Are the bright lights that cyclists now use safe for the eyes of onlookers?
鈥� Many people permanently damage their eyesight by looking at the sun through binoculars. The damage is done not by the light, but by the heat from the infrared radiation in the sun鈥檚 rays. The lens in the eye focuses both the visible light and the invisible heat on to the light-sensitive cells at the back of the eye. The heat is so strong that cells can overheat and be damaged in only a second.
The bright lights used by cyclists are LEDs, which produce a lot of light but almost no heat, so having one shine in your eyes won鈥檛 damage the cells in the eye. However, that doesn鈥檛 mean the experience is risk-free.
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The iris expands and contracts to alter the amount of light entering the eye through the pupil. It contracts quickly on exposure to bright light but expands slowly if the light is removed.
Night vision in humans occurs when the iris is dilated and the cells at the back of the eye have adapted to the low light. It can take a long time to establish, but can be suddenly lost with a flash of bright light. If this happens, visible things become invisible, and the risk of bumping into things grows.
鈥淣ight vision can take a long time to be established but can be lost in a second with a flash of light鈥�
This effect is sometimes manipulated in night-time orienteering, where it is important to retain night vision for running and reading a compass and map.
Incidents suggest people try to secure an unfair advantage by switching on their LED headband 15 seconds before the start and turning around, dazzling as many competitors as possible.
To combat this, some people wear dark glasses with side shields and only take them off after the start, or carry a mirror!
Andrew Carruthers, Beaconsfield, Quebec, Canada
鈥� When you step outdoors on a bright, sunny day, you often experience discomfort and partial blindness until your eyes adapt to the illumination. Temporary blindness also occurs when you walk indoors on a sunny day.
Using the exposure reading on my digital camera, I compared the illumination of a room with large windows on a shady side of my house with the outdoors on a bright sunny day. The outdoors scene is 400 times brighter (or 400 times as many lumens).
Looking directly at an LED reflector light bulb, probably brighter than a bicycle鈥檚, at a distance of 1 metre, it is just eight times brighter than my room on a bright day, or a 50th of the brightness of my outdoor scene.
Your eyes are in no danger from even the brightest bicycle light. It might appear painfully bright, but this is because you are usually looking at it with dark-adapted eyes. In daylight, it would not be so bright. In fact, I can barely make out the spot of light my powerful LED torch makes on a sunny wall.
Peter Bursztyn, Barrie, Ontario, Canada
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