The UV Intensity card enables individuals to gauge the strength of the Sun's ultraviolet rays by giving a general measurement against a colour scale.
The card incorporates a UV sensitive area that changes to a shade of purple when exposed to sunlight. This can then be compared to the purple printed scale which shows 4 levels from Low to Critical and an approximate UV Index.
When there is bright sunshine the photochromic area will change to a dark purple but when it is cloudy the area will only change very slightly as ultraviolet rays can still penetrate clouds.
The photochromic area takes about 15 to 20 seconds to take a reading and to settle.