Headlights: more light, less illumination

Yes I know exactly what you mean. I thought that when I got my cataracts sorted out, driving at night would be fine again. But no - it's just as bad. I've seen advertised some yellow(?) tinted 'shades' which allegedly cut down the glare. Has anyone tried them?

One of the main parts of the problem is that for streamlining (and fashion) purposes, headlights are made as small as possible, and this means that brightness must be higher for a particular amount of light. My first few cars had headlights seven or eight inches in diameter, whereas modern ones are often no larger than three inches.

The small size also means the beam-shaping lenses are less effective, and there is often a sloping glass outer cover, theoretically transparent but often causing further light scattering, so there is much more spill outside the wanted beam area.

Lamps called foglamps are often fitted on the front, and some drivers use them at night. They are not real foglamps, which provide an absolutely flat-topped beam, as any light spilled upwards from a foglamp defeats its purpose of not being reflected from fog or snow just in front of the car. The foglamps and dipped beams of an oncoming car usually look of roughly equal brightness, and the foglights should look much dimmer to an oncoming driver at average height.

Road humps don't help either, as well-adjusted dipped beams become main beams for an instant as the front of the oncoming car rises, enough to cause after-images which are not great on an otherwise fairly dark road.

And we're all getting older...

LEDs become semi-coherent at high current.

I first got to observe this, a long time ago, when I bought a 2 ampere Monsanto single Red LED with a threaded central slug under the anvil. The red light, if you shut off room lights and bounced the light off a painted plaster wall, it was "sparkly", which is the semi-coherence.

A laser if fully-coherent, and typically has a very small bandwidth as well (LED light is 200nm wide spectrally, a laser is a lot tighter on color, it's monochromatic). We try not to shine unlimited laser light at people, as today it is quite easy to make items that are not eye-safe. A laser would not typically be white, as it's a pure monochrome color. You could shine a blue laser through a phosphor of course, but that would be naughty.

Thus, not all illumination sources have identical photonic properties.

the light on an incandescent lamp is fully non-coherent. While the light can be bright, the sparkly effect seen with a LED is missing.

The sparkly effect is not necessarily "dazzle", as "dazzle" can indicate a health issue with "old eyes". I've had that. You can get dazzle, even with halogen lamps. But the sparkly effect is not really good.

White LED light starts with a blue LED. If the blue LED shines through a yellow phosphor, white light is emitted. Would the white light be semi-coherent as well ? It should not be, but then, I haven't attempted to verify this on a painted wall. I would never have suspected a LED could be semi-coherent, until seeing that 2 amp red one for myself.

To answer the question what are they going to do, they will pick the "most efficient" light source, over the pleasant or safe source. That's how the world works.

Paul

Some green lasers are actually infra-red lasers followed by a frequency doubler crystal and an infra-red blocking filter. Sometimes the filter gets left out to save money. Now that really is naughty!

Not always. Sometimes particular characteristics are required for brand consistency. For example, Audi require all their subcontractors to use red LEDs in the car cabin with a specified (and otherwise uncommon) wavelength to ensure that every module looks the same regardless of who made it.

John

Polarised headlights and windscreens are the answer.

Which means lights have to be brighter to compensate for the light absorption by the polarising filters in the headlights and windscreen. You also have the problem that as it gets towards dusk (too bright for headlights) it is more of a problem seeing things.

Otherwise, yes, an excellent idea.

ISTR there was a maximum wattage for the tungsten filament lamps fitted to cars when I took my driving test (1960). Clearly technical advances have by-passed this which, together with concentration into flat beams has made rather a mockery of any legal control measures.

Actually, motoring magazines and such quoted lamp power, which was easy to see written on the bulb, but the actual Construction and Use statutes stated used light units. When bulbs were either tungsten or quartz halogen, this correlated quite well with electrical power. I believe the unit used was brightness rather than luminous flux, but it has been half a century since I read them.

I have no idea what the current EU directive or regulation states, but it will be quite certain to still apply to the UK. OK, it appears there is no EU specification for maximum brightness. Clever. Headlight units must be 'type approved'.