H7 replacement lamp

That is part of the point that writer is making; the marketing hype is pseudo-science, not the real thing at all. The "... watts of light" thing isn't his choice of words. But Watts are a unit of "power", ie one Joule (of energy) per second (of time). Light is energy, so it isn't necessarily wrong to use Watts to measure light - but Lumens are a more useful unit for measuring the output of lamps, and Watts for measuring the power used to drive the lamp.

It's that latter usage (power consumption rather than light-energy produced) that is usually meant by the marketing types, to relate things to customers' familiarity with household incandescent bulbs which are traditionally classified by power consumption. You can find similar contortions on the packaging of 'high efficiency' (ie fluorescent) lamps.

Look at the phrasing here for what is being referred to :

"....plus 50 per cent category, which refers to the amount of light produced 75 metres ahead of the car compared with a standard bulb"

I admit that they don't say how they measured or in what units, but I suspect that this addresses the question of whether the gains are real or snake-oil/marketing hype.

T

Halfords and Auto Express don't seem to be talking about any unit of measurement in particular, and don't mention any. They just claim a vague comparison with an undefined "standard bulb".

I do take issue with Auto Express's statement "[...] But, for most of us, the best way of improving visibility in the dark is to fit brighter headlamp bulbs." Brighter headlamps may be of use to people anxious to drive as fast as possible on empty unlit roads (or off-road; rallying and racing, in other words) but in the real world, brighter headlamps just add to the useless glare and dazzle that all road users have to cope with.

I would hazard a guess that the magazine just held a photographer's light meter in the middle of the 'beam' to compare 'brightness'. That's a pretty meaningless comparison. What needs to be compared is how easily the driver can interpret what is illuminated by his lights - and what isn't directly 'in the beam', as well as what is. Too bright a beam actually makes that more difficult, unless the vehicle is fitted with several lamps to 'flood' the area with a spread of more or less even light - as can be seen on cars taking part in night rallies. At the same time, the effect of the lamps on what other road users can usefully see needs to be taken into account.

Photography is not a good guide to what the human eye and brain can perceive; cameras do not work the same way as eyes and brains.

My sympathy is with the French legislators who wanted everyone to have their headlamps tinted yellow to reduce glare and dazzle. Too much light, particularly in the wrong place, is at least as bad as too little.