Through The Past Darkly - Horrible Headlight History

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Re: Through The Past Darkly - Horrible Headlight History

Post by Junkman »

3. What does the science say?

Actually not much, believe it, or not.

Pierre Devaux, a scientist and member of the CIE, issued several papers on the subject, which are readily available online. Particularly his 1970 paper goes into great detail about why selective yellow light might be more suitable and less glaring than white light for night driving.

A research experiment done in the UK in 1968 using tungsten non halogen lamps found that visual acuity is about 3% better with selective yellow headlamps than with white ones of equal intensity.

A 1976 study done in the Netherlands found no significant benefit to either colour over the other, an interesting result, because it contradicts both, the claims of yellow superiority, and the counterclaims of the white light fraction.

Researchers note that tungsten filament lamps emit only a small amount of the blue light blocked by a selective yellow filter, so such filtration makes only a small difference in the characteristics of the light output, and suggest that headlamps using newer kinds of sources such as HID bulbs may, through filtration, give off less visually distracting light, while still having greater light output than halogen ones.

But no matter what scientists say, selective yellow lamps have consistently been subjectively preferred as better in bad weather and lower in glare than white ones. Even now, when most of the world's fog lamps emit white light, the superiority of yellow fog lamps still has traction.

So is there a real benefit?
Or is it just a subjective impression?

Because yellow light requirements are no longer on the law books, except in Monaco, where it is obviously not enforced, we will likely never know the vagaries of the answer to this question, since all scientific research has ceased. There are problems with drawing a conclusion from the 1936-1993 European experience with selective-yellow headlamps, since for one thing, car lighting technology during that time frame generally didn't give the driver enough light for safe night driving no matter which colour his headlights were. But even if this is dismissed, filtering out the blue, indigo, and violet, does reduce the absolute intensity of the beam by about 12-18 percent depending on the source. This, however, and I can't stress it enough, has no, or only a negligibly little part in reducing the glare, since a minimum of a 15% change is needed to cause an observer to even begin to notice any difference. No, it's the colour of the light that makes the difference. Even if a yellow and a white headlight emit exactly the same light intensity, the yellow light will be perceived as less glaring.

What, then, explains the persistent subjective preference among experienced poor weather drivers for selective yellow fog lamps, whether or not they happen to know that's the name of the colour, despite the white fog lamp prevalence for over two decades now?
Well, selective yellow light can indeed improve a driver's ability to see in fog (or rain, or snow), but not because it 'penetrates fog better' or 'reflects less droplets'. It's because of the way the human eye processes different colours of light.

Blue, indigo, and violet are difficult for the human optical system to process correctly. They are the shortest visible wavelengths and tend to focus in front of the retina rather than upon it. To demonstrate this to yourself, look at something that's a deep blue light emitter against a dark background in the absence of white light, such as a blue illuminated shop front, or a blue runway light at an airport. From any appreciable distance, it's almost impossible to see the blue lighted object as a sharply defined form. The edges blur. This blur effect is not present with nearby signs or lights of colours other than blue, try it out. I, for example, find it difficult to focus on the blue illuminated instruments when driving a VAG by night.

Blue light also stimulates the reaction called glare. Within the range of allowable white light, bluer headlamps have been shown to be up to 46% more glaring than yellower ones for a given intensity of light. So, it seems culling the blue out of the spectrum lightens the optical workload and reduces glare. For a more detailed examination of this effect with respect to driving in foul weather, see Bullough & Rea's study on the topic.

My personal experience is that driving with yellow headlights when it's really dark is indeed more relaxing for the eyes and they get accustomed to the reduced intensity very quickly. However, the glare effect is consequently even greater when encountering a car with white headlights, so the whole yellow light thing only really works when everyone has them.

Surprisingly I was unable to find any study on eye health. Whether French drivers from 1937 to 1993 suffered from less or more or equal eye problems than drivers in other countries has seemingly never been investigated. And this is despite medical students keep telling me there aren't any subjects left for writing a doctorate.
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Re: Through The Past Darkly - Horrible Headlight History

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4. How can selective yellow light be obtained?

Traditional methods involve simple subtractive filtration. Either the lamp lens is made of selective yellow glass, or a selective yellow glass balloon is put over the colourless bulb, or the a bulb itself is made with yellow glass, or a yellow reflector in conjunction with careful shielding, so none of the white bulb's light would shine through the colourless lens without first hitting the yellow reflector. The yellow glass contained cadmium, a toxic metal that came under strict control in the 1990s for environmental reasons, so the industry shifted towards yellow coatings applied to the inside surface of the lamp lens or reflector instead.

Another technique is to have a dichroic filter on the bulb or the lens. Sold under a variety of names ("Gold", "Irridium", "Ion Crystal", "All Season", "All Weather", etc). This is an irridescent multilayer interference coating which diffracts the blue-indigo-violet light so as to separate it out from the remainder of the light. That remainder (i.e. selective yellow light) passes straight through the filter. The blue-indigo-violet light, because it is not absorbed (blocked) but merely diffracted (bent to an angle) still leaves the lamp. It does so off axis, hence lamps with a dichroic filter on the bulb or lens tend to glow blue when viewed from outside the main portion of the light beam, and there can be an objectionable blue haze outside the brightest areas of the beam. The irridescence of these coatings also causes or amplifies second and higher order filament reflections, which can cause the lamp to emit more light into regions intended to be dark for control of glare or backscatter, such as above the cutoff of a fog lamp, or into the upward/leftward oncoming eyes zone of a low beam. In other words, with the mirrorlike dichroic coating reflecting images of the glowing filament, light goes where it doesn't belong.

Generally yellow bulbs are no longer available in a quality worth buying, unless you obtain NOS. The market demand for them in Europe quickly dwindled once France stopped requiring yellow light. While yellow lights are still permitted in France and a variety of other countries, they are mostly regarded as a retro or styling item and this no longer justifies first line manufacture, so most of the yellow bulbs now on the market are rubbish.
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Re: Through The Past Darkly - Horrible Headlight History

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5. What about light loss due to filtration?

Here is a transmissivity curve showing the amount of light of different wavelengths passed by a selective yellow filter:
SY_Transmissivity.jpg
SY_Transmissivity.jpg (18.33 KiB) Viewed 5559 times
You can see most of the red-orange-yellow-green light is passed, but most of the blue-indigo-violet light is blocked, so less than 100% of the light going into the filter comes out of the filter. Filtration loss is sometimes pointed to as a reason to prefer white lights or reject selective yellow ones. It's a debatable point, because while intensity is the primary factor in how well a light lets us see, the filtration loss involved in going from white to selective yellow is less than the smallest intensity difference that causes an observer to see a just about noticeable difference, and quite a lot smaller than output differences caused by other factors such as bulb quality and feed voltage. Moreover, the human visual system's difficulty processing blue light means the "missing" light wasn't very useful to begin with. The blue-indigo-violet being filtered out is only a very small part of the bulb's total output. The visible spectrum consists of all the colours of the rainbow - red, orange, yellow, green, blue, and indigo + violet. Glowing filaments produce a great deal of light in the red-orange-yellow-green wavelengths, and only very little light in the blue-violet wavelengths. To put very rough numbers on the matter, an H4 p43t 60/55W bulb produces about 1000 lumens, of which approximately 250 are red, 250 are orange, 250 are yellow, 175 are green, 50 are blue and 25 are violet.

To illustrate the relative effects of filtration losses depending on what colour is desired in the end, let's first look at doing the opposite of obtaining selective yellow light. Suppose we want to add a filter to the glass that makes the light look bluer/colder. How does it do that? Well, there's no such thing as a filter that adds light into the beam passing through it. So green-blue-violet light cannot be added, the only way to get the light to look colder is to suppress red-orange-yellow. To make the light to look, let's say, 20% colder, red-orange-yellow must consequently suppressed by 20%. According to what I wrote above, there is a total of 750 lumens' worth of red, orange and yellow. Cutting this by 20% leaves 600 lumens, plus essentially all of the bulb's original green-blue-violet output of 250 lumens, so the bulb now produces light that looks 20% colder and produces 850 lumens.

850 lumens happens to be the minimum legal output for an H4. To meet this compliance or performance, a bulb that produces only the bare minimum of light cannot be produced, because 50% of the production will emit only 849 lumens or less. A high-luminance filament would need to be employed to counteract some of the filtering losses, while at the same time the maximum allowable wattage specified the ECE regulations must not be exceeded.

This blue-bulb walk-through should bring clarity to why filtering for yellow does not significantly reduce light output. Take the 1000-lumen H4 as broken down by colour output above. The yellow is obtained by suppressing blue-indigo-violet (selective yellow contains all the green found in white light. If taking out green, it would be a turn signal type of amber light.) Cutting only blue-indigo-violet by 80% leaves 925 lumens' worth of red-orange-yellow-green, plus 15 lumens' worth of blue-violet after filtration. The total is 940 lumens, a much smaller loss.
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Re: Through The Past Darkly - Horrible Headlight History

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6. Conclusions

No scientific proof of selective yellow headlights being either superior, or inferior, to white ones has ever been delivered and likely never will, since, as I pointed out, scientific research ended with them being written out of the law books. However, the subjective perception that selective yellow lights are easier on the eyes and cause less glare, especially in inclement weather conditions, is held by the overwhelming majority of people who have used them. However, there never was a real reason for them being written into legislation, especially not in one country alone, which also happens to be a country that's among the most visited by foreign vehicles that gleefully emit white light.

Since at no point in the history of automotive headlights there was any evidence of a superiority of yellow over white, and neither was the opposite, the sensible thing would be to leave the decision to the individual motorist. But that would be sensible, so it is not going to happen.
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Re: Through The Past Darkly - Horrible Headlight History

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7. References and further reading

Auriot, Vincent (1936) Eclairage des automobiles, Journal Officiel de la Republique Française, Le ministre des finances.

Bullough, John, Mark S. Rea (2001) Driving in Snow: Effect of Headlamp Color at Mesopic and Photopic Light Levels, SAE Technical Paper Series.

Christie, A.W., Ashwood, J.E., Symons, R.D.H. (1968) Visual Acuity in Yellow Headlights, RRL Report No LR 156, UK Ministry of Transport Road Research Laboratory.

Devaux, P. (1970) State-of-the-Art Signaling and Lighting, SAE 700386, Society of Automotive Engineers.

ECE Regulation 19: Uniform Provisions Concerning the Approval of Power-Driven Vehicle Front Fog Lamps.

ECE Regulation 48: Uniform Provisions Concerning the Approval of Vehicles with Regard to the Installation of Lighting and Light-Signalling Devices.

Flannagan, M J, Sivak, M, Battle, D S, Sato, T, Traube, E C (1993) Discomfort glare from High Intensity discharge headlamps - Effects of context and experience, University of Michigan Transport Research Institute.

Japanese Industrial Standard JIS D-5500 (2007) Automobile Parts-Lighting and Light Signaling Devices p. 5, sec. 4.4.2, table #4.

New Zealand Vehicle Inspection Requirement Manual p. 4.1.2

SAE Standard J583: Front Fog Lamp

Schreuder, D.A. (1976) White or yellow light for vehicle head-lamps?, SWOV Publication 1976-2E, SWOV (Dutch Institute for Road Safety Research).

Sullivan, J. M. (2001) Visual effects of blue-tinted tungsten-halogen headlamp bulbs, University of Michigan Transport Research Institute.

United Nations Economic and Social Council, Economic Commission For Europe (2005) Proposal for collective amendments to regulations Nos. 37 and 99 regarding the colour "Selective Yellow", Working Party on Lighting and Light-Signalling (GRE)(Fifty-fourth session, 5 – 8 April 2005, agenda item 10.2.).

United Nations Regulations (Revision 3) (1958) Agreement concerning the Adoption of Harmonized Technical United Nations Regulations for Wheeled Vehicles, Equipment and Parts which can be Fitted and/or be Used on Wheeled Vehicles and the Conditions for Reciprocal Recognition of Approvals Granted on the Basis of these United Nations Regulations, E/ECE/TRANS/505/Rev.3.
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Re: Through The Past Darkly - Horrible Headlight History

Post by Warren t claim »

Junkman wrote: Mon Apr 15, 2019 3:41 pm 10. 1981-1990

In 1983 the NHTSA, again responding to a petition, approved the use of headlamps fitted with a standardised replaceable halogen bulb (9004) in the U.S. For the first time ever "architectural" headlamps could be fitted to new American cars for the 1984 model year. Practically immediately most American cars were designed with these new style headlamps with few exceptions. Plastic lenses and housings were the materials of choice from the onset, because they provided improved headlamp performance and increased styling flexibility. Nevertheless, further sealed beam sizes (e.g., 92 mm by 150 mm in 1986) were introduced, as well as additional halogen replaceable bulbs (e.g. 149005 and 9006 in 1987), which finally provided Americans stylists with sufficient flexibility to design practically any shape and form of headlamp assembly and legally install it on domestic market vehicles.

At the same time research and development was done on HID (high intensity discharge) lighting systems all over the world. Again, this wasn't new technology, but it had to be adapted for automotive use. In particular startup and quick restrike (that's switching between main and dipped beams) times had to be brought to within acceptable levels.

Although the early lighting literature indicates that attempts were made even back then to use something other than a parabolic reflector, most headlamps from the very beginning of automotive lighting employed such a reflector to collect the light from the source. By the early Eighties, European engineers began to experiment with segmented reflectors, although the basic surfaces were still parabolic. Their light source of choice was still the trusty H4 bulb, though. The 1983 Austin Maestro was the first vehicle equipped with Lucas-Carello's homofocal reflectors, which comprised parabolic sections of different focal length to improve the efficiency of light collection and distribution.

The next step was to develop multisegmented reflector surfaces, but this only became feasible with the emerging availability of CAD programs for optical applications. Smooth computer generated surfaces could now be designed, with the goal to ultimately form the required beam pattern without any optics in the lens. The 1987 Dodge Monaco/Eagle Premier twins and the Citroën XM were the first commercially available cars with complex reflector shapes, but they still had faceted optic lenses. General Motors' Guide Lamp division in America had experimented with clear lens complex reflector lamps already in the early 1970s and achieved promising results, but the U.S. market 1990 Honda Accord was first car ever with clear lens multi reflector headlamps. These had been developed by Stanley in Japan.

Ellipsoidal surfaces for headlamp reflectors were also first attempted early in the history of headlighting. I found an article about it issued by the Department of Commerce in 1925 and obviously it wasn't a new thing even then. In 1985, polyellipsoid headlamps (also called projector lamps) were developed in Europe and first installed as production headlights in a Neoplan city bus in Germany. Soon many other types of vehicles were equipped with this type of headlamp assembly. The advantages are a smaller headlamp vertical height, more uniformity in the beam pattern, and the ability to make a sharper cutoff, or gradient, which allows more accurate visual headlamp aim. They are particularly well suited for motorcycles and became almost universal on those for a while.
I often wondered why the yanks ruined the look of European cars like the XJS and Mercedes SL.
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Re: Through The Past Darkly - Horrible Headlight History

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Because guffament, of course. Or do you think they did it because they liked ruining cars so much?
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Re: Through The Past Darkly - Horrible Headlight History

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I assumed the latter as they try so hard to only produce crap cars themselves.
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Re: Through The Past Darkly - Horrible Headlight History

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Nonsense.
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Re: Through The Past Darkly - Horrible Headlight History

Post by Forddeliveryboy »

    Some interesting info there, JM. I think Britain was slower to use halogen lamps than continental Europe, I vaguely remember looking out for Saabs, Volvos and other forrin stuff with those bright, bright lamps which made the old ones look dirty yellow in comparison.

    One good reason SY headlights seemed so good was that French headlamps of the 1970s were often superb, in no small part because they had a massive reflector area - look at the depth of a Cit GS' units. I've never known better lamps, other than the massive things some 20s and 30s cars used.

    I continue to use yellow lamps November-March. When roads are wet they make a big difference, allowing you to see more clearly and tiring the eyes much less on a long, fast trip on unlit, poor roads. But yes, it's increasingly difficult to find decent bulbs.

    I only drove a couple of times in France before 1993, but the odd white headlamp really jarred on the eyes. Those were the days when the French had significantly better cars for driving at speed, they enjoyed making full use of this, it seemed most extremely in the dark.
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