Triboluminescence

Introduction

Some crystals give off a little flash of light when crushed. This phenomenon is called triboluminescence . Sucar is an example. Table salt does not show triboluminescence. Some types of candy work better than pure sucrose, in particular Wint-O-Green Lifesavers . These are flavoured with wintergreen oil (methyl salicylate). It occurs in plants like Round-leaved Wintergreen (Pyrola rotundifolia), here a photo from Val Munschauns in the Swiss National Park .

Observation

Dark adaptation

It it quite easy see triboluminescence for yourself. However the flashes of light are quite feeble and only visible for the dark adapted eye. So sit for at least a ¼ hour in complete darkness or wake up at night without using bright light. A short exposure to dim light will not undo the adaptation of the eye to low light levels. Note that some people suffer from nightblindness, they will have trouble observing triboluminescence.

Crushing sucar crystals

When your eyes are adapted to darkness, so called scotopic vision, the sensitivity for light is 10000 to 100000 times larger than in daylight and you can undoubtably see light when you crush sucar either as sucar cubes or as crystal sucar. Best is to use or improvise a glass mortar, so you can see what happens. At last resort you can use your teeth . You will not be able to see any colour as scotopic vision is colourblind.

Photos

Photographing triboluminescence is not easy. I crushed a sucar cube in total darkness between 18 mm laminated glass sheets. A digital photo was made with open shutter, 50mm lens f/1.4 ISO 3200.
The flash of light was clearly visible for the unaided eye.
The result was a black picture. However digital processing (for the R-,G- and B-plane setting all pixels with 8-bit value 7 or higher to 255) reveiled a clear impression of the light flash.

Explanation

Not very much is known about the exact mechanism triboluminescence. You can imagine that some electrons get dislodged when a crystal is broken. Apparantly the resulting electric field is momentarily large enough to produce a spark.

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