History of fluorescence 

Luminous materials were known in the times of the Greeks and Romans. Aristotle mentions the sea, meat and some fungi (rotting wood). Then in the 17th century phosphorescent substances were discovered “the marvellous light-absorbing and light-emitting luminous minerals”. Casciorolo (1602-4) working in Bologna, discovered that barium sulphide when put between red-hot coals became luminous. The Alchymyst by Joseph Wright.

In 1674 Christoph Adolph Balduin (1632–1682) first produced calcium nitrate. Everything in a glass vessel after being highly heated and dried up was found to be luminous. Named it ‘Balduin’s phosphorus’ (‘phosphorus’ means carrier of light). Since these minerals were stimulated to phosphorescence by a preceding exposure to radiation or sunlight and emitted a fairly bright light, they were looked upon as a magnet or a kind of sponge which could suck up light and give it out again. Dr. Brand in 1674-5 attempted to distil human urine and in this way discovered phosphorus.

In 1801 JW Ritter (1776-1810) discovered ultraviolet rays. When he covered paper with damp freshly prepared silver chloride and let the solar spectrum act on it in a darkroom he saw that the action began first beyond the ultraviolet and only then proceeded towards the violet. He also noted that silver chloride paper already exposed to diffused daylight that had turned slightly dark became darker in the violet end of the spectrum but lighter in the red end. This observation first pointed to the antagonism of the chemical effect of violet and red light.

Principles and theory 

Becquerel (1820-91) showed that nearly all fluorescent substances are phosphorescent although in some cases the phosphorescence may continue for only a fraction of a second. Phosphors are used on TV screens and monitors that used cathode ray tubes (CRT). Green phosphors are used with Oscilloscopes and for Scanning Electron Microscopy.

Two years after the invention of the daguerreotype, John William Draper (1811-1882) recognised that in every chemical change in a substance caused by light. Light rays of a definite wavelength are absorbed and that it is this absorption which produces the photochemical change. Stokes, employing fluorescent substances in 1852 found that quartz transmits most ultraviolet rays which led Crookes (1854) to the spectrography of the ultraviolet region with the wet collodion process.

In 1901 Max Planck (1858-1947) demonstrated that the absorption and emission of light, which is of a photoelectric nature, takes place in so-called quanta or packets of energy. Following on from Planck, in 1905, Albert Einstein (1879-1955) showed that radiation exists in packets in all circumstances and gave the name ‘photons’ to the free-travelling quanta of light. When light is absorbed an electron or electrons move to higher energy levels. This increases the energy level of the molecule.

The 1st Law of photochemistry is that no photochemical (or subsequent photobiologic) reactions can occur unless radiation is absorbed. Absorption of light involves the transfer of energy, hv, from light to individual molecules in the chemical.Substances all have their own absorption spectrum. The longer the wavelength the less energy. The shorter the wavelength the more energy. This is the main reason why ultraviolet and blue light are more likely to cause fluorescence because they have a higher energy.

Glossary of terms

Confusion of terminology

In the literature luminescence, phosphorescence and fluorescence are often used interchangeably which can lead to confusion.

Fluorescence/Luminescence

The emission of light from a substance when exposed to light. Fluorescence and luminescence are generally used to describe the effect of excitation caused by ultraviolet and blue light. Luminescence is usually associated with infrared luminescence, which is stimulated by blue/green light. In fluorescence and luminescence as soon as the light source is turned off the emission of light stops, ending within about 10-8 seconds after extinction if not faster. Substances that fluoresce on their own are said to ‘Autofluorescence’ in many uses of fluorescence fluorescent markers are used to make non-fluorescent things fluoresce. The image to the left showsx the fluorescence of rubies under ultraviolet light. Fluorescence can also include the action of blue or blue-green light on chemicals. One in common use is fluorescein which fluoresces green/yellow and is used in contact lens assessment and diagnostically in eye conditions.

Phosphorescence

The emission of light from a substance when exposed to light which continues after the light source is turned off. In phosphorescence:

1. Light is absorbed by the material
2. Light is released over a period of time

Barium sulphide, calcium sulphide or strontium sulphide. A card painted with calcium sulphide well known as luminous paint - fluoresces greenish/blue. In a darkroom it will continue to glow for a time depending on the length of exposure and intensity of the exciting radiation.

Bioluminescence

Light is created by physiological or chemical means within a biological organism for example the Angler fish, glowworms and fireflies.

Chemiluminescence

Light created by the mixing of two chemicals which react and energy is released in the form of light. Triboluminescence
Emission of light brought about by grinding certain crystalline substances. Sugar when crushed luminescent sparkles are visible.

Incandescence

Emission of light by a material solely because it is heated which occurs when thermal energy is transformed into light energy. For example carbon particle in candle flame, liquid molten metal, emits a continuous spectrum of colour.