Spectrophotometry

The absolute measurement of the colour of any object can only be obtained by determining wavelength by wavelength throughout the visible spectrum the ratio of the radiant flux leaving the object to that incident upon it. For transparent samples the ratio is that transmited flux to that incident upon it - generally at right angles to the surface of the sample.

Spectral reflectance = .

Spectral Transmission = .

Where = one narrow band of the spectrum, commonly called monochromated light.

The spectrophotometer is the instrument which enables this to be done. It has a built in illuminating device, a monochromator for separating light in the visible spectrum into narrow bands and a photoreceptor with a device to compare electrically the differing amounts of light incident upon it.

The incident light or reference beam on all spectrophotometers used in colour is measured from the light reflected from a reference standard of known spectral responce wavelength by wavelength. Some instruments have built in reference standards, others need a standard in position while a sample is being measured. For these latter instruments and for calibrating the former, the ideal standard would be a perfect diffuser which reflect 100% of the light incident upon it. Magnesium oxide or barium sulphate of 98% reflectance are mostly used.

The spectral measurement of transmitted light is a relatively simple matter with light incident at 90° to the surface of the sample and the transmitted beam being measured at the same angle on the reverse side of the sample. Should the sample be absorbing too strongly at some wavelenghs it is generally possible to reduce the thickness or increase its dilution if it is a liquid sample.

Such expedients are not possible with opaque surface colours so the spectrophotometer for reflectance measurements need to be designed differently from one built for simple transmittance measurement.