The Guide To How Lights Work, And Why You Should Care

[pope_face]

This is a write-up I did for a different forum, but I didn't see anything of the sort on here, so I thought I'd share it with you guys too. This is mostly directed to those people who like to put HID bulbs into halogen housings, which doesn't seem that common on here, but it might help people give some idea of how this all works before they start fooling around with their lights.

 

The first post is just general info on lights and how beam patterns are formed.

 

The second post is specific to projector housings and HID's.

 

The third post is a Cliff's Notes version (if you want to know why you shouldn't do it, but don't care about the science behind it).



The Guide To How Lights Work, And Why You Should Care (or How Beam Patterns Are Formed, And Why HIDs Are So Controversial)


Most (if not all) of us know that a light will turn on when we apply electricity to it, and will turn off when that electricity stops flowing. This is true for any light, whether indoor or outdoor, in your house or in your car, LED or halogen. All of us have had experience with indoor lights, like the kind you have in your desk lamp, or in the ceiling of your house. The old-style of these lights - the incandescent bulb - was basically a thin wire (called a filament) suspended on two metal posts, with a glass case (the "bulb") around it for protection. When you flip the light switch in the house, electricity (or an electrical current, to be more exact) is applied to the filament, causing it to heat up until it glows. The more current that's applied, the brighter it gets.



Now, most of us know not to look directly at the sun, as it can cause damage to your eyes, and most people tend not to look directly at lightbulbs either. If you did look at a light without any protection (like sunglasses or welding goggles), it would look like the entire bulb is lit up. For some time, lights were produced with a white coating on the inside of the glass. This coating diffused the light, scattering it in all directions and making it more pleasant. However, halogen lights nowadays are made with clear glass, and you can see the filament inside them. These bulbs don't have the coating to diffuse the light, so when you look at them with sunglasses or welding goggles (but don't try this at home!), you can see that it's just the filament that glows, not the entire bulb.



The lights in your car work on the same basic principle: A bulb basically contains a filament suspended on metal posts, and you connect it to your electrical system by plugging the base into the socket. When you apply power to the bulb, it heats up the filament until it begins to glow. In this case, the glass is clear, so it doesn't diffuse the light. Instead, all the light comes directly from the filament, like in the picture above. However, light has one very interesting property: It only travels in a straight line (at least as far as we're concerned). While it might seem like the light gets everywhere when you turn on the lights in your house, it's not because the light curves around until it gets there, but because it bounces off of things. The easiest way to see this is by looking at a laser, which is a form of very focused, high intensity light. It only travels in a straight line, and the easiest way to get it to change direction is by bouncing it off of a surface, like a mirror.



You can see this for yourself at home by making shadow puppets on the wall. When you shine a light on a wall in a dark room, it makes a bright spot on the wall. When you put your hand, a cardboard cutout, or anything else in front of it, you get a shadow on the wall. This is because the light doesn't go around your hand, but instead is blocked by it, preventing the light rays from hitting the wall. On the other hand, if you take a flashlight with a focused beam and shine it into a mirror in your bathroom, you can actually light up the wall opposite where the flashlight is facing. In fact, you can even light up yourself, even if the flashlight is pointing away from you, because you're bouncing the light off the mirror.



Now, "Why should I care?", you ask? Well, we already know that light comes from the filament, and that it only travels in a straight line, so the only way to get it to go where we want is by redirecting it. For most of the lights in your car, it doesn't really matter where the light goes; For things like turn signals, brake lights, or reverse lights, you just need to see that the light is on, not direct it anywhere. However, when it comes to headlights, the direction of the light is important. We need the light to reach (relatively) long distances so we can see what's in front of us, but we also need to prevent that light from blinding anyone coming towards us. If you've ever been on a dark road, and someone was coming towards you with high-beams on, you know how annoying (and dangerous!) that is; It becomes almost impossible to see what's in front of you.



But, wait a minute, if light is emitted from the filament, and only travels in a straight line, then how do we get such a nice beam pattern with low beams? The answer is: Reflection. Headlights essentially have three parts: The bulb, the reflector, and the lens. The reflector is essentially a giant curved mirror. As the light emitted from the bulb (the filament, to be exact) hits the mirror, it bounces off and changes direction. However, this change in direction isn't random. In fact, the light will bounce off at exactly the same angle as it hit the mirror. It's kind of like throwing a bouncy ball at the ground; If you throw it straight down, it'll bounce straight back up. If you throw it at an angle, it'll bounce up at an angle. The lower the angle you throw it at, the lower the angle it'll bounce back up with.



In the case of the reflector in your headlights, the mirror is curved around the filament, so every ray of light that hits it gets reflected pretty much straight ahead and down. In fact, the way the reflector is designed puts the filament at the focal point of the mirror. That is, if you were to shine a light straight at the headlight from outside, the light rays would all converge at the filament of the bulb.



This gives you lots of light straight ahead and on the ground when you have your low beams on, but still has a nice cut-off higher up to prevent light from shining directly at other drivers.



But, what about high beams? If the light is reflected from the bulb to make the low beam pattern, how can you have high beams by just pulling on the switch? Wouldn't you have to change the bulb? The answer is: Because the high beam filament is placed in a different position than the low beam. Remember that the reflector is very carefully curved to put the filament in your bulb at its focal point. As mentioned above, this means that (almost) all the light given off by the filament is reflected straight ahead and down. If you change the position of the filament, it's no longer at the focal point; This means that the light given off by the filament will now hit the reflector at a different angle, and then will get reflected at a different angle. This changes how the light is focused. Some good examples of this are flashlights with an adjustable beam, like a Maglite: If you shine a flashlight at a wall, the light emitted from the bulb bounces off the reflector and makes a pattern on the wall. If you turn the top of the flashlight (which contains the reflector), you can change the pattern, from narrow/focused, to wide. What you're doing is actually moving the reflector back and forth, while the bulb stays in the same spot. Because the reflector stays the same, the focal point never changes, but the position of the filament relative to the focal point does, and this gives you the different beam patterns.



Now, in most new headlights, the lens is clear and you can see the reflector. Like mentioned above, the reflector does the job of focusing the beam. However, a lot of old lights don't have clear lenses. Instead, they have patterns cut into them. In this case, it's not reflection that causes the beam pattern, but refraction. Refraction is the change in angle of a ray of light when it passes from one material (a medium) into another material. To be exact, it's the change in angle of a light ray when it passes from one medium of a given density, into a medium of a different density. For example, when the light ray goes from air into water, or air into glass.



While this may sound strange, it's actually pretty common... If you've ever looked at a glass of water with a straw in it, you may have noticed that the part of the straw in the water doesn't seem to line up with the rest of the straw. This is because the light is refracted as it passes from the higher density medium (the water) into the lower density medium (the air). The same applies to the headlights in older cars: The light from the filament is bounced off the reflector, but not perfectly. If you were to take the lens off an old headlight, the beam pattern would still be pretty scattered, even though the reflector aims it forward. Instead, the lens does the rest of the work, fine-tuning the light rays bounced off the reflector and making the nice pattern.



Now, most things are cheaper when you can make and sell lots of them. If you've ever had to get custom parts made up for your car (like brackets, exhaust, or anything), it's usually more expensive than buying something already made. This is because a company (or individual) has to hand-make everything. However, if you're going to be making a few, it's usually cheaper to make a mold. Still, if you want to produce lots, you need to start investing in machinery and setting things up properly to make the part. The same thing applies to light bulbs. While there are many different types of headlight bulbs available on the market (H3, H4, 9003, 9005), they tend to be used in many different vehicles. However, they each have a different plug and mounting system; You can physically put a 9005 bulb into an H4 housing, but you won't be able to use the mounting system to make it stay in, and you won't be able to plug it in to your wiring harness. This is because each different type of bulb has the filament in a different location, and headlights are designed in such a way that the filament of that particular bulb will be in the right spot. If not, then you wouldn't be able to get a consistent beam pattern with a given bulb. However, all bulbs of a given type do have the filament in the same location. This is why you can replace the bulbs in your headlights, and the beam pattern will still look the same.





Projectors and HIDs are covered in the following post.

 

[pope_face]

Now, as you can tell, there are only two basic styles of headlight: Those that use the reflector to make the beam pattern, and those that use the lens. However, all these lights tend to be quite large: The rectangular ones tend to be about the size of a postcard, while the circular ones are usually larger than a CD. So, what happens if you want a smaller diameter light, but don't really want to lose output? In that case, the best option might be a projector. These, like normal headlights with a lens-based beam pattern, use a reflector at the back of the light to direct the light forward, and a lens to fine-tune the light. However, the reflector in a projector light is much deeper than a regular headlight. This makes the reflector much more efficient at collecting the light and aiming it forward. Also, these lights use a rounded lens, rather than one with patterns cut into it.



Also, while a normal lens-based low beam uses the pattern to refract the light  and get the cut-off, projector lights use a physical shield inside the  housing to prevent some of the light from hitting the lens. In other  words, rather than "redirecting" the light passing through the lens  towards the road, projector lights use a little piece of metal to create  a shadow above a certain point. See how nice and crisp the top of the beam is, compared to the halogen low beam pattern above?



Projector lights are available in two styles: Halogen, and HID. Halogen projector lights work as mentioned above: A filament has an electrical current applied to it, which causes it to heat up and glow. As with all other halogen lights, the filament is located at a certain point in the light, and the housing/lens is designed around that. HIDs use a similar process to create light. HID stands for High Intensity Discharge, and the bulb is basically made up of two electrodes embedded end-to-end in a little glass capsule. Much like halogen lights (which are filled with a gas), the capsule is also filled with a gas. However, unlike a halogen bulb, the electrodes are not connected by a filament; Instead, they are positioned close enough to each other to produce a spark when a current is applied to them, much like a spark plug in your engine. Although a spark can be visible, this isn't what produces the light in an HID bulb. Instead, sparks are repeathttp://cliff.hostkansas.com/pffimages2/Hella_H9_Low_Beam_%28640x320%29.jpgedly produced in order to heat up the gas inside the capsule, and it's this heated gas that emits light.



In the case of cars, that gas is Xenon, which is why most HID lights are referred to as "Xenon" or "Bi-Xenon" lights. While Bi-Xenon makes it sound as if there's two bulbs inside, that's not true; There is only one bulb in a Bi-Xenon projector. But, remember that little shield in the projector housing that blocks part of the light to make it a low beam? In the case of regular Xenon projector, that shield is either there (for a low beam light), or it's not (for a high beam light). In a Bi-Xenon, the shield is there, but it can be moved out of the way by a little solenoid. The bulb stays on all the time, but when you're driving around the city, the little shield is in front of the light so that you don't blind other people. When you head out onto a dark country road and turn your high beams on, you're not actually turning on a different bulb, you're just moving the little shield out of the way to let more light out.



Now, if you've been paying attention, you'll remember that halogen bulbs use a glowing filament at the focal point of the reflector, and that the reflector (and the lens, in some cases) are designed to collect that light and focus it into a certain beam pattern. When you switch between high beam and low beam, you're switching between the filaments, and that changes the focus of the light on the reflector and lens, which causes a change in beam pattern. But, when you switch between high beam and low beam in a Bi-Xenon light, you're physically blocking part of the light from reaching the lens. These are two completely different systems: One "moves" the light back and forth to change the beam, and the other blocks part of the light from reaching the lens. With such huge differences in function, why would you want to use one type of bulb in a housing designed for the other? That would be like trying to hammer screws into wood, or trying to use a screwdriver to drive nails: They might seem similar, and they serve the same purpose, but they function very differently and you'd look pretty ridiculous doing it.



Instead, what you should do is retrofit the projector housing into your stock headlight housing. However, remember that there are two types of projector lights: Halogen and HID. Both use a little shield to make the beam pattern, so the million dollar question is: Can you use HID bulbs in halogen projectors? The correct answer is... No! If you've been paying attention, this should be obvious to you. Remember near the beginning, when I said that some bulbs for your house used a coating on the inside of the glass to diffuse the light, and that other bulbs didn't? Well the same thing applies here: Halogen bulbs use a filament that acts as a very localized light source. When a projector is designed for a halogen bulb, it is made with the assumption that the light will only be emitted from that very localized point. HIDs, on the other hand, act more like the "frosted" bulbs: It's not the filament that glows, it's the gas inside the capsule. The capsule has to be much bigger in order to fit both the electrodes and the gas, and because it's filled with hot, glowing gas, the light appears to come from the entire capsule at once. The light from a certain point in the HID bulb might be at the focal point of the projector, but most of the light would be off and bounce around unpredictably. This would be like having a lot of filaments in a halogen bulb: Even if you could focus the light from one of the filaments properly, all the light from the other filaments would negate that and just "spray" out randomly.



Say you've decided to go the HID route anyway; You've got your stock housing, you've got the proper projector housing, and you've got the proper bulbs. Can't you just install that in your stock housing and be done with it? Well, maybe. Remember the two basic styles of headlights? One style uses the reflector to focus the beam, while the other uses the lens and the reflector. And what are the three basic components of a headlight? The bulb, the reflector, and the lens. We already know how important the bulb is, and both basic styles of headlight use the reflector to focus the beam, but I only mentioned the lens for one type of headlight. What about the other? Well, in the case where the reflector creates the beam pattern, the lens is only there as a protective cover; It doesn't actually do anything. These are the "crystal clear", or just "clear", headlights that people talk about.



So, why does this matter, you ask? When the light leaves the projector, it's only going straight ahead, so it shouldn't be hitting the stock reflector anyway. And if the stock housing uses the lens to make the beam pattern, won't that just refocus the light from the projector and make it better? Technically, no... In fact, it'll make it worse. We already know how the lens can refract the light to make a nice beam pattern. The lens doesn't care how the light hits it, and will refract it regardless of where the light is coming from. However, it's designed to refract light hitting it in a very specific way: This is how the reflector focuses the light from the filament on the lens. A projector is already designed to do that: It aims the light hitting the reflector at the lens, and the lens then focuses the light into the beam. If you then put another lens designed to focus light in front of that, that lens would refocus the beam in a different way, and you could end up with some very awkward beam patterns. It would be like having 20/20 vision, and putting on someone else's glasses... You can still see, but your vision would be all weird, and you'd still look ridiculous.



What's the moral of the story, you ask? Well, the moral of the story is that most lights aren't designed by some 16-year old in a basement, and they're more than just a mirror and a piece of glass. Automotive lights are carefully designed around existing bulbs to provide the best illumination for their intended purpose, whether it's high-beams, low beams, fog lights, or driving lights. When new types of bulbs or lighting are invented that provide better illumination at a lower weight, cost, or power consumption, often times the old designs won't work, and engineers must work to redesign the existing standard to adopt this new technology. While DIY is great for saving money and learning about your vehicle, there are some things that no sane person would do without having a lot of knowledge and experience. For example, you wouldn't take your engine apart and start putting in parts from other cars "just because they fit", and "you think it'll work better". Headlights fall into the same category; They are very carefully designed so that all the components work together as a system, just like an engine. While messing up your lights won't cause your car to explode, having lights that blind other drivers can get you a ticket, into an accident, or into a fist-fight. If you're going to do it, understand what you're doing. Even if you think you know what you're doing, you're better off talking to a professional.

DISCLAIMER: While I made this thread to give people some information on how lights work, and how Xenon bulbs and HID projectors can interact with stock halogen housings, this is not an invitation or challenge to install HIDs (or any other type of light) into a housing not designed for them. The author (myself) takes no responsibility for any damages including (but not limited to): Emotional, mental, physical, or financial damages, injury, or death that arises for any attempts by you or anyone else to dismantle, modify, destroy, or otherwise do anything with any part of your or anyone else's vehicle. While the author strives for accuracy, the information above cannot be guaranteed and all individuals are recommended to do their own research. Again, the author is not liable, nor takes any responsibility, for anything you or anyone else does. If you don't know, DON'T DO IT.

[pope_face]

CliffsNotes Version:

- In the past, lighting in your car consisted of halogen-filled bulbs containing a filament. Each light took a particular style of bulb, and each bulb had it's own socket. While it doesn't matter as much for most lights, this is very important for headlights.

- Headlights consist of a reflector and a lens. In the older style cars, the reflector collected the light and aimed it ahead, and the lens was cut to refract the light and form the beam pattern (either high-beam or low-beam). In newer, "crystal clear" headlights, the reflector is precision cut to provide the beam pattern, and the lens is only a protective cover. In either case, the reflector (and lens, in some cases) is designed to work with light that hits it at a very particular angle. This light has to come (essentially) from the focal point of the reflector.

- As we mentioned above, the socket for headlights is very important. This is because there is a variety of halogen light bulbs available. Each of these lights is a different size and length, and the filament is in a different position compared to the mounting point. Because the filament is what makes the light, and because the reflector is designed to collect light from the focal point, this means that the filament needs to be at the focal point. So, if you use one bulb in a housing it wasn't designed for another, the filament won't be in at the focal point, and the light from your lights will be scattered.

- Unlike halogen bulbs, HID bulbs are filled with a gas (like Xenon) that's heated up by a pair of electrodes. Rather than the light coming from (essentially) a point source (the filament), the gas itself glows. This is like having thousands of individual point sources. If you put an HID bulb in a housing designed for a halogen bulb, you're essentially doing the same as putting a halogen bulb in a reflector designed for a different halogen bulb. However, it's much worse in the case of the Xenon bulb, since you get much more light output per watt, so not only are you scattering a large proportion of the light (making it useless and annoying), you're scattering a lot more light (making it even more useless and annoying, and making you a douchebag).

- So, if you want HIDs, you need to use a projector housing and lens designed for the HID bulb. You can then install the projector into your stock housing, as long as you have the "crystal clear" lenses. If you've got the lenses with the beam pattern cut into them (like the early GC/GF Impreza), then it's useless to put projectors in because the lens is just going to scatter the light anyway.

Long story short: If you want HID lights, make sure you have stock headlights with clear lenses (like the GC RS headlights), and use a projector housing and projector lens designed for the HIDs you want. Install the projector housing and lens into your stock housing (make sure you get the alignment right), reinstall your headlights onto your car, and enjoy your new light output. If you do it any other way, you're just wasting your money.

[pope_face]

Reserved for extra info and future expansion.

[TENDRIL]

nice read

[Laecaon]

What other forum did you post this on?

 

I know I have read it before... All of the pictures are very familiar.

 

And yes, I agree 100% Thats why I have HID projectors.

[pope_face]

Thanks for the support guys.

 

It was originally posted on dirtyimpreza.com. I've been more active on there for a while than on here, while I built my Subaru. I haven't read through it again, so I apologize to any Subaru specific references (I've caught at least one, just skimming through it). I'll tweak it again at some point to remove those references (for those people who aren't Subaru-savvy).

 

Most of the pictures are off the internet... I just searched for images that suited what I was trying to indicate. The write-up I did myself (with some help from the internet, a university Physics textbook, and williaty on DI).

[Monacar]

"The more current applied, the brighter it gets." You can't apply more current to a circuit.

[MikeRL411]

"The more current applied, the brighter it gets." You can't apply more current to a circuit.

 

Yes you can.  With a constant voltage source [like a lead acid battery] a series rheostat [google it if you don't know what that is] or stepped resistor can and will regulate current to a bulb.  I = E/R as Herr Ohm posited.

[Monacar]

You can add resistance but that lowers current. The only way to increase current would be to remove resistance in the circuit which you couldn't do unless you added extra in the first place.

[Monacar]

And Yeah I know what a rheostat is