Definitions of Spectrum, Spectroscopy and Spectrophotometer
In this blog we look at some terms that are commonly used in the sensing science industry but can sometimes cause confusion depending on the application, customer and level of colour science understanding.
SPECTRUM:
It was originally thought that visible light was the only part of the electromagnetic wavelengths, as it was clearly visible as white light, which under certain circumstances (splitting the light through a prism) 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.
As science has developed, we can confidently say that the electromagnetic spectrum extends far beyond the visible area and includes wavelengths that are much longer and much shorter than the visible area between 400-740nm.
Spectrum – The whole range of wavelengths that are known to man.
You can view the electromagnetic spectrum as we know it today on the image below to get a better understanding of the different wavelengths of energy that we can detect on the spectrum.
SPECTROSCOPY:
Spectroscopy 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-Infrared (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.
SPECTROPHOTOMETER:
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.
Spectrophotometer – an apparatus for measuring the intensity of light in a part of the spectrum, especially as transmitted or emitted by particular substances.
Spectrophotometers are often used to measure the intensity of light in a specific area of the electromagnetic spectrum and often these are grouped into the area of the spectrum that they measure. Most of our colour measurement instruments detect energy within the visible spectrum (400 – 740nm) although they are sometimes extended down into the UV area for certain applications (360 – 740nm).
This is why you might find a UV-VIS-NIR Spectrophotometer that will report a spectral curve in those wavelengths. Our instruments are unique for colour measurement and so as well as reporting the spectral information they also report C.I.E colour data.
COLORIMETER:
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.
A colorimeter does not report spectral data however there is a close correlation between these instruments and how human perception of colour. We still receive accurate information about the colour of a sample without an understanding of how that colour is made (wavelengths).
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.
