HID vs Halogen vs LED - The Shootout!
LED Lights
Cyclists who only occasionally ride at night opt for an inexpensive LED front light and rear LED flasher. Red or yellow LEDs suitable for use as rear lights have been available for many years. Recently, white LEDs which satisfy the requirements for a front light have come on the market, and some jurisdictions have made or are considering making these legally acceptable. Very high-power LEDs are sensitive to overheating and over-driving, if the enclosure or driving electronics are poorly designed. Both of these conditions significantly shorten the LED's lifespan, causing them to dim or completely burn out, and LEDs are expensive to replace. Most LEDs have a higher luminous efficacy than halogen lights, but poorly designed driving electronics can negate the advantage.
Low-power LEDs are adequate for riding on well-lit streets, but do not generally project a very bright beam as it is difficult to collimate the output from multiple LEDs into a single usable beam. This can be overcome by using a few very high-power LEDs - each with their own optics. It is now possible to buy LED equivalents for halogen rechargeable systems (including drop-in replacement bulbs), and LED lights for dynamos. on On a dynamo, LEDs produce more light than halogen lights at very low speeds (down to 3 km/h according to one manufacturer).
High-power LED systems often include an option to dim the LEDs. LEDs are well-suited to dimming, as halving the brightness usually more than doubles the battery life. By contrast, halving the brightness of a halogen bulb only slightly increases battery life.
Efficiency is set to increase, as white LEDs switch to emitting red, green and blue light to form a white output (current designs use a blue LED, with a fluorescent coating, much like conventional strip lights use). As very high power LED lights become available and start to replace halogen designs, they may replace incandescent lamps whatever the power source, and will probably challenge HID lights. A 3-Watt LED offers similar light output to a basic 10-Watt halogen bulb - compare this with current generation "very high brightness" LEDs at 0.05 Watts.
Advantages of LEDs
* Its Cheap
* Long battery life in flashing mode
* Very much visible
* Efficient. A few AAA batteries last for several months.
* Last nearly indefinitely if the light is well designed
* Option to Dimmed, usually with a slight gain in efficiency
* Are small, so easily be removed from the bike and into ones pocket to stop them getting stolen. Also means that they are less onerous to carry so it is more likely that you will have them with you when it gets dark.
Disadvantages
* Limited light output, especially in steady mode for many models
* Some models do not run as long on rechargeable cells as on alkalines; environmental concerns
* Illegal in some jurisdictions
* Attractive to thieves. Especially removable types, particularly if not removed when parked.
* Current designs do not have a "safety" switch, so they can get switched on in your bag, draining the battery.
Low power LED lights are mainly for "being seen", or as an emergency backup, and are the dominant choice for rear lights; higher power LEDs are now moving into the core market for illumination and are subject to rapid technical development.
Halogen Lights
Although these lights were primarily designed for off-road use, where they are almost universal, many commuters and transportational cyclists now choose to use high-power halogen front lights which operate from a NiMH, lead-acid, or Li-ion rechargeable battery pack.
The lights used by most halogen rechargeable systems are cheap, bright but fairly simple: they project a cone of light (wide and narrow beam options are available) which is good for off-road use but not ideal for road use as it can dazzle oncoming road users. This is why rechargeable halogen lights do not meet legal requirements in some jurisdictions.
Because they use standard commercial prefocused optics, a wide range of power and beam width combinations is available. Most systems allow simultaneous connection of different lamps - for example, a wide and a narrow beam for off-road riding, or a high- and a low-power beam for road riding.
Advantages of rechargeable halogen systems
* Bright, sometimes very bright
* Readily available
* Can connect multiple lamps to one battery pack; provides flexibility
* Reasonable battery capacity
* Very reliable
* Can usually be easily removed from the bicycle or to prevent theft
* Lamps are cheap, widely available, and come in many combinations of power and beam width
Disadvantages
* Relatively heavy battery
* Limited run-time between battery-recharges
* Hassle of being certain to keep batteries charged
* Batteries have limited life, typically 500-1000 recharge cycles
* Optics not optimised for road use
HID Lights
High-intensity discharge (HID) lights are the brightest lights currently available for bikes. They are very efficient, very bright, but expensive. They also tend to have high power consumption (although they use less power than halogens for higher output), so a relatively limited burn time. Otherwise they have the same advantages and disadvantages as rechargeable halogen systems, and like halogen systems they are designed primarily for off-road use, having rotationally symmetrical beams which cast as much light up as down. An additional disadvantage compared with halogen or LED lights is that the HID lamp does not tolerate repeated strikes, and in many cases does not relight immediately after shutting down. Likewise, should the battery level fall too low, the lamp will shut down rather than dimming. But the longer battery life than halogens tends to negate these problems, as many riders simply switch the light on and leave it running throughout the ride.
For the uninitiated, HID stands for High Intensity Discharge, commonly called arc lamps or metal halide lamps. HID lamps are 300% to 400% more efficient than a halogen lamp at converting electricity to light. This makes them an excellent choice for applications requiring a lot of light with great battery life. Eg. a 10W HID lamp is equivalent to a 30-40W Halogen Lamp.
As HID lamps utilise an Arc, there is no filament inside the lamp, thus making them less prone to shock and g-forces therefore making them last longer. Obviously dropping one is not a good idea. It is basically the same technology that is used in "Xenon" car headlights as used in some BMW's, Lexus, Mercedes etc.