REF: Automotive Lighting, Color, and You - Honda-Tech


REF: Automotive Lighting, Color, and You

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Old 09-30-2006, 11:21 PM   #1
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Default REF: Automotive Lighting, Color, and You

This is information I have collected from various sources on automotive lighting. This should be a manditory for everyone who ever asks why people have yellow bulbs or if they should get HIDS, or why thier aftermarket projectors suck.

Perception of Color:



Given the same output of power at each wavelength, the visual system will sense the yellow-green region as the brightest and the red or blue regions as the dimmest. This is why, among equally efficient light sources, a light source that has most of its power in the yellow-green area will have the highest visual efficacy, i.e., the highest lumens per watt. However, without a reasonable proportion of red or blue in its output, a light source will not be able to render colors satisfactorily.

How we see color depends on the wavelengths emitted by the light source, the wavelengths reflected by the object, the surroundings in which we see the object, and the characteristics of the visual system. Our conception of the color of an object is a constantly changing, highly dynamic process. It depends on what colors surround the object, how long we have been exposed to the scene, what we were looking at before, what we expect to see, and perhaps what we would like to see.

In sumary: The more blue/red the light is, the shorter/longer the wavelength, and the less responsive the human eye is to it (less visibility). The more yellow/green light is, the shorter the wavelength, and the more responsive the human eye is (more visibility), but the light lacks color definition. The perfect light is a happy medium between the two - white. This will allow acurate lighting with adequate visibility.

THIS DOES NOT MEAN YELLOW HEADLIGHT BULBS GIVE THE BEST VISIBILITY. FOR AN EXPLANATION, READ THE SECTION ON DICHORIC FILTERS.



Color Temperature:



Correlated Color Temperature (measured in Kelvins)-or simply Color Temperature-is a scientific scale to describe how "warm" or how "cool" the light source is. It is based on the color of light emitted by an incandescent source. As a piece of metal (a theoretical Blackbody) is heated, it changes color from reddish to orange to yellowish to white to bluish-white. The color of light emitted by an incandescent object depends only on the temperature. We can use this scale to describe the color of a light source by its "Color Temperature."

When we say a lamp has a Color Temperature of 3000 Kelvins, it means a glowing metal at 3000 Kelvins would produce light of about the same color as the lamp. Instead, if the metal is heated to 4100 Kelvins, it will produce a much whiter light. Direct sunlight corresponds to about 5300 Kelvins while daylight, which has the blue from the sky mixed in, is typically 6000 Kelvins or above. A standard incandescent lamp has a filament at 2700 Kelvins, and therefore (by definition) a Color Temperature of 2700 Kelvins.


Why Do Colored Bulbs Produce Less Light?

To understand this concept, a person must understand one simple fact: colored bulbs are made using dichroic filter coatings directly on the glass. Dichroism is based on the principal of interference:

Dichroic Filters

A dichroic filter or thin-film filter is a very-accurate color filter used to selectively pass light of a small range of colors while reflecting other colors.

Used in front of a light source, a dichroic filter produces light that is perceived by humans to be highly saturated (intense) in color.

Used behind a light source, dichroic reflectors commonly reflect visible light forward while allowing the invisible infrared light (radiated heat) to pass out of the rear of the fixture, resulting in a beam of light that is "cooler". Modern quartz halogen incandescent light bulbs frequently contain an integrated dichroic reflector.

Dichroic filters operate using the principle of interference. Alternating layers of an optical coating are built up upon a glass substrate, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. The layers are usually deposited using a process carried out in a vacuum. By controlling the thickness and number of the layers, the frequency (wavelength) of the passband of the filter can be tuned and made as wide or narrow as desired. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters don't absorb much energy during operation and so don't become nearly as hot as the equivalent conventional filter (which attempts to absorb all energy except for that in the passband).

Interference

Interference is the superposition of two or more waves resulting in a new wave pattern. As most commonly used, the term usually refers to the interference of waves which are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. Two non-monochromatic waves are only fully coherent with each other if they both have exactly the same range of wavelengths and the same phase differences at each of the constituent wavelengths.



The principle of superposition of waves states that the resultant displacement at a point is equal to the sum of the displacements of different waves at that point. If a crest of a wave meets a crest of another wave at the same point then the crests interfere constructively and the resultant wave amplitude is greater. If a crest of a wave meets a trough of another wave then they interfere destructively, and the overall amplitude is decreased.

Interference is involved in Thomas Young's double-slit experiment where two beams of light which are coherent with each other interfere to produce an interference pattern (the beams of light both have the same wavelength range and at the center of the interference pattern they have the same phases at each wavelength, as they both come from the same source). More generally, this form of interference can occur whenever a wave can propagate from a source to a destination by two or more paths of different length. Two or more sources can only be used to produce interference when there is a fixed phase relation between them, but in this case the interference generated is the same as with a single source; see Huygens' principle.

Total phase difference is derived from the sum of both the path difference and the initial phase difference (if the waves are generated from 2 or more different sources). Hence, we can then conclude whether the waves reaching a point are in phase(constructive interference) or out of phase (destructive interference).

To sum up the above theorys, using a dichoric filter on a halogen bulb is absolutely destructive to light output. Being as the white/yellow light outputted by a halogen lamp and the x color of said dichoric filter, the crests and troughs of the two wavelengths intersect, and lower the amplitude of the overall wave. So not only is the overall wavelength of the light lengthened (thus lowering the lumins) due to the blue/purple hues in the dichoric filter, but the magnitude (overall energy) of the lightwaves are decreased as well.


Types of Frontal Lighting:

Headlamps

Dipped-beam (low-beam, passing-beam, meeting-beam) headlamps provide a distribution of light designed to provide adequate forward and lateral illumination with limits on light directed towards the eyes of other road users, to control glare. This beam is intended for use whenever other vehicles are present ahead. The international ECE Regulations for filament headlamps and for high-intensity discharge headlamps specify a beam with a sharp, asymmetric cutoff preventing significant amounts of light from being cast into the eyes of drivers of preceding or oncoming cars. Control of glare is less strict in the North American SAE beam standard contained in FMVSS / CMVSS 108.

Main-beam (high-beam, driving-beam) headlamps provide a bright, center-weighted distribution of light with no especial control of light directed towards other road users' eyes. As such, they are only suitable for use when alone on the road, as the glare they produce will blind other drivers. International ECE Regulations[4], [5] permit higher-intensity high-beam headlamps than are allowed under North American regulations[6].

Auxiliary lamps

"Driving lamp" is a term deriving from the early days of nighttime driving, when it was relatively rare to encounter an opposing vehicle. Only on those rare occasions when one did briefly face opposing traffic would one use the dimmed or "passing beam". The full or "bright" beam was therefore known as the driving beam, and this terminology is still found in international ECE Regulations, which do not distinguish between a vehicle's primary (mandatory) and auxiliary (optional) upper/driving beam lamps.[7],[8],[9]. The "driving beam" term has been supplanted in North American regulations by the functionally descriptive term auxiliary high-beam lamp. They are most notably fitted on rallying cars, and are occasionally fitted to production vehicles derived from or imitating such cars. They are common in countries with large stretches of unlit roads, or in regions such as the Nordic countries where the period of daylight is short during winter.

Front fog lamps

Fog lamps provide a wide, bar-shaped beam of light with a sharp cutoff at the top, and are generally aimed and mounted low. They may be either white or selective yellow. They are intended for use at low speed to increase the illumination directed towards the road surface and verges in conditions of poor visibility due to rain, fog, dust or snow. As such, they are often most effectively used in place of dipped-beam headlamps, reducing the glare-back from fog or falling snow, although the legality varies by jurisdiction of using front fog lamps without low beam headlamps.

Use of the front fog lamps when visibility is not seriously reduced is often prohibited (for example in the United Kingdom), as they can cause increased glare to other drivers, particularly in wet pavement conditions, as well as harming the driver's own vision due to excessive foreground illumination.

The respective purposes of front fog lamps and driving lamps are often confused, due in part to persistent misapprehension by the public at large that fog lamps are necessarily selective yellow, while any auxiliary lamp that makes white light is a driving lamp. Automakers and aftermarket parts and accessories suppliers frequently refer interchangeably to "fog lamps" and "driving lamps" (or "fog/driving lamps"). In most countries, weather conditions necessitating their use are very rare, and there is no legal requirement for them, so their primary purpose is frequently cosmetic. Studies have shown that in North America more people inappropriately use their fog lamps in dry weather than use them properly in poor weather.


Dual-beam headlamps

Night driving has long been dangerous due to the glare of headlights from oncoming traffic which temporarily blinds drivers approaching from the opposite direction. Therefore, headlamps that satisfactorily illuminate the road ahead of the automobile without causing this effect have long been sought. The first attempts to address this problem involved resistance-type dimming circuits, which decreased the brightness of the headlamps when meeting another car. This gave way to mechanical tilting reflectors and later to double-filament bulbs with a high and a low beam. Automatic headlamp dimmers were also introduced.

In a two-filament headlamp, there can only be one filament exactly at the focal point of the reflector. There are two primary means of producing two different beams from a two-filament bulb in a single reflector.

American system

One filament is located at the focal point of the reflector. The other filament is shifted axially and radially away from the focal point. In most 2-filament sealed beams and in 2-filament replaceable bulbs type 9004, 9007 and H13, the high beam filament is at the focal point and the low beam filament is off focus. For use in right-traffic countries, the low beam filament is positioned slightly upward, forward and leftward of the focal point, so that when it is energized, the light beam is widened and shifted slightly downward and rightward of the headlamp's axis. Transverse-filament bulbs such as 9004 can only be used with the filaments horizontal, but axial-filament bulbs can be rotated or "clocked" by the headlamp designer so as to optimize the beam pattern or to effect the traffic-handedness of the low beam. The latter is accomplished by clocking the low-beam filament in an upward-forward-leftward position to produce a right-traffic low beam, or in an upward-forward-rightward position to produce a left-traffic low beam.

The opposite tactic has also been employed in certain 2-filament sealed beams: placing the low beam filament at the focal point to maximize light collection by the reflector, and positioning the high beam filament slightly rearward-rightward-downward of the focal point. The relative directional shift between the two beams is the same with either technique—in a right-traffic country, the low beam is slightly downward-rightward and the high beam is slightly upward-leftward, relative to one another—but the lens optics must be matched to the filament placements selected.

European system

The traditional European method of achieving low and high beam from a single bulb involves two filaments along the axis of the reflector. The high beam filament is on the focal point, while the low beam filament is approximately 1cm forward of the focal point and 3 mm above the axis. Below the low beam filament is a cup-shaped shield (called a "Graves Shield") spanning an arc of 165°. When the low beam filament is illuminated, this shield casts a shadow on the corresponding lower area of the reflector, blocking downward light rays that would otherwise strike the reflector and be cast above the horizon. The bulb is rotated (or "clocked") within the headlamp to position the Graves Shield so as to allow light to strike a 15° wedge of the lower half of the reflector. This is used to create the upsweep or upstep characteristic of ECE low beam light distributions.

This system was first used with the Bilux/Duplo bulb of 1954, and later with the halogen H4 bulb of 1971. In 1992, U.S. regulations were amended to permit the use of H4-style bulbs. Named HB2 or 9003, for the U.S. market, and with very slightly different production tolerances stipulated, these bulbs are physically and electrically interchangeable with H4 bulbs. Similar optical techniques are used, but with different reflector and/or lens optics to create a U.S. beam pattern rather than a European one.

Each system has its advantages and disadvantages. The American system historically permitted a greater overall amount of light within the low beam, since the entire reflector and lens area is used, but at the same time, the American system has traditionally offered much less control over upward light that causes glare, and for that reason has been largely rejected outside the U.S. In addition, the American system makes it difficult to create markedly different low and high beam light distributions; the high beam is usually simply a rough copy of the low beam, shifted slightly upward and leftward. The European system traditionally produced low beams containing less overall light, because only 60% of the reflector's surface area is used to create the low beam. However, low beam focus and glare control are easier to achieve. In addition, the lower 40% of the reflector and lens are reserved for high beam formation, which facilitates the optimization of both low and high beams.

Complex-reflector technology in combination with new bulb designs such as H13 is enabling the creation of European-type low and high beam patterns without the use of a Graves Shield, while the 1992 US approval of the H4 bulb has made traditionally European 60% / 40% optical area divisions for low and high beam common in the US. Therefore, the difference in active optical area and overall beam light content no longer necessarily exists between US and ECE beams.


Optics Technology:

Lens Optics



A light source (filament or arc) is placed at or near the focus of a reflector, which may be parabolic or of non-parabolic complex shape. Fresnel and prism optics moulded into the headlamp lens then shift parts of the light laterally and vertically to provide the required light distribution pattern. The lens may use both refraction and TIR to achieve the desired results. Most sealed-beam headlamps have lens optics.

Reflector Optics



The optics required to give the proper light distribution pattern is designed into the reflector itself, with such a unit being known as an "optic reflector". The reflector design starts as a parabola standing in for the size and shape of the completed package. The optical engineer replaces the entire surface with individual segments of specifically calculated, complex contours. The precise shape of each segment is designed such that their cumulative effect produces the required distribution pattern.

Optic reflectors are commonly made of compression-moulded or injection molded plastic, though glass and metal optic reflectors also exist. The reflective surface is vapor deposited aluminum with a clear overcoating to prevent the extremely thin aluminum from oxidizing. Extremely tight tolerances must be adhered to in the design, tooling and production of complex-reflector headlamps.

Projector Optics



In this system a filament is located at one focus of an elliptical reflector and has a condenser lens at the front of the lamp. A shade is located at the image plane, between the reflector and lens, and the projection of the top edge of this shade provides the low-beam cutoff. The shape of the shade edge, and its exact position in the optical system, determines the shape and sharpness of the cutoff. The shade may have a solenoid actuated pivot to provide both low and high beam, or it may be stationary in which case separate high-beam lamps are required. The condenser lens may have slight fresnels or other surface treatments to reduce cutoff sharpness. Recent condenser lenses incorporate optical features specifically designed to direct some light upward towards the locations of retroreflective overhead road signs.

Take note that all of these different types of lenses process light different ways. This is why headlights designed for halogen lighting are not suited for HID lighting, and vise versa. Even HIDs designed for reflector housings instead of projector housings do not work well in non-hid housings. This is also true for using HIDs in halogen projector housing.


"Plug'n'Play" Hids VS. Projector Retrofit


Halogen Technology:

Halogen technology makes incandescent filaments much more efficient, and Europeans chose to use this extra efficiency to produce much more light than was available from nonhalogen filaments at the same power consumption. Unlike the European approach which emphasized increased light output, most U.S. low beam halogens were low current versions of their nonhalogen counterparts, producing the same amount of light with less power. A slight theoretical fuel economy benefit and reduced vehicle construction cost through reduced wire and switch ratings were the claimed benefits. There was an improvement in seeing distance with U.S. halogen high beams, which were permitted for the first time to produce 150,000 candelas per vehicle, double the nonhalogen limit of 75,000 candelas but still well shy of the international European limit of 225,000 cd. After replaceable halogen bulbs were permitted in U.S. headlamps in 1983, development of U.S. bulbs continued to favor long bulb life and low power consumption, while European designs continued to prioritize optical precision and maximum output.

The first halogen bulb for vehicle use, the H1, was introduced in 1962 by a consortium of European bulb and headlamp makers. This bulb has a single axial filament that produces 1500 lumens when operated at 13.2 volts. H2 (55 W, 12.8 volts, 1820 lumens) followed in 1964, and the transverse-filament H3 in 1966. H1 still sees wide use in low beams, high beams and auxiliary fog and driving lamps, as does H3. The H2 does not see wide use any more because it is complex to make and to service. The H2 bulb is no longer approved for new lamp designs. The use of H1 and H3 bulbs was legalized in the United States in 1997. More recent single filament bulb designs include the H7, H8 (35 W, 730 lumens), H9 (65 W, 2100 lumens), and H11 (55 W, 1200 lumens) bulbs.

The first dual-filament halogen bulb (to produce a low and a high beam with only one bulb), the H4, was released in 1971. The U.S. prohibited halogen headlamps until 1978, when halogen sealed beams were released. To this day, the H4 is still not legal for automotive use in the United States. Instead, the Americans created their own very similar standard (HB2/9003). The primary differences are that the HB2 sets more strict requirements on filament positioning, and that the HB2 are required to meet the lower maximum output standards set forth by the United States government.

The first U.S. halogen headlamp bulb, the 9004/HB1, is a transverse dual-filament design that produces 700 lumens on low beam and 1200 lumens on high beam. The 9004 is rated for 65 watts (high beam) and 45 watts (low beam) at 12.8 volts. Other U.S. approved halogen bulbs include the 9005/HB3 (65 W, 12.8 V), 9006/HB4 (55 W, 12.8 V), and 9007/HB5 (65/55 watt, 12.8 V). With their plastic bases, the 9004, 9005, 9006, and 9007 are simply not suited for higher wattage bulbs. The bulbs use electrical contacts that are much too small to handle the excess current. Further, many American headlamps are designed such that throwing more light at them will simply result in more glare for oncoming traffic.

The first halogen filament polyellipsoidal "projector beam" automotive lamp was the Super-Lite auxiliary low beam, produced in a joint venture between Chrysler Corporation and Sylvania and optionally installed in 1969 and 1970 full-size Dodge automobiles. It used an 85 watt transverse-filament halogen bulb and was intended to extend the reach of the low beams during turnpike travel when low beams alone were inadequate but high beams would produce excessive glare. Projector main headlamps first appeared in 1983. Developed more or less simultaneously in Germany by Hella and in France by Cibie, the projector low beam permitted accurate beam focus and a much smaller-diameter (though much deeper) optical package for any given beam output. The 1986 BMW 7 Series was the first to use projectors for low beams. Projector and CAD technology allowed the development of reflector headlamps with non parabolic, complex-shape reflectors. First made by Valeo under their Cibie brand, these headlamps would revolutionize automobile design. The 1987 Dodge Monaco/Eagle Premier was the first U.S.-market car with complex-reflector headlamps, while the 1990 Honda Accord was the first U.S.-market car with such headlamps employing a completely clear, nonfaceted front lens.


HID Technology

HID stands for high-intensity discharge, the technical term for the electric arc that produces the light. Automotive HID lamps are commonly called 'xenon headlamps', because of the xenon gas used in the lamps. The xenon gas allows the lamps to produce minimally adequate amounts of light upon startup and speed the warmup time. If argon were used instead, as is commonly done in street and other stationary HID lamps, it would take several minutes for the lamps to reach their full output. HID headlamps use a small, purpose-designed metal halide lamp and produce more light than ordinary incandescent light bulbs (including quartz halogen lamps). The light from HID headlamps has a distinct bluish tint when compared with normal headlamps. The high intensity of the arc comes from metallic salts that are vaporized within the arc chamber.

HID headlamp bulbs produce between 2,800 and 3,000 lumens from 42 watts of electrical power, while halogen filament headlamp bulbs produce between 700 and 2,100 lumens from between 40 and 65 watts. Because of the increased amounts of light available from HID bulbs, HID headlamps producing a given beam pattern can be made smaller than halogen headlamps producing a comparable beam pattern. Alternatively, the larger size can be retained, in which case the Xenon headlamp can produce a more robust beam pattern.

An HID headlamp requires a ballast. The ballast converts the 12 volts used in automotive electrical systems to the several thousand volts required to strike and maintain the arc.

Despite marketing claims to the contrary, HID headlamps' light output is not similar to daylight. The spectral power distribution (SPD) of an automotive HID headlamp is discontinuous, while the SPD of a filament lamp, like that of the sun, is a continuous curve.

The arc within an HID headlamp bulb generates considerable short-wave ultraviolet (UV) light, but none of it escapes the bulb. A UV-absorbing hard glass shield is incorporated around the bulb's arc tube. This is important to prevent degradation of UV-sensitive components and materials in headlamps, such as polycarbonate lenses and reflector hardcoats. The lamps do emit considerable near-UV light).

Vehicles equipped with HID headlamps are required by ECE regulation 48 also to be equipped with headlamp lens cleaning systems and automatic beam levelling control. Both of these measures are intended to reduce the tendency for high-output headlamps to cause high levels of glare to other road users.

The arc light source in an HID headlamp is fundamentally different from the filament light source used in tungsten/halogen headlamps. For that reason, HID-specific optics are used to collect and distribute the light. Installing HID bulbs in headlamps designed to take filament bulbs results in improperly-focused beam patterns and excessive glare, and is therefore illegal in almost all countries.


Comparison of HID vs Halogen:








Color Temperature Examples:








Modified by Eran at 1:45 PM 10/14/2006


Modified by Eran at 3:29 PM 10/14/2006


Modified by Eran at 11:24 PM 10/16/2006


Modified by Eran at 9:58 PM 1/23/2007
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Old 09-30-2006, 11:50 PM   #2
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wow thats alot of reading
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Old 09-30-2006, 11:51 PM   #3
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wow thats alot of reading
The good info always is
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Old 10-01-2006, 12:16 AM   #4
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wikipedia ftw
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Old 10-01-2006, 12:42 AM   #5
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not to sound like the normal hater but is it supposed to be in the eg-ek section?

very good info though
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Old 10-01-2006, 01:52 AM   #6
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good info, been looking for something like that.
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Old 10-01-2006, 05:45 PM   #7
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TTT, mods move this if you'd like.
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Old 10-01-2006, 06:55 PM   #8
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very nice! 8)
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Old 10-01-2006, 07:44 PM   #9
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I think this should be in the top of every car specific forum for non HID cars. Excellent find.
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Old 10-01-2006, 09:05 PM   #10
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Quote:
Originally Posted by broketuner-sol_si
I think this should be in the top of every car specific forum for non HID cars. Excellent find.
I compiled it from wiki, GE, Phillips, and some H-T Sponsor ads.
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Old 10-01-2006, 09:09 PM   #11
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I want some 20000k bulbs!
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Old 10-01-2006, 09:15 PM   #12
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I want some g bulbs!
Those would have such poor light output

But hey, they'd be purple as sin!
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Old 10-02-2006, 11:21 AM   #13
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Updated to include information about lens/reflector/projector tech.
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Old 10-02-2006, 01:24 PM   #14
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Get Me Stuck!
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Old 10-04-2006, 08:01 AM   #15
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Hope you all are reading this
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Old 10-04-2006, 08:29 AM   #16
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Default Re: REF: Automotive Lighting, Color, and You (Eran)

nice info (i think?)..bump. i only read your bold comments. lol
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Old 10-04-2006, 08:41 AM   #17
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Originally Posted by Eran
Hope you all are reading this
i read it, ALL of it. very informative indeed. i always like learning new things
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Old 10-04-2006, 08:42 AM   #18
 
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Nice
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Old 10-04-2006, 09:56 AM   #19
 
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now i know way to much about headlights
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Old 10-04-2006, 09:58 PM   #20
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Kick rocks, up top.
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Old 10-05-2006, 03:28 AM   #21
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Default Re: REF: Automotive Lighting, Color, and You (Eran)

sorry but i only looked at the pictures....
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Old 10-05-2006, 03:44 AM   #22
 
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Default Re: REF: Automotive Lighting, Color, and You (IT'S ME DAVID)

Good info and pics.
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Old 10-13-2006, 10:07 PM   #23
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TTTTTT
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Old 10-13-2006, 11:34 PM   #24
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Default Re: REF: Automotive Lighting, Color, and You (Eran)

I just read it and it was very informative. I am buzzed, so it's effectiveness was worn, but I have a better understanding about headlights, as well as a new look into how I will go about switching to H.I.D.

-Shane
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Old 10-14-2006, 12:41 AM   #25
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<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">Why Do Colored Bulbs Produce Less Light?

To understand this concept, a person must understand one simple fact: colored bulbs are made using dichroic filter coatings directly on the glass. Dichroism is based on the principal of interference:

Dichroic Filters

A dichroic filter or thin-film filter is a very-accurate color filter used to selectively pass light of a small range of colors while reflecting other colors.

Used in front of a light source, a dichroic filter produces light that is perceived by humans to be highly saturated (intense) in color.

Used behind a light source, dichroic reflectors commonly reflect visible light forward while allowing the invisible infrared light (radiated heat) to pass out of the rear of the fixture, resulting in a beam of light that is "cooler". Modern quartz halogen incandescent light bulbs frequently contain an integrated dichroic reflector.

Dichroic filters operate using the principle of interference. Alternating layers of an optical coating are built up upon a glass substrate, selectively reinforcing certain wavelengths of light and interfering with other wavelengths. The layers are usually deposited using a process carried out in a vacuum. By controlling the thickness and number of the layers, the frequency (wavelength) of the passband of the filter can be tuned and made as wide or narrow as desired. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters don't absorb much energy during operation and so don't become nearly as hot as the equivalent conventional filter (which attempts to absorb all energy except for that in the passband).

Interference

Interference is the superposition of two or more waves resulting in a new wave pattern. As most commonly used, the term usually refers to the interference of waves which are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. Two non-monochromatic waves are only fully coherent with each other if they both have exactly the same range of wavelengths and the same phase differences at each of the constituent wavelengths.



The principle of superposition of waves states that the resultant displacement at a point is equal to the sum of the displacements of different waves at that point. If a crest of a wave meets a crest of another wave at the same point then the crests interfere constructively and the resultant wave amplitude is greater. If a crest of a wave meets a trough of another wave then they interfere destructively, and the overall amplitude is decreased.

Interference is involved in Thomas Young's double-slit experiment where two beams of light which are coherent with each other interfere to produce an interference pattern (the beams of light both have the same wavelength range and at the center of the interference pattern they have the same phases at each wavelength, as they both come from the same source). More generally, this form of interference can occur whenever a wave can propagate from a source to a destination by two or more paths of different length. Two or more sources can only be used to produce interference when there is a fixed phase relation between them, but in this case the interference generated is the same as with a single source; see Huygens' principle.

Total phase difference is derived from the sum of both the path difference and the initial phase difference (if the waves are generated from 2 or more different sources). Hence, we can then conclude whether the waves reaching a point are in phase(constructive interference) or out of phase (destructive interference).

To sum up the above theorys, using a dichoric filter on a halogen bulb is absolutely destructive to light output. Being as the white/yellow light outputed by a halogen lamp and the x color of said dichoric filter, the crests and troughs of the two wavelengths intersect, and lower the amplitude of the overall wave. So not only is the overall wavelength of the light lengthened (thus lowering the lumins) due to the blue/purple hues in the dichoric filter, but the magnitude (overall energy) of the lightwaves are decreased as well.</TD></TR></TABLE>
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