When winter draws close and night driving becomes a part of the journey home, the thought of seeing clearly the hazards ahead, be it pedestrian, cyclist with no lights, horse or small child, becomes more worrying. It seems to be a good point to reflect on what the eye can and cannot see, before launching into the themes of special bulbs and spectacles. 

How the eye sees

The eye has two completely different systems for day and night vision, to deal with the enormous variation in brightness. Photographers will know that a daylight scene exposed at 1/250 sec in daylight may need 30 seconds or more at night, but in fact the maximum range with which the eye can deal is around 1,000,000,000 to 1. It cannot, however, deal with all this range at once. 

The bright end of the range is dealt with by the cone cells in the retina, which have 3 different pigments to absorb light, giving us colour vision. Cones do not respond to low light levels, where vision is dependent on the retinal rod cells. Rods have only one pigment, so colour vision at low light levels is impossible. It also follows that any discussion of bulb colours as a means of improving night vision is meaningless, as is the wearing of any kind of coloured spectacles at night. Ditto tinted windscreens. At night the only important feature of lighting is the spectral intensity around the 560 nanometer wavelength (a blue-green colour). A bulb with a blue-green filter will not perform any better than a clear bulb, and a yellow-filtered bulb will actually be worse. There is no justification for French-style coloured bulbs. 

Bright lights bleach the rod pigment, which is slowly regenerated in the dark Whereas bleaching is rapid, regeneration is slow. Full dark adaptation takes about 1 hour, which is why I am sceptical about claims to see far along the road when vision is temporarily disrupted by opposing headlights. You are blinder than you think. Incidentally the cone pigments are also bleached by the brightest of lights, but recover more rapidly, in about 10 minutes. 

Two useful tips when oncoming lights are blinding

1. Close one eye to begin the regeneration process earlier (also useful when entering a dark tunnel from bright daylight.

2. Look to one side of the blinding light. This bleaches the rods at the edge of the retina, leaving the central ones, which one uses more, less affected.

Special lenses

Polarising (Polaroid) lenses are useful for eliminating glare from reflections from metallic surfaces and water, but let less light through, not a good idea at night. Photochromic lenses, which darken in bright sunlight, might also be thought a bad idea, but in practice they are not a problem. Any other sort of coloured lens is a bad idea at night. 

Headlight bulbs

The best bulb is the brightest within the law. True xenon headlights are therefore the best, but very expensive. “Xenon-charged” and other phrases used to suggest that a bulb costing a little more than an ordinary quartz-halogen can be superior are simply a way of parting the credulous from their cash. 100 watt bulbs sold “for off-road use only” are illegal on the road. Premium bulbs cost a little more, but give off more light than regular bulbs for the same electrical consumption. Blue-filtered bulbs may give you street cred, but no better vision at night.

Copyright PHP Harris 2002

If you enjoyed that, you might like some of my other jottings. Click here.