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