Bicycle lighting the visibility of the bicycle rider to others in dark conditions, i.e. to increase the rider's conspicuity and to enhance the ability of the rider to see, illuminating the way forward. Both reflectors and active lights are used to make the rider more visible, and many jurisdictions require one or more types of light to be fitted to bikes ridden at night. White light in the front, red lights in the back, with orange reflectors on the side is the most common setup (Dell XPS M1210 Battery) .
Many types of light sources are used for bicycle lights.
The earliest bicycle and motor-car lamps were often powered by acetylene gas, the medium calcium of carbide now almost unused except by cavers. The light given was very bright, though they required constant maintenance and the arrival of battery-powered lamps in the 1890s was well received (Dell Studio XPS 1340 Battery) .
Early battery lamps generally used a lead-acid battery, but these were replaced by self-contained dry cells; lamps became smaller and more reliable. At the same time dynamos were developed which generated energy from the bicycle's own movement. These were more practical, as storage density in batteries was at this time very low.
Not all jurisdictions required use of lights after dark (Dell Studio XPS 1640 Battery) .
In the UK the law requiring use of rear lights was resisted by cyclists' groups on the grounds that it downplayed motorists' obligation to be able to stop well within the distance they can see to be clear.
Replaceable-cell battery lights had a renaissance on the invention of the alkaline battery, with a much higher storage density. Moulding techniques for plastics also improved, allowing lens optics to be refined at low cost thus making more efficient use of the light output (Dell Vostro 1710 Battery) .
During the 1980s the lighting market became more globalised: in Europe, the French "Wonder Lights" and Ever Ready brands gradually disappeared in favour of American, Japanese and German products.
In recent times there have been many advances: exceptionally efficient dynamos; cheap high-output sealed-unit halogen lamps originally developed for decorative lighting; improved storage density in rechargeable batteries driven by the computer industry; high-output light emitting diodes (LEDs); white LEDs;high intensity discharge (HID) lights crossing over from the automotive sector (Sony VGP-BPS13 battery) .
Batteries, either rechargeable or disposable, are often used to power electric bicycle lights. For bicycle touring, commuting or if battery operation is otherwise unwanted an electrical generator, or dynamo, may be used. (While "dynamo" generally refers to a generator that produces direct current with the use of a commutator, cycling usage normally refers to an alternating current magneto (Sony VGP-BPL9 battery) ).
Unlike batteries, a dynamo has unlimited duration, but the maximum power output is relatively low. This is somewhat mitigated by good optics and slight overvolting of the lamp; a good dynamo nominally rated at 6 V is generally allowed to produce 7 V or so by its voltage limiting circuitry. In Germany, it is illegal to use bicycles without a dynamo-based lighting system except on "racing bicycles" below 11 kg.This has stimulated the market for high quality dynamos such as B&M or Schmidt (Sony VGP-BPL11 battery) .
The simplest bicycle headlights use a tungsten filament lamp. A more expensive, but brighter and more efficient alternative is the halogen lamp. A high intensity discharge (HID) light provides greaterlight intensity, but requires special circuitry and a powerful battery. Filament based lamps have largely been superseded by high efficiency white light-emitting diode (LED) headlights, which combine lower expense, higher light output, and longer battery life (Sony VGP-BPL15 battery) .
The best LED systems may soon make even HID systems obsolete.
Red tail lights have traditionally combined a white light source with a red filter, which wastes power creating light that will not be transmitted. Red LEDs emit only red light in the first place, and hence consume much less energy (Dell Inspiron E1505 battery) .
The type of lights used may depend on the available power sources and the amount of light required, but as often reflects personal preference or local availability. A low power headlight and a rear red LED suffice for cycling at low speeds in familiar areas with street lights. Unlit and unfamiliar areas demand a more powerful headlight for safety, at the cost of greater weight, higher cost, or shorter run time (Dell Latitude E6400 battery) .
A common setup for a bicycle commuter might include:
- a quartz-halogen or high power LED headlight powered by a rechargeable battery pack or dynamo
- a LED tail light, either steady or flashing/pulsating
- a flashing/pulsating LED front light for conspicuousness and as backup (HP Pavilion dv6000 Battery)
Under the International Vienna Convention on Road Traffic (1968) of the United Nations, a bicycle is a vehicle. Article 44 of the Convention stipulates that: "Cycles without an engine in international traffic shall: (c) Be equipped with a red reflecting device at the rear and with devices such that the cycle can show a white or selective-yellow light to the front and a red light to the rear." In some countries, for example France, it may be an offence to even sell a bicycle not fitted with legally compliant lighting system (Sony VGN-FZ31S battery) .
Germany requires that all bikes over 11 kg be fitted with compliant dynamo lighting systems, but even lightweight bikes are required to be fitted with lights (battery powered lights allowed) except when racing. However not all countries impose this requirement on their domestic cyclists.
Many jurisdictions require bicycles to be fitted with reflectors at point of sale (Hp pavilion dv6000 battery) .
In the United States this is regulated by the Consumer Product Safety Commission. CPSC compliant reflectors (also commonly fitted in other markets) have three retro-reflective panels positioned at 30° angles. The standard requires a forward facing white reflector on the front of the bicycle, sideways facing white reflectors on each wheel, a red reflector mounted on the rear, and yellow reflectors mounted on the front and back of each pedal. Some interpret this as an endorsement of reflector-only night cycling (SONY VGN-FZ38M Battery) .
Many jurisdictions require the use of a headlight and a rear light or reflector after dark. Most European countries and some US states require front and rear lights at night, while others allow reflectors only at the rear. Individual jurisdictions define specific legal requirements in terms of light output and the size of lamp and reflector lenses, compliance with specified standards, or simply stipulate a minimum distance from which any lighting device should be visible (SONY VGN-FZ31z Battery) .
In some jurisdictions, some systems involving LED based flashers are illegal because blinking lights are only allowed on emergency vehicles, but those rules are rarely enforced.
In the UK, the regulations governing bicycle lights are set out in the Road Vehicles Lighting Regulations 1989 and their subsequent amendments (SONY VGN-FZ31E Battery) .
The regulations require a white front light, a red rear light, a red rear reflector, and amber/yellow pedal reflectors on the front and rear of both pedals. Reflectors must conform to BS 6102/2 or an equivalent European standard. The situation for lights is more complicated:
- a light with a steady mode is considered approved only if it conforms to BS 6102/3 or an equivalent European standard;
- a light without a steady mode is considered approved only if it flashes at a constant rate of between 60 to 240 flashes per minute and has a luminous intensity of at least 4 candela (SONY VGN-FZ31J Battery) ;
The majority of LED lights available are not approved for UK use since they have steady modes that do not conform to BS 6102/3. It is, however, legal to fit additional lights providing that they are of the correct colour, they do not dazzle other road users and that if they flash, they do so at a constant rate of between 60 and 240 flashes per minute (SONY VGN-FZ31M Battery) .
National cyclists' organisations such as LAB (US) or CTC Lighting Regulations (UK) are a source of lighting information.
The use of lights for night riding is generally recommended or required by authorities as a basic safety precaution, even in a well-lit urban context. Studies show a correlation between collisions and failure to use lights even during daytime (SONY VGN-FZ31B Battery) .
It is generally believed that reflectors alone are inadequate for night riding (almost all cycling organisations unconditionally advocate using a headlight at night), but the Consumer Product Safety Commission and other regulatory agencies have declined to mandate the inclusion of active headlamps with new bicycles, citing increased costs compared to benefits and the fact that most bicycles are not ridden at night (SONY VGP-BPS13 Battery) .
It has been estimated that 75% of bicycles used in the United States at night do not meet the legal requirements for lighting. This may be because the majority of unlit cyclists are riding in urban areas with many streetlights, and bicycles are not required to be sold with a headlight. In countries where bicycles are used widely for commuting and short trips, such as the Netherlands andScandinavia, bicycle head- and tail lighting regulation is heavily enforced by the authorities (Dell Inspiron 1320 Battery) .
Cyclists who only occasionally ride at night may 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 (Dell Inspiron 1320n Battery) .
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 (Dell Inspiron 1464 Battery) .
Low-power LEDs are sufficient 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 (Dell Inspiron 1564 Battery) .
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 (Dell Inspiron 1764 Battery) .
Efficiency is set to increase, as LED technology continues to follow a Moore's Law pattern of exponential efficiency and cost improvements. 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 (Dell Studio 1450 Battery) .
- High luminous efficacy leading to long battery life
- Lifetime around 50000 hours of operation if current and temperature is managed
- Can be dimmed, usually with a slight gain in efficiency
- Instantaneous turn on/off and blinking (Dell Studio 1457 Battery)
- Limited light output per single LED
- High power LEDs require a constant current source
- Illegal in some jurisdictions
Low power LED lights are mainly for "being seen", or as an emergency backup, and are the dominant choice for rear lights (Dell Latitude D610 Battery) .
Home made LED lights
Home brew system of high power one watt Luxeon LEDs for headlights and tail lights, and numerous colored LEDs for decoration and visibility
As with high-power halogen systems, high-power LED systems have attracted self-build enthusiasts, as top-end commercial products tend to be expensive due to low production volumes. Home brewers can incorporate state of the art technology months or years before it reaches the marketplace in retail products (Toshiba NB100 Battery) .
Low-cost battery lights
Low-cost battery lights can be a good choice for occasional use, and usually meet legislative requirements. But the saving can often be negated by poor battery life. These lights are low-power but compensate this to some extent with good optics. Some newer models have LEDs instead of halogen lamps, which are illegal in some jurisdictions (Toshiba Satellite M65 battery) .
Advantages of low-cost battery lights
- Low cost
- Readily available (both lights and batteries)
- Cost of batteries, unless they are rechargeable. This stops it being "low-cost" due to the amount of batteries used over time.
- Generally low power (Toshiba Satellite M60 battery)
There are some high-power flashlights with rechargeable lithium-ion batteries. Although not specifically designed for bicycle use, these are a viable alternative to rechargeable halogen lights, though they are still expensive. They can be fastened to the bicycle handlebars with various mounting devices such as a "lock-block". Although the optics of these flashlights are generally high quality they are still rotationally symmetrical (unlike motor vehicle lights) (Dell Latitude D830 Battery) .
Advantages of rechargeable flashlights
- Versatile - can also be used as a flashlight off the bike
- Portable, so theft resistant
- Limited burn time (Dell Studio 1735 Battery)
- Heavy (rechargeable systems generally separate the weight of the battery from the lamp); weight may be an issue for off-road riders due to vibration
- Optics not optimised for road use
- Sometimes difficult to mount on the bike, which also leads to a safety issue—if a light falls off its mount, it may fall into and block the front wheel and cause it to immediately stop, typically causing the bike to flip.
an orange beach cruiser bicycle modded with automobile headlights housed with halogen lamps (Dell Latitude D620 Battery) .
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 (Dell Inspiron Mini 10 Battery) .
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 (Sony VGN-FW11S Battery) .
Many systems use standard commercial prefocused optics, making a wide range of power and beam width combinations 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 (Sony VGN-FW11M Battery) .
Advantages of rechargeable halogen systems
- High power output
- Readily available
- Reasonable battery capacity
- Very reliable
- Can usually be easily removed from the bicycle or to prevent theft
- Lamps are cheap and widely available (Dell Studio 1555 battery) .
- Limited run-time between battery recharges
- Hassle of being certain to keep batteries charged
- Batteries have limited life, typically 500-1000 recharge cycles
- Power cycling reduces lamp life (Dell Latitude E5400 Battery)
A homebrew HID light assembly consisting of a 10 watt HID light (right) and a 30 watt HID light (left) as well as three 1 watt Luxeon high power LEDs
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 (Dell Latitude E4200 Battery) .
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 (Dell Vostro A840 Battery) .
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.
Advantages of rechargeable HID systems
- Very high power output
- High luminous efficacy (Dell Inspiron 300M Battery)
- Lamp requires warm up before reaching full output power and/or luminous efficacy
If using a HID lighting system on the road, lights should be adjusted to avoid dazzling oncoming traffic (Dell Studio 1737 battery) .
The only real advantage to these is that they are often omnidirectional, a quality useful in running lights which must be visible through a very wide arc. Newer LED lights have this feature, removing even this final advantage. Energizer once made a 2.4W halogen rear lamp, which was essentially a headlamp with a red lens, but most rear lights only need to be around 0.5W (Dell Inspiron E1505 battery) .
Rear LED light mounted on the seatpost of a road bicycle
Most LED lights will work in either flashing or steady modes. Some LED lights have multiple banks of LEDs allowing both flashing and steady light at once. This can also be achieved by having one flashing light and one steady one.
In many countries, LED flashers are the norm for rear lights. In others such as Germany flashing lights are forbidden by law (Dell Latitude E6400 battery) .
In the UK flashing LEDs (front and rear) are legal from October 2005. Many vendors claim EU compliance, however this provides no consistent safety or legal value across the Union.
The most common power source for rear LEDs is a set of alkaline cells and rechargeable cells. In both cases the battery tends to fail quickly when it goes; it is widely considered good practice to have two rear lights in case a battery fails en route (Dell RM791 battery) .
Advantages of LED rear lights
- Compared with incandescent lamps, near-infinite service life
- High luminous efficacy
- Usually a choice of flashing or steady (Dell XPS M1530 battery)
An innovation in bicycle lighting, though common in industrial applications, xenon strobes are brighter than LEDs and are used by some as rear lights.
Advantages of xenon strobes
- Very bright
- Relatively expensive
- High power consumption (short battery life) (Dell XPS M2010 battery)
- Lamps require fairly frequent replacement
- Intense light can be a serious irritation to other road users
- Brief flash followed by darkness, can interfere with night vision and make judgement of cyclist's position difficult.
Bicycles intended for police bicycle patrol are outfitted with special colored lighting, generally blue and red, to alert the surrounding public of the officer's presence (Dell Vostro 1000 battery) .
These lights can flash in a variety of patterns, sometimes supplemented with a siren, to help the officer make his or her way through crowded streets or sidewalks in case of emergency. A popular method of supplying power to police bicycle lights is to encase the battery in a bottle-shaped container which can fit into the bicycle's water bottle holder (Acer Aspire One battery) .
The introduction of the Low self-discharge NiMH battery (LSD-NiMH) in 2005 made rechargeable AA and AAA sized batteries more viable for powering LED bicycle lights. Previously, the self-discharge effect of NiCAD and NiMH batteries caused the battery to run down over a period of weeks or a few months, even when not in use (Sony VGP-BPS13A/B Battery) .
This was particularly a problem for low powered LED lights, and for users who only used their bicycle lights occasionally. The LSD-NiMH battery greatly reduced the self-discharge effect, allowing the battery to keep its charge for a year or more.
For higher-powered lights, an external battery pack of 12V sealed lead acid battery is often required (Dell Precision M70 Battery) .
These battery packs usually strap to one of the horizontal bars on the bike, or come in the shape of a water bottle to mount in the bike's water bottle holder. A cable connects the battery pack to the light.
Shimano Nexus hub dynamo
Dynamo systems require no batteries to recharge or replace, and may be permanently fitted to the bike. Bicycle dynamos produce alternating current, so are more correctly called "alternators" or "generators", but the word "dynamo" is also popular (Toshiba Satellite L305 Battery) .
There are three main types: hub dynamos are built into the front or rear wheel hub, and are generally the most efficient; bottle dynamos attach to the seatstay or fork and are rotated by a small wheel in contact with the tire sidewall, they are generally easiest to obtain and cheapest while European-designed tyres generally feature a raised "track" to run the generator's wheel; bottom bracket dynamos bolt between the chainstays behind the bottom bracket and are powered by a roller against the tyre, these are easy to fit and do not wear the tire sidewall (Toshiba Satellite T4900 Battery) .
A fourth type is a spoke-mounted electromagnetic system.
Output and optics
Dynamos are generally limited to 0.5 A output, nominally 3 W at 6 V, although 12 V dynamos are available and the best nominal 6 V hub dynamos can produce 6 W at speed, as detailed here (a bottle dynamo is likely to slip if run at twice the nominal power, a hub dynamo does not have this problem) (Toshiba PA3399U-2BRS battery) .
A dynamo behaves as a constant-current device, not constant voltage, so additional lamps added in series will draw the same current. However, this is only approximate, and the voltage can exceed the capacity of a single lamp at speed, causing failure. Historically this was a nuisance, but modern lamps and dynamos often incorporate zener diodes to prevent it, and dynamos can be designed to "saturate" beyond a certain voltage to protect the lamp (saturation is a feature of all permanent magnet generators) (Toshiba Satellite A200 Battery) .
Good dynamos can achieve efficiencies of up to 70% (i.e., under 5 W of the rider's output is diverted to produce 3 W of electricity) and provide full output at surprisingly low speeds, often 4 to 6 mph (6 to 10 km/h) is sufficient for full brightness.
To compensate for their limited output, dynamo headlights have good optics which focus the limited amount of light in a narrow beam that lights up the road directly in front of the bicycle; this can be seen in Andreas Oehler's side-by side comparison of beam patterns (Toshiba Satellite 1200 Battery) .
To produce light when the bike is stationary, some dynamo lights have a stand light facility, usually a single blue-white LED powered by a capacitor, which runs for around five minutes. It is now common to use all the dynamo's output to power the front light; the alternative is a 2.4 W headlight and a 0.6 W tail light. Most good dynamo rear lights now use LEDs instead of incandescent lamps (Toshiba Satellite M300 Battery) .
Increasingly, dynamo headlights use LED light sources for the main beam as well as the standlight; some manufacturers include B&M, Supernova (both German) and Solidlights (British). In these newer lamps, the dynamo output requires substantial cleaning up and rectification by electronics before it can be used to drive an LED, whereas a conventional filament lamp will run happily on the "raw" AC from the dynamo (Dell Latitude XT2 Tablet PC Battery) .
Pros and cons
Hub dynamos are generally considered the best but require a wheel to be rebuilt if retrofitted (the existing rim can usually be reused). Some bikes are available with hub dynamo lighting systems as a factory option; this can be very cost-effective.
Some riders consider the output of dynamo lighting insufficient. Others report that it is adequate to see on dark roads at speeds up to about 20 mph (32.19 km/h), (single 3 W) or more if a 12 V system is used (Toshiba Portege 335CT Battery) .
LED headlights typically give 2-3 times the light output of a halogen lamp for the same power, due to their greater efficiency .
Advantages of dynamo lighting
- Unlimited burn time
- Always available
- High-efficiency optics designed to maximize the available power for the lamp (optimised for road use)
- Reliable (see below for known issues with bottle dynamos)
- Environmentally friendly and inexpensive to run
- Unattractive to thieves, due to difficult removal . (Dell Vostro A90 Battery)
- Bottle dynamos may be noisy, can slip when wet, and may wear the sidewall of the tire
- No lighting available when stopping when equipped without standlights
- Limited power output (Toshiba Satellite P15 Battery)
- Cut out at very low speeds (below walking pace) (modern dynamos often add capacitors to keep the lights going for a short while when the bike slows or stops, or in-line rechargeable batteries for long-time use of unused real-time energy)
- Difficult to remove by the owner to thwart theft or vandalism
- Cheap or poorly aligned dynamos produce noticeable drag
- Halogen bulbs run by a dynamo have a relatively short life (around 100 hours) (modern dynamos tend to not suffer from this problem) (Toshiba Satellite Pro M10 Battery)
Supplementary lighting and conspicuity
Fore and aft lights
Headtorches are a useful adjunct to bicycle lights. They can be pointed without steering the bike, giving useful "fill-in" lighting especially on poor or very dark roads. They also allow the wearer to read road and directional signs placed on high signposts.
Some rechargeable systems offer a head torch option powered from the main battery pack (Toshiba Portege 3110 Battery) .
Automatic turn signals worn on hand
In recent times, automatic turn indicators became available. They react to a change of orientation (an angle) in relation to Earth gravity; they blink only when a cyclist's hand is out-stretched to indicate a turn (Toshiba Portege R600 Battery) .
Manually operated turn signals
Also in recent times, there have been manually operated signals that are available, usually having an LED tail light incorporated into the fixture, which contains the two signals found on the left and right respectively. An example can be found here (Toshiba Satellite 1900 Battery) .
Reflective and high-visibility materials
Retro-reflective materials, in the form of reflectors, reflective tape, and reflective clothing, are useful in making a cyclist visible to other road users. Reflective materials can be applied to bike, rider, luggage, and tyres are available with reflective sidewalls (Toshiba Portege R200 Battery) .
On the bike
Reflectors and reflective tape provide additional visibility (especially when applied to moving parts of the bicycle) and are mandatory in many jurisdictions. Pedal reflectors in particular are very visible to following traffic as they move up and down; unfortunately they are not compatible with most clipless pedal systems, although adaptors are available for some, mainly older SPD models, and a few single-sided designs are available with built-in reflectors (SONY VAIO VGN-FZ21m Battery) .
In the UK, where front and rear pedal reflectors are compulsory after dark, most cyclists with clipless pedals are therefore riding illegally. The law is rarely, if ever, enforced, but could potentially be used in court to reduce financial compensation if the cyclist was hit by another vehicle (SONY VAIO VGN-FZ18m Battery) .
The CTC have suggested that the requirement could be waived if the cyclist fits an additional rear reflector and/or lighting, but this was not changed in the last revision of the UK vehicle lighting laws (which permitted flashing LEDs). Riders of recumbent bicycles have pointed out that the pedal reflector requirement is nonsensical for them, since the reflectors point straight up and down in use, and are invisible from other vehicles (Dell Vostro A90 Battery) .
As of 2008, California law allows white or yellow shoe reflectors (front and back), or reflective ankle bands, in lieu of pedal reflectors.
Reflectives are visible only when in the beam of a headlight, and even then only within a narrow locus. Importantly, they do nothing to light up the road. Reflectors are not a substitute for lights (Dell Vostro A860 Battery) .
An illuminated bicycle jacket made out of clear vinyl, fiber optics, LED's, and Christmas Lights that improves rider visibility
On dark roads reflective materials such as 3M's Scotchlite will show up boldly in car headlights. Evidence shows that bright material on the sleeves can prompt drivers to give more passing distance. Retroreflective materials can discourage some oncoming motorists from dipping their headlights, as the rider become less visible if they do (Dell Vostro 2510 Battery) .
The colour of lighting should be checked in the rider's area. A single solid colour can disappear under artificial light, particularly yellow sodium vapour lighting, and colour blindness is common; red/green colourblindness can make yellow fluorescent vanish against a green background (hedges or grass), although people with red/green colourblindness dispute this. Vests with both yellow and orange fluorescent plus wide strips of reflective may be the best solution (Dell Vostro 1700 Battery) .
In recent times electroluminescent clothing has become available to add to the existing array of LED-illuminated armbands and helmet blinkies. Anelectroluminescent helmet has been patented.
Measures of light output
Three units are used to measure light output. Manufacturers do not always quote the most appropriate figure—for example, watts are commonly quoted, but wattage alone is a poor measure since (a) it reports the consumption of power rather than the output of light, and (b) lamp optics will significantly impact the portion of the light which is delivered where it is needed, for example concentrated in a spot beam or dispersed as in running lights (Dell Vostro 1510 Battery) .
Candelas, measuring the intensity of a beam, are more appropriate when the aim is to illuminate brightly a small spot, while lumens, measuring the entire production of light, are more relevant to the purposes of broad beams or non-directional running lights.
For more details on this topic, see Watt.
The watt (W) is the unit of power, and is usually quoted for the electrical power input, not the light power output (Dell Vostro 1400 Battery) .
Electrical power is the product of voltage and current (watts = volts × amperes). Input power is only useful when comparing lights of similar technologies. A 3 W halogen dynamo headlight will light the road up about the same as a rechargeable light of around 7–10 W, but the rechargeable usually outputs much more light to the sides, which is useful on trails (although wide and narrow beam versions are available) (HP Pavilion DV7 Battery) .
A 3 W LED is somewhat brighter than a halogen lamp but generally less well focused. HID lights put out large amounts of light and are often quoted in terms like "80 W halogen equivalent".
For more details on this topic, see Candela.
The candela (cd) is the SI unit of luminous intensity, that is power per unit solid angle, weighted according to the sensitivity of the human eye to various colours of light (HP Pavilion dv9000 battery) .
A typical candle produces light with about 1 candela of luminous intensity. A lamp can produce higher luminous intensity either by producing more light, or by focusing it tighter. The luminous intensity of a light depends on many factors, including the colour of the light and the eye's sensitivity to that colour, the optics involved, reflector and lens (Dell Vostro 1000 battery) .
Despite its complexity, it is a more useful measure than watts, because it defines how much usable light is shed in a given place: a dynamo headlight designed for road use and focused for seeing the road makes more efficient use of the power of the lamp than lights using rotationally symmetrical optics (Dell INSPIRON 1525 battery) .
For more details on this topic, see Lumen (unit).
The lumen (lm) is the SI unit for luminous flux, the total amount of light emitted by a source, weighted according to the sensitivity of the human eye to various colours of light. Lumens per watt is a common measure of the efficiency of a light source. The luminous flux is of less value for bicycle lighting due to the importance of directionality. Luminous intensity is much more useful, but lumens per watt is a handy way to compare the output of otherwise similar lights (HP Pavilion DV4 Battery) .