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Old 06-22-2001, 10:17 AM   #1
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HID & Halogen Lights, And your eyes.

Was reading something interesting and thought I'd share it with you guys. For those that doubt the HID lights and those that don't. Enjoy, it's a lot of reading but well worth it!

Actually, Joyce Kilmer’s poem “The Twelve-Forty-Five” is about a train. Nevertheless, its engine’s shriek and headlight’s glare are most automotively appropriate these days, what with controversies concerning High Intensity Discharge lights, their faux brethren, blue-blocker glasses, red instrument illumination and related night-driving issues. All of these, in turn, got me interested in the physiology of driving at night. And, the next thing I knew, I was gleaning information from the American Academy of Ophthalmology, in the person of spokesman Dr. Martin Mainster, Professor of Ophthalmology, University of Kansas Medical Center; and Dr. Michael J. Flannagan, Assistant Research Scientist at the University of Michigan’s Transportation Research Institute; as well as specialists at BMW and Ford. Here’s what I learned.

The human eye

To say the eye is like a camera dramatically underrates human physiology, but it’s not an absurd starting point. Light enters the eye through an adjustable iris and encounters a controllable lens. The iris opening depends on the intensity of the light. The lens focuses this light onto the retina, the film, sort of, sensitive areas of which are wired to the brain through the optic nerve. The sensitive areas are of two types: cones and rods. Cones give our perception of color; rods, our vision in low light, albeit purely black and white.

We see fine print by focusing it onto the center of the retina, a cone-rich region called the macula. At its very center is the fovea centralis, which has only cones. By contrast, our peripheral vision depends on the far reaches of the retina. Just as the macula has mostly cones with some rods, this region has mostly rods with some cones.

Our day vision (precisely: photopic vision) relies mostly, but not exclusively, on cones. Our pure night vision—stargazing, for instance, or the night watch at sea—is termed scotopic vision and depends mostly, but not entirely, on rods (which, of course, is why cats at night are all shades of gray). In between is a regime called mesopic vision, what our cars’ headlights offer us in night driving. In fact, specialists suggest that night driving is actually high-end mesopic, not far from the photopic regime.

Our eyes’ sensitivity, loosely, our ability to detect a target against a background, depends not only on the regime of vision—photopic, mesopic or scotopic—but also on the wave length of the light, on its color. The visible portion of the electromagnetic spectrum stretches from blue, with a wave length of around 400 nanometers, through green, yellow, orange to red, at around 700 nm.

Photopic vision has a peak sensitivity at around 555 nm, in the yellow range. And it’s no surprise that R&T covers often feature yellow cars (to help them jump off the newsstand). By contrast, scotopic vision has its sensitive peak closer to 500 nm, in the blue-green portion of the spectrum. Mesopic evidently shares characteristics of both, though more subtly.

Red at night, drivers’ delight?

There’s interesting implication, not to say a bit of myth, in the matter of red illumination for instruments and controls. Since scotopic vision is most sensitive in the blue-green range, this night vision would seem least compromised by gauge illumination from the other end of the spectrum, namely red. We all recall movies of submariners and night fighter pilots bathed in red illumination. What’s more, it is not unfashionable today to see the same in automotive instrument panels.

And, in fact, if you’re driving by moonlight in the middle of nowhere with your car’s headlights off, it would be good to have instruments illuminated in red. In less bizarre settings, your eyes are functioning high-end mesopically, and the color of instrument illumination is essentially irrelevant.

Airline pilots gave up red illumination years ago when studies showed that the primary factor was brightness—not color. In modern aircraft “glass cockpits,” i.e., those replete with video displays, the panels are multicolored as well as multifunctional, day and night.

There’s nothing wrong with red displays, unless you’re one of the 8 percent of males who are red/green color-blind (curiously, the percentage of females so affected is much lower, 0.5 percent). Or maybe it’s presbyopia (“old eyes”) creeping up on you. On a personal note, my own 58-year-old lenses have lost their youthful talent for instant-zoom. An inherent characteristic called chromatic aberration causes red light to focus at a slightly different distance from white, and this leaves me with fuzzy red tach and speedo.

So is blue better?

Enter the xenon arc-discharge bulb with its characteristically bluish cast. Is this High Intensity Discharge lighting inherently better?

Xenon HID headlights are certainly more efficient. They produce some four times the illumination compared with filament lights of the same wattage (70–80 lumens/watt versus 18–20 for typical halogens). What’s more, having no filaments to burn out, HID lights may well outlast the car to which they’re fitted. True, this premium illumination is not without its tradeoffs (see “Technology Update: On The Beam,” R&T, August 1995), principal among these being cost—by maybe a factor of 10—and controversy concerning glare.

There’s interesting physics involved in HID illumination contrasted with incandescent lighting. Within an automotive HID bulb two electrodes reside in xenon gas. High voltage generates an arc, and light is produced as the xenon between these two electrodes is ionized. By contrast, in a conventional bulb, electricity passes through its tungsten filament, and the wire glows with the heat of electrical resistance.

If it’s halogen lighting, as fitted to the vast majority of today’s cars, the filament can take increased heat because the halogen gas promotes an internal recycling, a redeposition of evaporated tungsten. This greater heat, in turn, produces a light that’s less orange than that of the classic sealed beam. But the best of halogen bulbs exhibit a “color temperature” of around 3800 degrees Kelvin, compared with sunlight’s 5250 deg K. A xenon HID’s color temperature is around 4500 deg K, and thus its illumination is closer to daylight conditions.

This needn’t be true of all HID sources, however. Sodium or mercury lamps, for instance, are the sort common in urban lighting, and they have a tendency to turn reds into muddy browns and flesh tones positively creepy. But, at least in part because of its bluish tint—toward that sensitive area of our scotopic regime—xenon HID illumination does an excellent job of heightening contrasts in night driving, of making reflective paint of road signs and lane markers pop into view.

What about faux-HID?

If blue is good, then why not give a conventional bulb a blue tint or add a dose of xenon to its halogen atmosphere? Indeed, such faux-HID bulbs are on the market. I’d think of them as the headlight equivalent of red instrument illumination: a neat fashion statement, if you like, but not particularly functional.

If anything, the tint decreases the bulb’s illumination and increases its surface temperature. There are no electrodes, no arc discharge and no xenon ionization of note. The light is still produced by a filament, and this is what dictates its color temperature.

Maybe less blue is better?

Another fashion statement is implicit in “blue-blocker” glasses, tinted to absorb the blue end of the spectrum and leave everything tinged in yellow. Shooters favor these, with claims of sharper definition of target.

Whatever their merit in daylight target practice, such glasses are nonsense for night driving. Any tint between the eye and its nighttime target reduces brightness and requires more contrast for a target to be perceived against its background. What’s more, blue-blocker reduction is in that regime in which we’re most sensitive in the dark, namely the scotopic.

Another anti-blue contention is that xenon HIDs are worse in their scattering of light in fog and similar conditions. Like any good myth, this one has an interesting origin: The sky is blue because of Rayleigh scattering, a color-dependent phenomenon in which the blue portion of the sun’s spectrum gets scattered more than the red. (Without our atmosphere, the daylight sky would be black, with point-sources of sun, [reflected] moon and stars.)

And, so says the myth, the light reflected by fog droplets in our headlights gets scattered the same way. Ergo, blue illumination would make this scattering all the worse.

But reflected fog droplets (like the whiteness of clouds) are examples of Mie scattering, and this phenomenon characteristic of larger droplet size is chromatically-independent. That is, the color of the illumination has no effect whatsoever on the scattering.

A win/win for HID?

Xenon HID headlights would be a win/win, but for one pesky aspect—the other driver. If he’s directly behind you, you’ll be glad to have an anti-glare setting on your car’s rearview mirror. This is especially true if he’s in an SUV pushing the limits of regulated headlight height. And side-mirror glare may still be annoying, whatever the vehicle behind. If he’s coming toward you, you’re wise to look away from the glare of the oncoming lights. Otherwise, your eyes take time to recover from the ensuing overload of “dazzle.” It’s generally believed that recovery time lengthens with age.

Also, studies at the University of Michigan’s Transportation Research Institute indicate that the bluish tint of xenon HIDs produces substantially more dazzle. Subjects rated glare after being confronted by halogen or HID illumination at different intensities. Researchers collected subjective ratings as well as intensities measured at the subjects’ eyes. Halogens operating at 11¼2 times the intensity of the HIDs gave the same perceived glare.

The reasons for this are not completely understood, but test methodology suggests that scotopic sensitivity plays only a minor role, if any. It’s thought that the differences may lie at the other end of the optic nerve—in the brain itself. In a sense, human beings spent millennia of evenings illuminated by the orange glow of a hearth. Now, for less than a decade, we’re being bombarded by something bizarrely blue, and our brains don’t know quite what to make of it.

Controversies of HID glare—and, most welcomed, research efforts that may resolve them—continue.

A No. 4 pencil or a broad brush

The BMW 750iL was first in the U.S. with xenon HIDs, low beams only, in 1993. The Lincoln Mark VIII offered optional HIDs in 1996, first U.S. marque to do so. And this brings up another complex issue of night driving—the beam pattern. With only a bit of simplification, think of the European pattern as being drawn with a sharp and very hard No. 4 pencil, whereas the classic U.S. sealed-beam pattern would have been best rendered with a broad brush. Efforts are underway to homogenize these standards, but it’s still easy to recognize a sharper cutoff in illumination bearing European origins even though it meets U.S. standards (which have evolved since sealed-beam days).

The European standard prohibits illumination above the horizontal plane containing the bulb; the softer U.S. image allows only a modicum of illumination above this plane.

European standards also require automatic leveling and self-cleaning of HID headlights. The latter is typically achieved by high-pressure spray. Self-leveling suspension systems, such as Mercedes-Benz ABS, suffice for the former. Porsche’s Litronic system, by contrast, accomplishes the leveling within the light itself in conjunction with its low- and high-beam coming from a single arc source.

U.S. standards have yet to require these features. Automaker responses are mixed, some fitting HIDs without them, others taking advantage of their cars’ self-leveling, others incorporating active light control from their home-market standards

HID light engines for all

Let’s conclude with a look at what’s coming down the road. Before long, maybe within the decade, there’ll be a single source of illumination, its light piped through fiber-optic cables to multiple points of the car. There, it’ll be fine-tuned, directed, focused and tinted appropriately.

Having a single source, likely a xenon HID “light engine,” reduces the cost of arc-discharge illumination dramatically. Fiber-optic cables are inexpensive, durable and provide great packaging opportunities.

In time, it might be appropriate to celebrate our breaking away from that orange glowing hearth.
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Old 06-22-2001, 10:58 AM   #2
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Wow . . . that was a long article . . . I remember reading it . . . Road and Track, right?

Helped me to decide on Xenon bulbs . . . and I'm considering yellow fogs . . .
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Old 06-22-2001, 11:12 AM   #3
R.I.P. DAD 3/25/11
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Originally posted by i_rebel
Wow . . . that was a long article . . . I remember reading it . . . Road and Track, right?

Helped me to decide on Xenon bulbs . . . and I'm considering yellow fogs . . .
Yup! Just noticed it and read through it. I have the real HID's for my car, but haven't had the time to install. Think I'm just going to pay someone to have them installed while I am at work and just pick it up at the end of the day. I think they are definitely worth having.
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