Sometimes the colours we see may not be what they seem!
Firstly, the colour we see is basically the way a combination of light waves within the visible spectrum reflects back from a surface whereas others get absorbed by the surface (see also “How we see light?”). This is referred to as luminescence. So far, so good. All seems fine when the light waves bounce back in exactly the same way they approach the surface.
But .. in some cases things change! There is a phenomenon called Fluorescence. This occurs when light waves undergo a slight change before they bounce back from a solid surface and therefore produce a different colour to the one that was initially emitted from the light source.
Typically the shift in wavelength is from a shorter wave to a longer wave, meaning from the blue end of the spectrum towards the yellow and red.
A bit of technical info that will hopefully explain this phenomenon in some detail without delving too much into the chemistry knowledge needed to fully understand this complex process:
The life cycle of fluorescent light follows the following sequential steps:
A light source (a range of light waves) encounters a surface containing fluorescent molecules (examples of these molecules are ethidium bromide, alexafluor350 and fluorescein)
Excitation: The molecules enter an excited state for a very short period
Loss of energy: During the excited state a certain amount of energy gets lost
Emission: The remaining energy bounces back from the surface – this is the light we see
Fluorescence: If the initial energy from light waves hitting a fluorescent surface is beyond our visible spectrum, then it may become visible after losing some of this energy and subsequently leaving the surface as light waves within our visible spectrum and therefore causing the surface to appear brighter.
This phenomenon also appears in nature
Commonly displayed in sea-living creatures, fluorescence can be spotted in certain species of fish and other living sea creatures, like corals. The reasons for organisms to have developed fluorescent colouring are believed to be either for defense purposes where a bright colour signals a thread to the enemy or as a mating ritual, where a brightly coloured mate is thought to be more virile and thus a more promising match.
Another well-known example of fluorescence is the presence of fluorite in gemstones. The quality of certain gemstones are often determined after examination under UV light in order to display the amount and colour of fluorescent materials present in their structural make up. For instance the famous “Hope” diamond fluoresces red.
Returning to the world of print, a well-known example of fluorescence in the printing industry is when UV light reaches a surface that contains optical brightening agents (OBAs). UV or Ultra Violet light, meaning “above” violet, sits just outside our visible spectrum. Therefore we can’t see it but if UV light waves reach a surface with optical brighteners, this can cause the light waves to lower their frequency so that they become part of our visible spectrum. This is what makes the surface appear brighter.
Paper producers aim to make the papers look brighter than their natural fibres allow them to be by adding OBAs – however, the downside is that this often has a long term damaging effect with papers yellowing with age.
For the record: Most of our fine art papers do not contain OBAs. This means they do not change colour depending on the amount of UV light they’re exposed to and also don’t tend to yellow with age.