Re: Through The Past Darkly - Horrible Headlight History
Posted: Tue Apr 16, 2019 3:08 pm
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.
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.