiance and radiance measurement. In orderto quantify the radiation emission from source, radiance measurement is normally used. On the otherhand, when the level of exposure is of concern, the irradiance or the integrated irradiance measurementis then carried out. 3.1.1 APPLICATIONS OF RADIOMETER
Radiometer is commonly used in industry to quantify light which is outside the visible spectrum, i.e., ultravioletand infrared. Ultraviolet (UV) light is widely used in the industry for various applications, for example,•Curing of photoresists in semiconductor manufacturing•Curing of emulsions for printing or plate-making•Colour-fastness testing•Biological applicationTo conduct UV measurement by radiometer, either radiance or irradiance measurement, the spectral response(wavelength range and peak wavelength) should be specified to match the specific application.Beside UV, infrared energy is also a common parameter in the field of radiometric measurement. Infraredmeasurement is useful as all material emits infrared radiation according to their thermal energies. Infraredthermometer utilises the principle of infrared radiance measurement to determine the temperature of object by non-contact means. Hence, such infrared radiometer is also commonly known as “Radiation Thermometer”.Different filters with specific spectral responses are used for different applications and temperature ranges. Formore details about temperature measurement by infrared detection, please refer to our publication on ‘TheWonders of Temperature’. Standard Illuminant D65: Averagedaylight (including ultravioletwavelength region) with a correlatedcolour temperature of 6504K.Standard Illuminant C: Averagedaylight (not including ultravioletwavelength region) with a correlated colour temperature of 6774K.Standard Illuminant A:Incandescent light with a correlatedcolour temperature of 2856K. 200 –150 –100 –50 –0 –|300|400|500|600|700 22a: Standard Illuminants Wavelength (nm) Fig. 2.4.4.3 - Spectral Distribution of CIE illuminants

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