It seems that there are many misinterpretations about aftermarket halogen bulbs. Whenever someone expressed a concern about them, they would be directed to other websites that offered some information, but not the entire picture. After extensive research, I've decided to pool the knowledge I've discerned into this simple, easy to read post, for anyone to consult should they have a question about their aftermarket halogen bulb. A noble reason, I must admit, but in reality ... I am just bored out of my mind and can't sleep and light bulbs are really the only parts of the car that I know anything significant about. 
Anyhow ... let's first start with definitions:
<b>K</b> - [Kelvin] A standard unit of measurement of <i>heat</i>.
<b>Tungsten</b> - Metal used in the filaments of bulbs. Extremely high boiling point, resistivity and tensile strength at temperatures greater than 1700 degrees celsius.
<b>CCT</b> - [Correlated Color Temperature] temperature at which one would have to heat a "black body radiator" to produce certain colors of light.
<b>black body radiator</b> - a theoretical object that emits 100% of its energy at a certain temperature
<b>lumens</b> - measurement of illumination
In order to dispel the myths revolving around halogen bulbs, it's important that we understand how it works:
<i>(this is boring ... skip if needed)</i>
Today, all non-HID bulbs work in the same, simple fashion. An electric current is driven through a Tungsten filament, which (due to its high resistivity) heats up to insane temperatures. The wire then radiates light.
Halogen bulbs derive their name from the presence of trace amounts of halogen gas within the bulb. Its function is to sweep (through a series of chemical reactions) Tungsten atoms from the walls of the bulb, back onto the filament, increasing its life.
<i>(ok, start reading here)</i>
Now to the part that pertains to the average consumer ...
(thought)
Most consumers believe that Kelvin is a measure of how "blue" a bulb's light is. (8500K=purple, 6000K=blue, etc.) In a way that assumption is correct, however, there is more to it than that. What the "8500K" label really means is that the bulb will produce the same color light as a black body radiator that is heated to 8500K. This is known as correlated color temperature. Lower CCT's have a reddish hue ... and higher have blueish. <i>Halogen bulbs with tungsten filaments will never achieve a CCT of 6000K (blueish) or greater</i>. Why? Because tungsten, the metal that most resembles a black body radiator, has a boiling point of 5700K. The filament would damn near vaporize at any temperature greater! Therefore, to achieve the "blue" look, companies coat the outside of the bulb with heat-resistant film that filters out lightwaves at the lower end of the spectrum (reds and yellows). What does this mean to you?
1. The "brightness" (measured in lumens) of the bulb is significantly decreased ... leading to decreased visibility for the driver.
2. <i>Bulbs that are coated in this way do not give out white light</i>. The definition of white light is the presence of all colors in the visible spectrum. In filtering out the reds and yellows, companies achieve "bluer" hues, not white.
3. In order to combat the decreased output in lumens, some companies increase the watts their bulbs use. Everyone knows the dangers of bulbs like these, by now.
4. Blue colored light places much more strain on the retina of the eye due to its high-energy state (e.g. it's much more tiring to catch a fastball than a slowball), leading to eye-fatigue.
I'm sure there are more disadvantages, but it's late, and I'm finally getting tired, so I can't think of any others. Most of you, undoubtedly, have known the cons of the "blue" aftermarket bulbs ... but at least you now know the rhyme and reason behind it. The bottom line is to stay away from halogen bulbs that market themselves as 6000K or greater... They're not worth your hard earned dollar. Save it for HID bulbs ... which are another discussion entirely.
Thanks for letting me ramble, and I hope I wasn't too boring ... now, where's my bed ...* (sleep)
Anyhow ... let's first start with definitions:
<b>K</b> - [Kelvin] A standard unit of measurement of <i>heat</i>.
<b>Tungsten</b> - Metal used in the filaments of bulbs. Extremely high boiling point, resistivity and tensile strength at temperatures greater than 1700 degrees celsius.
<b>CCT</b> - [Correlated Color Temperature] temperature at which one would have to heat a "black body radiator" to produce certain colors of light.
<b>black body radiator</b> - a theoretical object that emits 100% of its energy at a certain temperature
<b>lumens</b> - measurement of illumination
In order to dispel the myths revolving around halogen bulbs, it's important that we understand how it works:
<i>(this is boring ... skip if needed)</i>
Today, all non-HID bulbs work in the same, simple fashion. An electric current is driven through a Tungsten filament, which (due to its high resistivity) heats up to insane temperatures. The wire then radiates light.
Halogen bulbs derive their name from the presence of trace amounts of halogen gas within the bulb. Its function is to sweep (through a series of chemical reactions) Tungsten atoms from the walls of the bulb, back onto the filament, increasing its life.
<i>(ok, start reading here)</i>
Now to the part that pertains to the average consumer ...
(thought)
Most consumers believe that Kelvin is a measure of how "blue" a bulb's light is. (8500K=purple, 6000K=blue, etc.) In a way that assumption is correct, however, there is more to it than that. What the "8500K" label really means is that the bulb will produce the same color light as a black body radiator that is heated to 8500K. This is known as correlated color temperature. Lower CCT's have a reddish hue ... and higher have blueish. <i>Halogen bulbs with tungsten filaments will never achieve a CCT of 6000K (blueish) or greater</i>. Why? Because tungsten, the metal that most resembles a black body radiator, has a boiling point of 5700K. The filament would damn near vaporize at any temperature greater! Therefore, to achieve the "blue" look, companies coat the outside of the bulb with heat-resistant film that filters out lightwaves at the lower end of the spectrum (reds and yellows). What does this mean to you?
1. The "brightness" (measured in lumens) of the bulb is significantly decreased ... leading to decreased visibility for the driver.
2. <i>Bulbs that are coated in this way do not give out white light</i>. The definition of white light is the presence of all colors in the visible spectrum. In filtering out the reds and yellows, companies achieve "bluer" hues, not white.
3. In order to combat the decreased output in lumens, some companies increase the watts their bulbs use. Everyone knows the dangers of bulbs like these, by now.
4. Blue colored light places much more strain on the retina of the eye due to its high-energy state (e.g. it's much more tiring to catch a fastball than a slowball), leading to eye-fatigue.
I'm sure there are more disadvantages, but it's late, and I'm finally getting tired, so I can't think of any others. Most of you, undoubtedly, have known the cons of the "blue" aftermarket bulbs ... but at least you now know the rhyme and reason behind it. The bottom line is to stay away from halogen bulbs that market themselves as 6000K or greater... They're not worth your hard earned dollar. Save it for HID bulbs ... which are another discussion entirely.
Thanks for letting me ramble, and I hope I wasn't too boring ... now, where's my bed ...* (sleep)
I eat up this stuff .... I could go into more detail about how Catz HID uses thin film capacitors in series to help with current loads... but then I'd lose everyone 
