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AUTOMOTIVE HEADLIGHTS, PART 2
THE BOSCH AUTOMOTIVE HANDBOOK devotes 37 pages of fine print to the subject of Lighting Equipment. Wikipedia’s Headlamp entry is also extensive, with 116 references. Here I offer highlights, with comments on several units and techniques of headlight measurement and the continuing differences between Europe and North America regulations. This follows up on descriptions of hardware given in “Automotive Headlights, Part 1.”
ECE versus FMVSS/CMVSS regulations. The Economic Commission for Europe, the ECE, is a United Nations group working for world harmonization of vehicle regulations. By contrast, regulations in North America are embodied in a U.S. Department of Transportation Federal Motor Vehicle Safety Standard, FMVSS 108, and its Canadian counterpart, CMVSS 108. These are based on standards established by SAE International, formerly the Society of Automotive Engineers.
ECE regulations involve Type Approvals issued by government. FMVSS/CMVSS 108 compliance relies upon manufacturer self-certification.
Here’s a selection of scientific units involved in illumination measurement. The International System of Units, SI, has all but replaced traditional English units of candlepower and the like.
Candela. The candela is the basic SI unit of luminous intensity, of how strong a point of light is shining in a particular direction. Its name is Latin for candle; an ordinary candle emits light with the luminous intensity of about 1 cd.
An ordinary candle’s intensity of emitted light is about one candela. Image by Bangin.
This luminous intensity is weighted across the full spectrum of wavelengths to account for the sensitivity of the human eye. It’s akin to the dBA sound scale and its weighting with regard to our hearing.
Lumen. The lumen is the SI unit of luminous flux, of the total visible light emitted by a source. Its definition opens up a wonderful can of mathematical worms: 1 lm = 1 cd·sr, where cd is candela and sr is a steradian.
Er…, what’s a steradian? This is short for square radian. Ok, but then what’s a radian?
At left, an angle of one radian. At right, a steradian is its solid-angle counterpart.
The radian has an intuitive (and tasty) interpretation: It’s the angle of a pie slice whose crust length equals the pie’s radius.
In a lot of mathematics, radian measure is more useful than degrees. The fact that 1 rad is a tad less than 60 degrees (indeed, about 57.3 degrees) is evident from recalling that an equilateral triangle has interior angles of 60 degrees.
The arc length associated with this equilateral triangle is obviously a bit greater than the circle’s radius.
Back to illumination: Lamp output can be specified in lumens. In fact, by European Union law, lumens must be used (rather than watts).
Light output versus power consumption for three sources. Data from www.energystar.gov.
Luminance efficacy. How well a source produces its light can be quantified by lumens/watt. For example, data from the table above gives average efficacies of 14, 60 and 85 lm/w for incandescent, fluorescent and LED technology, respectively.
Illuminance. The SI unit of illuminance, of luminous intensity, is the lux; 1 lx = 1 lm/m2. The complexities of headlight patterns can be seen in the following descriptions from The Bosch Automotive Handbook.
Above, luminous intensities of Europe/ECE headlamps. Below, FMVSS counterpart for low beam. Source: The Bosch Automotive Handbook, 8th Edition, 2011.
Briefly, patterns of ECE light distribution differ from those promulgated by FMVSS. In fact, different units of luminous intensity are used: ECE R 112 is defined in terms of lux; FMVSS 108, in terms of candela. (From RapidTables, 3 lx at 50 ft. is equivalent to about 700 cd.)
On low beams, North American headlights throw more light straight down the road. The ECE pattern has a sharp asymmetric cutoff that avoids glare for on-coming drivers, but also reduces illumination of overhead signage. By contrast, ECE high beams are stronger than those permitted by FMVSS.
At left, an ECE low beam seen from above. At right, an ECE high beam.
Controversy exists as to which automotive headlights are better or safer, the ECE or FMVSS variety. However, in a sense, the differences reflect realities of the two driving cultures and environments. ds
© Dennis Simanaitis, SimanaitisSays.com, 2015
Simanaitis Says 
Wonder how many pages the Lucas Automotive Handbook gives to headlights.
Ha! And how stained the pages are…. I love the argument that Lucas wires transmit smoke. And, of course, when the smoke seeps out, the electrics don’t function.
Which reminded me of the joke: “Why do the Brits drink warm beer? Because they have Lucas refrigerators!” – i recall Peter Egan writing about various challenges with Lucas wiring in old English sportscars. Guess it’s better today. The new Opel Astra (Europe only?) can be had with some fancy powerful LED matrix headlamps that intelligently and automatically adapts and adjusts to whatever situation you may encounter. Test drivers in car mags love them and I look forward to testing them myself.