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 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.
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 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.
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