Bicycle lighting

File:Commutebike.jpg

A commute bicycle with an inexpensive, homemade, rechargeable battery powered, quartz-halogen lighting system

Bicycle lighting has two main purposes: seeing and being seen. There are several types of bicycle lights available, each with its own advantages and disadvantages, and each with its own enthusiastic advocates! Different technologies address these two purposes in different ways. There is no one "best" solution for any rider, and many riders mix and match different technologies to provide the balance that works for them.

History

The earliest bicycle and car lights were generally powered by acetylene (carbide) lamps, now almost unused except by cavers. They were dim and temperamental, and the arrival of battery lamps was warmly welcomed.

Early battery lamps generally used lead-acid batteries, but these were replaced by self-contained 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 much more practical, as storage density in batteries was at this time very low.

This was the state of play when most bicycle light laws were passed; not all jurisdictions mandate use of lights after dark. This was not always uncontroversial: for example, 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 something of a renaissance on the invention of alkaline batteries, with much higher storage densities. Moulding techniques for plastics also improved, allowing lens optics to be refined at relatively low cost thus making more efficient use of the light output. During the 1980s the lighting market became more globalised: European riders would have noticed the gradual disappearance of the French "Wonder Lights" and Ever Ready brands in favour of American, Japanese and German products.

In recent times there have been several major 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 LEDs; white LEDs; High Intensity Discharge (HID) lights crossing over from the automotive sector.

In many cases legislation has failed to keep up, mandating outdated standards.

Choosing an Appropriate Lighting System

If you want to cycle at low speeds, in familiar areas, with street lights, other ambient lighting, or when there's a full moon on a cloudless night, a low power LED headlight, and a rear LED flasher may be sufficient. If you actually want to see obstacles in darker areas, stand out in heavy traffic, or travel at higher speeds (>10 mph) you probably need a more powerful, quartz-halogen front light. Added power comes with penalties of greater weight, greater expense, and/or shorter run time. If you are doing bicycle touring, or if you don't want to hassle with batteries, you can use dynamo-powered bicycle lights.

Before choosing lights it is important to check your local legislative framework - if you are involved in a collision and found not to have been using legal lights (and/or reflectors) this can affect your right to compensation.

Many regular cyclists recommend multiple lights for different purposes or in case of battery or lamp failure. A common setup for a commuter might be:

• flashing rear LED (illegal in some jurisdictions)
• steady rear LED (mandated in some jurisdictions)
• halogen front light powered by a rechargeable battery pack or dynamo
• flashing LED front light for conspicuity (again, illegal in some jurisdictions).

Decisions may be driven by the power source (e.g. tourists choosing a dynamo system) or by the amount of light required (e.g. off-road riders choosing a HID rechargeable), but often it is a matter of personal preference or local availability. When deciding on an appropriate system it is wise to check all the factors, such as burn time (will the battery last for your entire commute, both ways?), cost of batteries for battery lamps or LEDs, lifetime of battery packs for rechargeable systems and so on.

Each type of lighting system has its pros and cons. The brightest lighting systems are the high power halogen lights and High Intensity Discharge (H.I.D.) lights, powered by a rechargeable battery. Dynamo systems are more convenient, because there are no batteries to worry about, but provide less light. Self-contained LED front lights have reached a stage where some are viable alternatives to conventional bicycle lights although the majority are more suitable for being seen or emergency use only.

Legal requirements

Under the international Vienna Convention on Road Traffic (1968) of the United Nations, a bicycle is defined to be 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." However not all countries impose this requirement on their domestic cyclists. The U.S. has a reputation for permitting unlit cycling at night, there is no federally binding regulation and many States have laws requiring only the use of front, but not rear, lights after dark. In some countries, for example France, it may be an offence to even sell a bicycle not fitted with legally compliant lighting system. Individual jurisdictions define specific legal requirements in terms of light output and the size of lamp and reflector lenses, compliance with specified Standards or they may simply stipulate a minimum distance from which any lighting device should be visible.

Some commonly available lighting systems such as LED based flashers may not conform to the local legal requirements, and some jurisdictions mandate specific standards approvals which may mean that a system which is both safe and practical may be, technically speaking, illegal - though these rules are rarely enforced. In other cases a single LED blinkie, or even reflectors alone, is enough to meet the mandated requirements, but most cyclists would argue that this is inadequate.

You should base your choice of lighting on the circumstances that prevail for your ride, in particular ensuring that you have enough light to see on the darkest part of your route on the darkest nights.

Wherever you ride you should familiarise yourself with local legal requirements, especially if you are touring abroad. In the UK, for example, the law requires a light approved to BS 6102 Part 3: 1986, but allows the use of additional non-standard lights; in practice this means that users of powerful rechargeable systems need to add an additional light to meet legal requirements. Ensure that you have lights which meet what requirements exist, and if you use additional lights, ensure that these are legally permitted (e.g. that they are of the correct colour, do not flash if that is prohibited etc.) National cyclists' organisations such as LAB (US) or CTC (UK) are a good source of this kind of information.

Safety

Cycling without lights is generally considered to be unsafe. Most cycle safety advocates agree that reflectors alone are insufficient (and lights are required by law in many jurisdictions).

That said, and without wishing to condone or encourage unlit riding, it must be said that solid evidence is hard to come by. This may be because the majority of unlit cyclists are riding in urban areas where the major function of lights is in being seen; it is possible that the cyclist is sufficiently visible under street lighting that the additional conspicuity afforded by lights is not significant - on unlit roads most cyclists will use lights out of simple practicality.

The view among experienced cyclists certainly amounts to a strong consensus in favour of use of lights. Some claim that only very bright lights will do; this is more contentious.

Front lighting

Modern LED headlight

LEDs

Cyclists that ride at night only occasionally might 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 or are considering making these legally acceptable. Very high-power LEDs are sensitive to overheating (e.g. 5W Luxeon Vs), and over-driving, if the enclosure, and driving electronics are poorly designed, both of these conditions result in significantly shortened lifespan (dimming, or complete burn-out), and the LEDs are expensive to replace. Efficiency of the LEDs in lumens/Watt is superior to most halogen lights, but the driving electronics can eat up all of the advantage unless well designed.

Most riders will use alkaline cells to power inexpensive LED lights; these can have a life of anything from under a week to a year or more depending on the amount of riding, and brightness of the LEDs - life will be longer in flashing modes (even though many lights make the LEDs brighter when flashing than when steady to conserve battery life).

Low power LEDs are adequate for riding on well-lit streets, but they 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 small number of very high power LEDs - each with their own optics - and it is now possible to get LED equivalents for halogen rechargeable systems (including drop-in replacement bulbs), as well as LED lights for dynamo power - Solidlights offer a range, as do NightLightning, Cateye, Busch & Muller and others. One particular advantage of LEDs in dynamo-powered lighting is that they produce more light at very low speeds (down to 3km/h according to one manufacturer).

High power LED systems often include an option to dim the LEDs. LEDs are very well suited to dimming (running at half brightness will normally give you more than double the battery life), unlike halogen lights, where running a single bulb at half brightness will only give you a slight increase in battery life.

The adoption of high-power LEDs by the commercial lighting industry and automotive industries is driving prices down and brightness up all the time. Efficiency is also set to increase as white LEDs switch to emmiting 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, such as the Lumileds Luxeon range become available and major manufacturers start to adopt these high power LEDs to replace their halogen designs (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), there are grounds to believe that LEDs will replace incandescent lamps whatever the power source. In the future they may even challenge HID lights.

Advantages of LEDs

• Can be cheap
• Long battery life in flashing mode
• Can be very visible
• Efficient
• LEDs last pretty much indefinitely (as part of a well-designed light)
• LEDs can be dimmed, usually with a slight gain in efficiency

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

In general, then, 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 at present.

Home Made LED lights

As with high-power halogen systems, high-power LED systems have attracted self-build enthusiasts, top-end commercial products tend to be similarly high priced - due to the relatively low production volumes for these high-end parts. Some examples - [1] [2] [3] [4]

Low-cost battery lights

For occasional use a low-cost battery light such as the Cateye HL-500 can be a sound choice and usually meets legislative requirements. A complete set of halogen front light and LED rear light, including batteries, can be had for well under $20. These lights are low power but compensate to this to some extent by using surprisingly good optics. Some newer models are available with LEDs instead of halogen lamps; this is not legal in all jurisdictions.

Advantages of low-cost battery lights

• Low cost
• Easily moved between bikes
• Readily available (both lights and batteries)

Disadvantages

• Cost of batteries
• If using rechargeable cells, limited burn time
• Generally low power

In summary, then, a standard halogen light can be a good choice for the less demanding occasional user. It can also be a useful emergency light if your usual lights are more exotic.

Rechargeable Flashlights

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 (US$80). They can be fastened to the bicycle handlebars with a device called a "Lock Block." Look for a flashlight that has an adjustable spot to flood beam. These are sold by police supply stores, under the Streamlight brand. Although the optics of these flashlights are generally high quality they are still rotationally symmetrical (unlike motor vehicle lights).

Advantages of rechargeable flashlights

• Versatile - can also be used as a flashlight off the bike
• Portable, so theft resistant

Disadvantages

• Limited burn time
• 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

Rechargeable Halogen Systems

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 unsophisticated: 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. Road vehicle lights generally have beam patterns which are relatively flat, and most road vehicles are subject to legislation restricting the amount of light spilled above the horizontal axis. This also means that 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 offroad riding, or a high and a low power beam for road riding.

Cost for these systems ranges from US$50 to US$200+.

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

In summary, rechargeable halogen lights are the most common solution for offroad night riding, and increasingly popular for road riding due to their brightness, moderate cost and widespread availability.

Home-Made Rechargeable Halogen Systems

Some cyclists build their own lighting systems with commercially available lamps, batteries, and chargers. For a person with the necessary technical background and workshop skills, this can be an enjoyable challenge. Hardware costs can be lower than equivalent commercial systems (although "intelligent" chargers can be quite expensive). However, some components are not typically available in local hardware stores, and time spent searching for suppliers should not be underestimated.

Besides locating appropriate batteries and chargers, the biggest challenge is attaching the lamps to the bicycle. Resulting systems often have a decidedly "homemade" look, relying on blocks of wood, pipe fittings, etc. for housings and mounting hardware. (See illustration above.) Many web sites have detailed instructions, including parts sources (search for: "build bicycle lighting systems" to find these sites).

Advantages of home built systems:

• Less hardware expense
• For hobbyists, the pleasure of designing one's own system

Disadvantages

• Necessity of searching for appropriate components
• Need for appropriate home shop tools and skills, both mechanical and electrical
• Construction time

Examples - External Links

• How-to build a halogen bike headlight with readily available components

Rechargeable H.I.D. Systems

HID lights are the brightest lights currently available for bikes. They are efficient, very bright, but comparatively expensive (US$250 and up). They also tend to have high power consumption, so a relatively limited burn time. Other than this 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.

If you use a HID lighting system on the road, you should be particularly careful to adjust your lights to avoid dazzling oncoming traffic.

Dynamo Systems

Dynamo systems are probably the most widely-used worldwide, although they are uncommon in North America. They provide a self-sufficiency which is valued by their users: no batteries to recharge or replace, and permanently fitted to the bike so no worries about being out unexpectedly late without your lights. (Note: bicycle dynamos produce AC, and so are more correctly referred to as alternators or generators, but the word 'dynamo' has stuck in universal usage.)

Dynamos are generally limited to 0.5A output, nominally 3W at 6V, although 12V dynamos are available and the best nominal 6V dynamos can produce 6W at speed, as detailed here. A dynamo behaves as a constant-current device, not constant voltage; this means that the voltage can exceed the capacity of the lamp at speed, causing failure. Historically this was considered an inherent nuisance but these days lamps and dynamos often incorporate zener diodes to prevent it. In practice it is not an issue for most users as the electrical design of the dynamo itself tends to self-limit the output voltage at normal riding speeds. Good dynamos are extremely efficient and provide full output at surprisingly low speeds, often 4-6mph 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. To compensate for the fact that dynamos produce no power when stationary the best lights (and some dynamos) have a stand light facility, usually a single blue-white LED powered by a capacitor which will run for around five minutes. It is now common practice to use all the dynamo's output to power the front light; the alternative is a 2.4W headlight and a 0.6W tail light. Most good dynamo rear lights now use LEDs instead of incandescent lamps.

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 20mph (single 3W) or more if a 12V system is used. Whether the output is sufficient or not will be a personal choice based on the roads you ride and perhaps how good your night vision is. Around town the quantity of light output is generally less important.

Like most lights, dynamo systems range in cost from around $30 for a basic system to $250 or more, especially if you aim for 6W output. Hub dynamos are generally considered the best but require a wheel to be rebuilt if retrofitted (your 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.

Cheap dynamo lighting systems are available both from bike shops and non-specialist retailers. These can be made to work quite well; simply changing the lamp to an halogen one will produce a big improvement in light output. As usual though, both efficiency and longevity tend to improve with price. At present the SON hub and Dynosys bottle dynamos set the standard by which others are judged.

Advantages of dynamo lighting

• Unlimited burn time
• Always there
• High-efficiency optics that are designed to maximize the available power for the lamp (optimised for road use)
• Reliable (although see below for issues with bottle dynamos)
• Environmentally friendly and cheap to run
• Unattractive to thieves

Disadvantages

• Bottle dynamos are noisy, can slip when wet, and wear the sidewall of the tire
• Without standlights, when you stop your lights are off
• Limited power output
• Cut out at very low speeds (below walking pace)
• Difficult to remove where theft / vandalism are a problem
• Cheap dynamos produce noticeable drag
• Halogen bulbs run by a dynamo have a relatively short service life (around 100 hours)

The best dynamos come from Europe and Japan. Busch & Müller and Dynosys make higher-end systems; Marwi, Axa-Basta and Nordlicht offer some low-cost models. SON, Shimano and Renak Enparlite make hub dynamos. Head and tail lights are made by a number of manufacturers including Busch & Müller, SON, Hella and Axa-Basta. The SON E6 headlight has the same excellent optics as the highly-regarded but now unavailable Bisy-FL. This is available with and without a glowring (which is visible from the sides but can interfere with night vision if mounted on the handlebars).

There is a NEWLY invented no-friction bicycle lights dynamo. It overcomes most "Disadvantages" on normal dynamo. details on: http://www.freelights.co.uk

Rear lighting

Filament lamps

The only real advantage to these is that they are often omnidirectional, being visible through a very wide arc. Newer LED lights have this feature, making lights based on filament lamps pretty much obsolete.

LEDs

In many countries LED flashers are the norm for rear lights. In others such as Germany flashing lights are forbidden by law. In the UK flashing LEDs (front and rear) are legal from October 2005. It has been found that people tend to underestimate the distance to blinking lights and also that drunken drivers are attracted by them, and there is evidence that they are harder to place than a steady light; on the other hand they have also been shown to be between three and five times as visible as a steady light of equivalent brightness. Most LED lights will work in either flashing or steady modes; some have multiple banks of LEDs allowing both at once. The obvious answer is to have one of each, or a light which will do both simultaneously. Cyclists who habitually ride in groups at night, should use a steady rear light, to avoid discomfort to other members of the group.

Low-end flashers sold in discount stores (typically using AAA cells) have low brightness and are not especially robust. The best LED flashers are solidly built, weather-proof, have very bright LEDs, have a solid mounting system (which may be a cost option), and are often designed for visibility from the side, for example by having LEDs pointing to the sides as well as to the rear (although these may be of questionable benefit, being very low power). Good rear LEDs generally cost in the region of $25+.

The most common power source for rear LEDs is a set of alkaline cells; rechargeable cells are less environmentally damaging but have a much shorter run time. 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.

Advantages of LED rear lights

• Compared with incandescent lamps, near-infinite service life
• Bright
• Efficient
• Cheap
• Usually a choice of flashing or steady

Disadvantages

• Cheap ones are not very bright and have poor battery life
• Flashing not an unequivocal benefit

Xenon strobes

A relatively recent 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

Disadvantages

• Relatively expensive
• High power consumption (short battery life)
• 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
• Brief flash, followed by darkness, can make judgement of cyclist's position difficult

Supplementary lighting and conspicuity

Headtorches

Headtorches, such as those made by Petzl 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 headtorch option powered from the main battery pack, but if judiciously used you can also get good life out of alkaline batteries or rechargeable cells.

Reflective and high-visibility materials

Retro-reflective materials, in the form of reflectors, reflective tape, and reflective clothing, are thought to be useful in making a cyclist visible to other road users. Reflective materials can be applied to bike, rider, luggage - you can even get tyres with reflective sidewalls.

Of course, none of this absolves drivers of their fundamental responsibility to drive within the distance they can see to be clear: deer and fallen trees, to name but two, rarely bother to wear reflectives.

On the bike

Reflectors and reflective tape consume no power, weigh little and 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.

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.

Clothing

No bike light will ever win the arms race of vehicle light brightness and in the dazzle and light pollution of a city street the cyclist's body is a far bigger target for drivers to see; 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. So when choosing cycle clothing, especially jackets, it makes sense to go for the most garish option available. Be aware, though, that retroreflective materials can discourage some oncoming motorists from dipping their headlights, as you become less visible if they do. A peaked cap of some form can be quite useful to prevent being dazzled by oncoming headlights. If all else fails close one eye until the oncoming vehicle has passed - this preserves the night vision in the other eye.

Check the colour of lighting in your 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). Vests with both yellow and orange fluorescent plus wide strips of reflective may be the best solution.

In recent times electroluminescent clothing has become available, an improbable crossover from the rave scene, to add to the existing array of LED-illuminated armbands and helmet blinkies. An electroluminescent helmet has been patented.

Repairs in the Dark

It's a good idea to keep a flashlight for use when making running repairs (a headtorch works, too). Although you can use a headlight if removable, a self-contained flashlight is more convenient. Tiny, inexpensive LED flashlights are now available, and are suitable for this purpose. It is also considered prudent smart to carry spare lamps if you use a system where the lamps are replaceable separately, and spare batteries for non-rechargeable battery lights.

Measures of light output

There are three main units used in the measuring of light output. Manufacturers are in the business of selling product, so do not always quote the most appropriate figure - for example, Watts is commonly used, but wattage alone is a poor measure since the light optics will have a significant impact on the proportion of that power which is delivered where you want it.

Watts

The Watt (W) is the unit of (in this case electrical) power, the product of voltage and current (Watts = Volts x Amps, in other words). This is only really useful when comparing lights of similar technologies, and even then only in a limited sense. A 3W halogen dynamo headlight will light the road up about the same as a rechargeable light set of around 7-10W, but the rechargeable will put out much more light to the sides, which is useful on trails (although wide and narrow beam versions are available, so even this is a generalisation). A 3W LED is somewhat brighter than a halogen lamp but generally less well focused. HID lights put out incredible amounts of light and are often quoted in terms like "80W halogen equivalent". One thing does not change: the more Watts the lamp uses, the faster the battery will run down.

Candela

The candela (cd) is the SI unit of luminous intensity. Formally, it is the luminous intensity in a given direction of monochromatic light source of frequency 540 × 10^12 Hz having a radiant intensity in that direction of 1/683 Watt per steradian. The measured intensity of a given light is thus dependent on many factors, including the colour of the light and the eye's sensitivity to that colour, the optics involved, reflector and lens, and of course where in the beam you are standing. This is a more useful measure than Watts, despite its complexity, because it defines how much usable light is shed in a given place: a dynamo headlight, which is designed for road use and is focused to place light where it is needed for seeing the road, makes more efficient use of the power of the lamp than lights using rotationally symmetrical optics.

Lumens

The lumen (lm) is a derived unit for luminous flux, formally the luminous flux emitted into unit solid angle by a non-directional point source having a luminous intensity of 1 candela. Solid angle, in case you didn't know, is expressed in steradians (sr); the solid angle subtended by a surface at a given distance is defined as the surface area divided by the distance squared - in other words, you draw a sphere of 1m radius, place a 1cd light source in the middle, and the luminous flux expresses how much light (1 lumen) will fall on each square metre of the inner surface of the sphere; i.e. 1cd = 1lm/sr. Lumens per Watt is a common measure of the efficacy, or "efficiency", of a light source. Clearly the luminous flux of a light source is of little value in isolation; the light will be changed by the optics. Luminous intensity is a much more useful measure, although lumens per watt is a handy way of comparing the output of otherwise similar lights.

External links

• Photos of various lighting systems