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Photochromic ink darkens, as the light level increases. Some photochromic inks change colour. In fact, it is UV light that causes the darkening of the ink, which means the ink works best in natural light. This special ink has two main applications; sunglasses and spectacles. However, it is also used in novelty items such as embroidery thread and toys, where colour change takes place according to light level. Nail lacquer / varnish is also available with photochromic technology.
|The diagrams below, show a special UV torch shining ultra violet light on a test tube filled with photochromic ink. Diagram one, shows the torch off and the ink remains clear in colour. In diagram two, the UV light has been switched on for five seconds and the ink begins to darken. In diagram three, the light remains on for fifteen seconds and consequently the ink darkens considerably.|
|HOW PHOTOCHROMIC LENSES ARE MANUFACTURED|
|Modern photochromic lenses are manufactured from plastic (polycarbonate). The ‘blank’ lenses are carefully heated in a special oven, so that photochromic ink is absorbed by the surface layer. Each lens is ‘ground’ precisely, according to the optician’s prescription. It is the ink molecules in the surface layer of the lens that change shape, due to a chemical reaction caused by UV light. The brighter the natural light (sunlight), the darker the lenses appear.|
|A PRACTICAL APPLICATION OF PHOTOCHROMIC INKS|
|The tint of a photochromic lens changes according to the intensity of natural light (UV light). Inside a building, the lenses remain clear but they darken when outside in sunlight. Spectacles with this type of lens are comfortable to wear, in all conditions. However, they are more expensive than spectacles with normal, clear lenses.|
|COLOUR CHANGING NAIL
It is possible to buy nail varnish that changes colour according to the intensity of natural sunlight. The nail varnish / lacquer takes advantage of photochromic technology.
|NOVELTY TEDDY BEAR
The teddy bear seen opposite is an example of a novelty toy. The sunglasses and ribbon change colour when exposed to natural light (sun light). The ribbon on the bowler hat, also changes colour.
|The sun bed seen below,
has UV light tubes for tanning. In normal use, the lights are turned on
and controlled by a timer. After a set time, the lights turn off
automatically. UV light can be dangerous, if a person is exposed to it for
As an extra safety precaution, an arm band worn by the user, has a warning sign (badge). It changes colour, when the user has experienced more than the safe amount of UV light.
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