The perception of colour can be confusing especially when the same process can have different names depending on its application. This short Blog will help clarify what each name refers to.
It was originally thought that Visible Light was the only Electromagnetic Wavelength, as it was clearly visible as white light, which under certain circumstances could also be discerned as the colours of the rainbow. Visible Colour is also related to temperature and William Herschel noticed, in 1800, that not only does each colour of the rainbow have a different temperature, the wavelength above Red was the hottest, and this is called Infrared. A year later Johann Ritter noticed at the other end of the visible spectrum, an energy that could cause changes to chemicals, and this become known as Ultraviolet.
Since then multiple frequencies have been discovered on the spectrum, from X-Rays below ultraviolet to radio waves above visible light, and many many more. We could say the whole universe is represented by some sort of wavelength or frequency.
Spectrum is, therefore, the whole range of wavelengths that are known to man.
Spectroscopy, in simple terms, is the study of any wavelength and its reaction to matter. When light strikes an object, or if other wavelengths strike an object, the wavelength changes, not the object. If this change is repeatable, and diagnostic of a property of the object, then light may be used to identify objects or properties of an object. In simplest terms, if white light strikes a red apple, and only the red colour is reflected, then this will appear to be a red object/apple. Using non-visible light, like Near Infra-Red (NIR), and if we know through chemical tests that the object contains iron, then when we project NIR wavelengths onto an object and get the same graph as we obtained from iron, then we can deduce this object is iron.
Spectroscopy is quite a mature science, yet still in its infancy for non-invasive identification of matter, especially remote matter on planets in the solar system.
While Spectroscopy can identify matter or components of an object, it does not quantify that component in matter. This is where a Spectrophotometer is required. In this case, a known Illuminant is passed through a prism splitting white light into discrete wavelengths. The quantity or intensity of light transmitted or reflected is quantified by measuring the intensity, which in turn can define a colour or compound over a range of wavelengths.
A colorimeter characterises colour samples to provide an objective measure of colour characteristics as perceived by the human eye. Utilising filters rather than a detector, a Colorimeter emulates the human experience. Robust and accurate these devices are preferred in factories or on-site where “Laboratory” conditions are not to be found.
While a Spectrophotometer can create a Reflectance Curve graph, characterising the details of a colour, a Colorimeter can only deliver a CIE norm like CIE L*a*b, which is usually accurate enough for a PASS/FAIL decision, as well as to check raw materials as well as the final product.
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