There is a problem. At CTC we have been getting an increasing number of complaints about dazzlingly bright cycle headlamps, from drivers, pedestrians and from other cyclists. When I get these lamps to review, I think it's great that they light up the world so well, but embarrassing when oncoming cars stop in their tracks at my approach! Yes this does happen, and no I don't think it's a good thing to blind the person steering an oncoming ton of metal!
Aiming the lamp downwards helps, but some of these lamps are so bright and so optically simple, that they have to be aimed down to only a few feet in front of the wheel before they cease to dazzle other road users. That makes the lamp useless to ride with. So what can you do? Some riders fasten the bracket a bit loose, so they can swivel their lamp down and up again, or shade the lamp with one hand, neither of which seem entirely satisfactory answers to the problem. Most lamps can be dimmed to a lower setting, but dimming and/or switching back to maximum, is invariably a whole lot more complicated, requiring several button-presses, than flipping the dip switch in a car. It's got to be that convenient, I think, before most people will do it.
Until recently I've only had the evidence of my eyes and the reactions of other road users. So when the Sunday Times asked if I was aware of the problem and offered to get some lights tested to see if there was any substance in the allegations that bike lamps are getting too bright, I had to agree.
Rest assured, the test was done properly. For your full information here's the technical basis of the tests conducted with my assistance, by Dr Gareth John, at the laboratory of Photometric & Optical Testing
Glare is a subjective matter. It can be measured, but depends upon the brightness of the general scene and position within that scene of the source of glare, as well as its ‘luminance’. Luminance is the intensity of light emitted (in candela) in a particular direction, divided by the area it comes from.
According to the Road Vehicles Lighting Regulations, it is an offence for any light on any vehicle (that includes bicycles) to “cause undue dazzle or discomfort to other persons using the road”. So the law is subjective also. To put some numbers on how bright a light can be before it’s likely to dazzle, we must refer to design standards.
Motor vehicles must be ‘type approved’ before they can be used on the road and as part of that approval their lighting must conform with ECE regulations. These regulations ensure that headlamps do not dazzle other road users by putting a top limit on the intensity of light emitted above the horizontal (and to the offside) by a dipped headlamp. For technical reasons different types of lamp have different limits, but the most an oncoming road user should see from any ECE approved headlamp, when dipped, is 625 candela.
When it comes to bike lamps, the British Standard is out of date to the point of irrelevance. Headlamps are no longer made to BS (so I told the Sunday Times to forget about its over-cautious anti-dazzle limit of 70 candela, but unfortunately the DfT dragged that up
) and most of those on the market conform to no recognised standard. A few lamps on the UK market however, are made to conform with German traffic regulations, which limit the intensity of light emitted by a cycle headlamp, above the horizontal, to 200 candela. A lower limit than motor vehicles is justified by the smaller size and hence brighter appearance (i.e. luminance) of cycle lamps, and 200cd is a reasonable figure bearing that in mind. Since bike lamps cannot (usually) be dipped, this regulation (and our defunct BS) assumes that the lamp is aimed downwards at an angle of 3.4 degrees. An international (ISO) standard for cycle lamps is currently in preparation, which most countries will probably ignore - except Germany and France. This has the German anti-dazzle limit of 200cd and aims the lamp down at 3.5 degrees. Note: the lamp can be as bright as anything in the beam centre, neither BS nor German nor ISO standards put any limit on a bike headlamp's peak intensity, it's only above 3.5º up from that hotspot that there is any limitation.
Most cyclists will probably aim their lamps a little higher in order to see far enough ahead (I checked the headlamp on my commuter bike and it's aimed down at only 2 to 2.5º), but to be fair we measured at 3.5 degrees above the centre of the beam’s hot-spot. I recommended reserving our sternest criticism for lamps that could not easily be dimmed to 625cd, the same as a dipped car headlamp, even though car lamps are bigger and hence exhibit less glare at the same intensity. Most of the bike lamps we tested exceeded that nevertheless, and some by a huge amount!
Fortunately perhaps, it takes a lot more light to look a little brighter and to represent the way in which these differences are actually perceived I charted our measurements on a logarithmic scale. The red spots indicate the intensities of every lamp’s beam centre when it’s set for maximum brightness, green spots are the maximum values 3.5º above there, and green boxes mark the corresponding values on the lamp’s minimum setting.
I said we should be happy with a lamp that could be dimmed to less than 200cd, or even 652cd, in one simple movement and likewise back to maximum, just like dipping a car headlamp. But with most lamps it takes several button-presses to go from max to min, or back again.
We didn’t measure the flash modes, but some of these lamps appear to flash at maximum intensity. This might not be a problem in daylight, but even their minimum 3.5º intensities seem likely to cause discomfort if flashed at night.
So that's it. There is a problem and you'd better believe it. Sure the Sunday Times has exaggerated it a bit, like they do, but the manufacturers of cycle lamps seem to need a stronger incentive than we've so far been giving them, to get a bit cleverer in the optics department and/or provide us with simple dimming facilities, so we can use their products more responsibly.
There's one lamp in that line-up, the B&M Ixon IQ, that conforms with German regs and has a much more sophisticated optical design than any of the others, with a very sharp cut-off a couple of degrees above the beam centre. Admittedly its beam centre isn't as brilliant either, as any of the others in the test, but look at the difference: this lamp is more than 15 times brighter in the beam centre than 3.5º above. Combine this optical design with a more powerful LED and it would be possible to equal the performance of some of the brightest lights in test without exceeding 625cd at 3.5º up. Combine it with an easy dimmer switch to one-quarter power and it would pass the proposed ISO standard.
Another lamp in that test, the Supernova Airstream, has fairly clever optics and achieves a less abrupt cutoff. On lowest setting it just scrapes under 200cd at +3.5º, but it takes several button presses to get to/from that setting. This lamp also comes in a version that's legal in Germany, which I guess must be programmed to get there in one. But the UK importer sells the 'international' version. Pity.