The whole idea of ‘black’ light would seem to be an impossibility yet using ‘blacklight’ is a common technique in science capturing not the reflection of light from the subject but the emission of light by the subject due to the action of the blacklight.
The oddest subjects emit light when hit by blacklight from tonic water to almonds, from the skin to eggs, from oils to paints. This is termed ‘fluorescence’ and the blacklight ‘ultraviolet’ radiation.
Professor RW Woods reported on his creation of a convenient source of blacklight in 1919 using a dense cobalt-blue glass, often called a Wood’s lamp in his honour. Since then the light has been used in medicine for the detection of clinical conditions like Tinea capitis (Ringworm) and in criminal investigations for the detection of forgeries and for finding bodily fluids at crime scenes.
In looking through a camera’s viewfinder without something that fluoresces under the blacklight nothing is visible. It is only when a fluorescing object comes near, from the photographer’s shirt or hands (as above) to the perennial bane of photographer’s life dust that something becomes visible.
As the light captured by the camera, once the reflected ultraviolet has been removed, is that emitted by the subject the image is often soft in appearance.
My first introduction to blacklight was during my training and work as a medical photographer recording electrophoresis plates and later when teaching scientific photography trying as many subjects as I could find to fluoresce from the literature or trial and error.
One intriguing group of fluorescent materials in view of the growing concern over obesity are oils and fats. The oils in nuts fluoresce as does olive oil with colours depending on purity or additives and country of origin.
Its use to enhance commerical products is inescapable as fluorescent paints and inks entice us as they visually jump out shouting ‘Buy me!’ and our bank notes are checked for forgeries at the till.
We may not ever see blacklight with our eyes except the very hint of indigo or violet but we can see and wonder at its effects.
This whole text with photographs is available as a pdf
Confusion of terminology
In the literature luminescence, phosphorescence and fluorescence are often used interchangeably which can lead to confusion.
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.
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.
The emission of light from a substance when exposed to light which continues after the light source is turned off.
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.
Light is created by physiological or chemical means within a biological organism for example the Angler fish, glowworms and fireflies.
Light created by the mixing of two chemicals which react and energy is released in the form of light.
Emission of light brought about by grinding certain crystalline substances. Sugar when crushed luminescent sparkles are visible.
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.
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.
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.
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.
Ultraviolet light is directed onto the specimen by Woods Lamp so the fluorescence is visible or using an ultraviolet transmission filter over a flashgun termed the excitor. The barrier filter is an ultraviolet cut-off filter for example 2E.
Effect of using different UV cut-off filters
By changing the ultraviolet cut-off filter to one which cuts off more ultraviolet light it is possible to enhance the quality and definition of the colour image produced.
1A Skylight - Not recommended
2B Useful for cutting off excessive UV from flash and sky easy to obtain for UV fluorescence work.
2E Usual barrier filter for fluorescence work.
3, 6,9 or 12 Increasing cut-off deeper yellow can however cut-off some blue fluorescence.
It is important to make sure your filters do not fluoresce under UV as this reduces image quality.
Kodak colour compensating filters can occasionally improve the purity of colours CC20Y + light balancing 81EF (Wratten 81EF is brownish used to lower colour temperature).
Exposure times are in the order of seconds 1 - 10 seconds, f/5.6 - f/8, but may be longer with faint fluorescence so would need to allow for the reciprocity characteristics of the film.
Routinely I use Velvia 50 ASA slide film for greater colour saturation but to begin with it is sensible to use a 400 ASA colour negative film which has a greater exposure latitude.
Daylight film is the preferred choice but tungsten balanced film can also be used but it is more sensitive to blue light so gives a profound blue cast in the presence of any stray UV.
A digital camera can also be used but should be set on daylight rather than auto white balance.
If doing comparative work it is also good to take a control photograph under normal light as above showing normal and fluorescent photographs of a stalagmite.
|Colourless solution of eosin and acriflavine||Brilliant green|
|Mustard made into a solution and then dried||Green|
|Hen’s egg - brown fresh||Brilliant scarlet|
|Quinine detected weak solutions||Electric blue|
|Sodium salicylate||Like a star|
|Tinea - ringworm||Bright metallic green|
|Squamous cell carcinoma||Glows like hot coals|
|Secretions of the skin, fingers and nails||Delicate blue|
|Decay, plaque or false teeth||No fluorescence|
|Seborrheic eczema||Dull brownish-yellow|
|Psoriasis (on underside of scales)||Silver white light, (pink)|
|Paraffin wax + paraffin oil||Blue|
|Quinine in drinks like tonic water||Blue|
|Alcoholic solution of chlorophyll||Red|
|Crystalline lens of eye under suitable conditions||Bright blue|
Forensic uses include drugs e.g. Lysergic Acid (LSD) which can be detected by absorption and excretion of quite small quantities. Mineral oils fluoresce differently from vegetable oils e.g. compare mineral oil, linseed oil, paraffin, vaseline etc. using oil spots on paper. Precious stones and pearls fluoresce differently depending on their origins. Wool can be distinguished from cotton and silk.
Baudot P Andre JC. (1985) Identification and quantitative determination of LSD by fluorescence: new data United Nations Office on Drugs and Crime Bulletin January 1st : 79 –93. Accessed July 5th 200 url http://www.unodc.org/unodc/en/bulletin/bulletin_1985-01-01_1_page007.html
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