Light and Lighting Facts and Bits of Data!

Updated slightly 1/14/2011, minor cleanup 7/11/2013.

Table of Contents - linked to the relevant sections

Incandescent light bulbs can last a century! You can buy ones that do!
Low voltage use of 120V incandescents
Incandescents at less voltage with color/brightness of a candle
Economies of scale affecting incandescent efficiency, why higher wattage incandescents are more efficient and why lower wattage ones are designed to last longer
Low voltage incandescents are more efficient than line voltage ones
Color temperature of incandescents
Efficiency (Overall Luminous Efficacy) of various light sources

Incandescent light bulbs can last a century!

There is one that has done so, viewable to the public in a fire station at 4550 East Ave, Livermore, California USA. It is known as the Centennial Bulb. It is also well enough known to have energy efficiency being a fraction of that of same-wattage "standard incandescent lamps". This lightbulb is widely mentioned as being a 4 watt one and sometimes as a 3-watt one, although I suspect its light output is close to or somewhat less than that of a 4 watt "nightlight bulb" while its actual power consumption is a few times that.

Now, why have incandescents that last a century been achieved but are not available?

First, incandescents lasting 8,000 or 10,000 or 20,000 operating hours or even typically a century are not so unobtainable after all - see below!

Second, there is a big reason why most are designed to burn out sooner. It is not simply a conspiracy to make you buy more lightbulbs. It is easy to make an incandescent lamp (lightbulb) last longer by having the filament run cooler. A big drawback of this is lower energy efficiency. In typical home use most of the cost of most incandescent lighting is from the cost of the electricity consumed, so it usually costs more to use 100 watt lightbulbs with extra long life rather than equally bright 75 watt ones of "standard" life.

Now for the longlife ones, with sources! (In approximate order of increasing life expectancy)

Philips "Double Life" has been consistently available at Home Depot in my experience. Light output and energy efficiency are reduced roughly 8-9% compared to "standard" light bulbs.

40A/99, 60A/99, 75A/99, 100A/99: These are 40, 60, 75 and 100 watt 120 volt lightbulbs with design life expectancy of 2,500 hours.

"Standard" in the USA is 750 hours for 75 watts or more, 1,000 hours for 60 watt, and depending on the manufacturer 1,000 or 1,500 hours for 40 watt.

The 2500 hour /99 types have light output and energy efficiency compromised about 13-13.5% from 750 hour versions of 75 and 100 watt 120V "A19" lightbulbs. The compromise is 10% for 1000 hour 60 watters, and 7% for 1500 hour 40-watters.

Where to get: Electrical/lighting supply shops, maybe some online sources. However, minimum order may be a case of them and they may need to be ordered.

130V versions: These have roughly 2.5 times their rated life expectancy at 120V, while at 120V consuming typically 88-89% of their rated wattage and producing typically 75-76% of their rated light output.

I have seen these at Lowes and I have seen these as being available at Other online sellers likely have these and these are typically available from electrical/lighting supply shops, but may have to be ordered and ordered by the case.

Pricing of these is not bad - sells 100 watt 130V ones for 68 cents apiece plus shipping (if total order meets/exceeds their minimum) for as few as 2 bulbs, and for 59 cents apiece plus shipping for 48 of them as of "wee hours" 12/23/2006.

"/35" 3500 hour versions: Life expectancy is indeed 3500 operating hours, and in addition the filament has a different design and has more supports to better withstand vibration and mechanical shocks. These light bulbs are often referred to as "industrial service" bulbs.

Light output and energy efficiency are typically reduced 32-36% compared to "standard" versions.

Where to buy: Online sellers such as, or electrical/lighting supply shops. Minimum order is often by the case, although has a much lower minimum (but higher prices). It's probably best to order by the case from one of those electrical/lighting supply shops of the kind that contractors go to.

Traffic signal lamps: 120 volt ones are normally designed to last 8,000 hours. In addition, there is little question about their ability to be turned on and off even hundreds of thousands of times!

Keep in mind that many of these are in oddball wattages with 92 and 116 watts being common.

Energy efficiency is typically around 37% less than that of "standard" incandescents. A 116 watt 120V "traffic siagnal lamp" has light output only a few percent more than that of a 75 watt "standard" one rated to last 750 hours.

There are even 130V traffic signal bulbs. A 130V traffic signal bulb when operated at same voltage as a 120V "standard" lightbulb of same rated wattage will typically have a life expectancy around 20,000 hours, consume 10-11% less power, have light output roughly 46-50% of the "standard" one, and have energy efficiency roughly 50-55% of that of the "standard" one.

Atlanta Lightbulbs has some superlonglife bulbs including several rated to typically 20,000 operating hours here.

230V lightbulbs! These are what European ones usually are. However, sells these, with E26 bases to fit standard North American "medium" / "Edison" sockets!

As of 4/13/2010, had 13 different incandescents rated anywhere from 220 to 277 volts available at: here.

It is indeed true that life expectancy of these is in the ballpark of a couple million operating hours, as in around or over 100 years!

Just keep in mind some facts of operating 230 V lightbulbs at 120 volts:

1. Power consumption is typically about 37-38% of their 230V wattage. If the filament's resistance did not vary with temperature, this percentage would be 27.2% (square of 12/23).

2. Light output of a 230V version at 230 volts is usually slightly less than that of a 120V version at 120 V. The lower amp filament is thinner and has to be operated at a slightly lower temperature for same life expectancy, and that reduces energy efficiency.
Light output of a 230V lightbulb at 120V is roughly 9% of what is achieved at 230V, or close to 8% of that of a 120V "standard" light bulb of same rated wattage.

3. Energy efficiency ends up being about 20-25% of that of "standard" lightbulbs of same rated wattage, or about 25-35% of that of a 120V one whose power consumption is the same as the 230V one in question consumes at 120V, or about 30-40% of a 120V one of the same light output. This means that a 200 watt 230V light bulb at 120V consumes close to 75 watts while having light output close to that of a 25 watt 120V lightbulb! Would you triple (maybe more) your lighting-related electric bill to make your lightbulbs last a century or two, along with making the color of their light less white and more like that of candle flames?

More extreme - 277 volt lightbulbs! Yes, there is such a thing as 277V lightbulbs available at and other sources. These are available with E26 "regular North America" "medium" "Edison" screw bases!

The 100 watt 277 volt one at 277 volts produces a little less light than the 120V version does at 120 volts. The 277V 100 watt one at 120 volts has power consumption close to 29-30 watts, and light output maybe 4-5% of its 277V output, and maybe 3.5-4.4% of the 120V output of a "standard" 120V 100W lightbulb, or a little more than the light output of a 7.5 watt 120V lightbulb and significantly less than the light output of a 15 watt 120V lightbulb. Energy efficiency is roughly 24% of that of a 120V lightbulb of the same 120V power consumption and roughly 30% of that of 120V one having the same 120V light output.

But life expectancy gets into several centuries, possibly a millennium or two should no breakage or significant air leakage occur in that much time!

Use of 120V incandescents at low voltage!

Do you like the glow of a 120V incandescent at 12 or 18 or 24 or 30 or 36 volts? Do you have a science fair project involving this?

I recommend "tubular"/"refrigerator"/"showcase" "lamps" (lightbulb types) of 25 or 40 watts. These are better than most others for giving some sort of nice glow at really low voltage since they have a vacuum rather than a gas fill.

Vacuum incandescents have lowest variation of filament temperature with applied voltage, and as a result have lower variation (although still large) of efficiency with applied voltage. Also, unlike gas filled 120V incandescents, their current requirement is low at low voltages, so that they are easy on batteries in any battery-powered project.

My experience is that 25 and 40 watt 120V tubular "refrigerator bulbs" easily visibly glow at 15 volts even in moderately bright room lighting of 240 lux, and should glow dimly visibly at 15 volts in a classroom having illumination level of 1,000-2,000 lux. The glow of these is fairly easily visible at 12 volts in a somewhat typical home illumination level of 100 lux. In a dimly illuminated room at 1 lux, these easily visibly glow at 8 volts. In a dark room, these easily visibly glow at 6 volts, moderately easily visibly glow at 5 volts and very dimly visibly glow at 4.1 volts. The current requirement of the 25 watt version is a little less than 60 mA at 15 volts, a little less than 50 mA at 12 volts, and a little less than 40 mA at 8 volts.

Second-best for efficiency decreasing less as voltage decreases are high wattages, such as at least 100 watts for 120 volt versions. They are better at glowing at low voltages if they have coiled-coil filaments aligned vertically, and if they are not extended life versions.

Should a battery-powered project be in question, I recommend considering "average voltage" of nominally 1.5V cells and NiMH cells in most use while in "average state of charge" to be 1.3V per cell even with a fairly light load. With a "medium load" such as what a 100W 120V incandescent draws at 12 volts, plan on 1.25 volts per cell. Of course, at times you will get more - but be prepared for what may easily occur.

Incandescents at less voltage with color/brightness of a candle

So you want an incandescent to roughly match both the color and brightness of a candle??

One could use a low power incandescent with a color filter such as a CTO, but chances are that with only minor use it will be overall better just to dim an incandescent to candle-like brightness, and to use incandescents that when dimmed to candle-like brightness have candle-like color.

So what is the color and brightness of a candle? This varies, but I have some figures!

A "spherical candlepower" is 12.566 lumens, and lumen output from a linear source or narrow cylindrical source producing a candela (formerly candlepower) perpendicularly is 9.87 lumens. So I say anywhere in or near the 10-12.5 lumen range counts for brightness.

As for color, definable by color temperature: Candle flames vary, but fairly typical is 1900 Kelvin. A smaller cleaner-burning one can exceed 2000 Kelvin, and a larger sootier one could be about or possibly less than 1800 Kelvin. So I consider 1900 Kelvin typical.

Dimming 120 volt incandescents having rated wattage of 40 watts to "candle-like brightness" of 10-12.5 lumens usually achieves a candle-like color temperature not far from 1900 Kelvin.

Dimming a "USA-usual" "standard" 100 watt 120V "A19" lightbulb with rated light output 1670-1750 lumens and rated life expectancy 750 hours to "candle-like brightness" achieves a warm-side color with a color temperature of about 1800 Kelvin.

On the other hand, 25 watt incandescents should have color only slightly whiter than that of a candle flame when dimmed to "candle-like brightness".

As for voltage - the 100-watt example above achieves this at 36-38 volts. Incandescents of lower wattages down to 40 or possibly 25 watts achieve candle-like brightness at somewhat higher voltages usually close to 50 volts.

As for 12 volt options - I favor using an 1156 at voltage in the 5 to 5.5 volt range.

Keep in mind that energy efficiency of such use of dimmed incandescents is low. If there is need for spending large amounts of time long term with candle-like color illumination, especially if brightness has to be like that of multiple candles, then the best solution becomes more likely one involving a combination of undimmed or mildly dimmed incandescent lamps and filters along the lines of CTO filter gels.

Economies of scale affecting incandescent efficency - why higher wattage incandescents are more efficient and why lower wattage ones are designed to last longer

1. For a given filament temperature, a thicker filament is more durable than a thinner one, so expect life expectancy to increase with wattage.

2. If the filament design is adjusted for same life expectancy for different wattages of incandescent lamps of the same design voltage, then ones of higher wattage will have a slightly higher efficiency since thicker filament wire permits a higher filament temperature giving a spectrum that has more visible light and less infrared (but still mostly infrared).

3. As design wattage increases, the lightbulb manufacturing cost increases less than proportionately. This means that not only it will cost you less to use fewer higher wattage lightbulbs in place of more lower wattage ones, but furthermore:

The percentage of cost of usage that is from electricity cost increases since the per-lumen cost of lightbulbs decreases. This shifts optimization to make higher wattage lightbulbs more energy-efficient at the expense of shorter life expectancy.
This is why most 120V "standard" incandescents of wattage 75 to 300 watts are designed for average life expectancy of 750 hours, 60 watt "standard" 120V incandescents are designed for average life expectancy of 1000 hours, and most 120V incandescents of wattage less than 25 watts are designed for average life expectancy of at least 2,000 hours, and most 120V incandescents of wattage 7 watts or less are designed for average life expectancy of at least 3,000 hours.

With higher wattage incandescent filaments usually being designed for shorter life expectancy and lower wattage ones usually being designed for longer life expectancy, expect even more variation of efficiency with wattage.

4. In gas filled incandescent lamps, a thicker filament has a thicker "boundary layer" of hot gas around the filament. The temperature gradient in this boundary layer is less when the boundary layer is thicker, so a thicker filament has less heat conduction loss per unit area of filament. This is another reason why higher wattage incandescents tend to be more efficient than lower wattage ones.

In fact, with very thin filaments, a fill gas would cause a heat conduction loss severe enough to outweigh advantages of a fill gas, so incandescent lightbulbs with very thin filaments (such as most designed for current less than .2 amp) have a vacuum.

Incandescents with a vacuum are designed to have a lower filament temperature because the filament evaporates faster in a vacuum than in a gas. Gas atoms/molecules bounce most evaporated tungsten atoms back towards the filament and this slows filament evaporation. With a vacuum instead of a fill gas, the filament needs to be operated roughly 200-300 degrees C cooler for the same life expectancy. If the filament's wattage is less than roughly 8-10 watts per centimeter of visibly apparent (without uncoiling) overall filament length, then a fill gas would reduce energy efficiency more than operating the filament at this lower temperature would.

Low voltage incandescents are more efficient than line voltage ones

Yes, it is largely true. By and large, low voltage incandescents are more efficient than 120V ones of the same wattage and life expectancy. The thicker filament can be run hotter. And when comparing between gas filled lower voltage and higher voltage incandescent lamps of the same wattage, the thicker filament is associated with a lower temperature gradient in the immediately surrounding gas - and has less heat conduction by the gas per unit area of the filament.

How much of a difference does this make? A 120V 100 watt "A19" lamp designed to last 750 hours produces 1710-1750 lumens, while a 12 volt one designed to last 1,000 hours produces 2050 lumens - 17% more. A 50 watt 12 volt halogen produces about the same amount of light as a 60 watt 120 volt halogen of the same life expectancy.

But before you run out to get transformers, consider the losses of the transformers, and the amount of time needed for their cost to pay for itself with any electricity savings of just a few watts.

There is an "optimum design voltage" for incandescents which maximizes efficiency, should your voltage requirement be flexible. A higher than optimum design voltage has disadvantage of a longer, thinner filament, while a lower than optimum design voltage has the disadvantage of heat being conducted through the ends of the shorter, thicker filament.

Color temperature of incandescents

Most household incandescents have color temperature of 2600 to 2900 K at rated voltage.

The 100W 750 hour 120V A19 lamps rated to achieve 1670-1750 lumens have typical color temperature of 2875 K. An older Kodak reference says 2865 K, but a slight revision of the blackbody radiation formula after that raised this very slightly. The lamps have also been made extremely slightly hotter-running afterwards. 60 watt 120V A19 lamps with life expectancy of 1000 hours and rated light output of 845-890 lumens achieve close to 2800 K.

As for extremes: Osram's version of the EHJ (a 24V 250W halogen lamp with rated life expectancy of 40 or 50 hours) has been mentioned as achieving either 3500 or 3550 Kelvin. 120V nightlight bulbs tend to achieve about 2300 Kelvin, maybe slightly less for 4 watt ones rated to last at least 5000 hours. The 682 or 682AS15, with its 60,000 hour life expectancy, low design current of .03 amp, vacuum fill and low luminous efficiency, probably has a color temperature of about 2000 Kelvin.

Of course, incandescents have reduced color temperature when operated at less than rated voltage. For an extreme, a 300W 120V 750 hour incandescent with a coiled-coil filament, when operated at 12V, achieves approximately or slightly over 1200 K. At 12V it also produces somewhat less light than a 1/3 watt neon nightlight, while consuming nearly 9 watts of power. Tungsten filaments even visibly glow at 800 K or a little less than that, but with obvious lack of useful illumination.

Efficiency (Overall Luminous Efficacy) of various light sources

Incandescent: This varies widely!

The 682 (or 682AS15) is a 5 volt .03 amp incandescent designed to last 60,000 hours and has a light output of .03 spherical candlepower, or .38 lumen. This is approx. 1.3 lumens per watt.

Chicago Miniature's 8-3995 is a 130V .02 amp lamp rated to produce .2 spherical candlepower, approx. 2.5 lumens. This works out to approx. .97 lumen/watt.

On the other extreme, a 500 watt 120V photoflood lamp designed to last 4 hours achieves about 35 lumens/watt. Osram's version of the EHJ, a 24V 250W halogen lamp with 40 or 50 hour life expectancy, is rated to produce 10,000 lumens, which works out to 40 lumens/watt.

However, "standard" 120V incandescents achieve 14.5-14.8 lm/W for 60 watt ones and 17.1-17.5 lm/w for 100 watt ones.

Fluorescent: This varies also!

On the low end is the 4 watt F4T5, where a "cool white" one achieves about 24 lm/W. After ballast losses in a ballast that can lose a couple watts, this can be 16 lumens/watt. On the high end are 32 watt F32T8 ones with modern electronic ballasts. Overall luminous efficacy even after accounting for ballast losses can be about 95 lumens/watt.

Most compact fluorescents 13 watts or more with integral electronic ballasts achieve about 60 lumens/watt. Most 15 and 20 watt linear (straight tube) fluorescents achieve about 60 lumens/watt before accounting for ballast losses and about typically 50 lm/w after. 40 watt 1.5 inch diameter F40T12 achieves about 80 lumens/watt before accounting for ballast losses and typically about 67 lm/W after.

Keep in mind that fluorescent lamps have degradation of their phosphor as they age, and one should plan for typically about 10% less light and less efficiency when aged to "average condition" (my words).

Mercury Vapor - Varies with wattage and whether or not the lamp is a phosphored one. The common phosphored 175 watt mercury achieves about 48.5 lumens/watt when brand new but broken in for 100 hours, and this is down to about 43 lumens/watt when "aged to average condition". This is before accounting for ballast losses.

Extremes before accounting for ballast losses, after a 100 hour break-in period but before any aging beyond that, are about 31 lumens/watt for the 50 watt phosphored one (a little less if you can get an unphosphored one) and 63 lm/w for a phosphored 1000 watt H36 type. Ballast losses and aging will reduce these figures.

Metal Halide - again, this varies with wattage. This varies with other factors, including compromising of efficiency for the lamp to be operated in different operating positions and for startability without a high voltage ignitor (typically included with a "pulse start ballast"). Efficiency will be a little higher for pulse start versions and for ones that require specific operating positions.

The extreme lows are about 45 lumens/watt for a 10 watt specialty lamp and about 51 lm/W for a 39 watt general purpose one. Lower still are some colored ones and ones designed to specialize in producing wavelengths with special applications, such as ultraviolet. The extreme high is 104.5 lm/w for a 1000 watt one. This is after a break-in period but otherwise no aging, and without accounting for ballast losses.

A 175 watt one that takes the non-pulse-start M57 ballast and is designed to be operated in any position achieves about 74 lumens/watt after a 100 hour break-in period and about 57 lm/w after being aged to "average condition". 175 watt ones with operating position restrictions achieve after break-in about 85 lm/W. 175 watt ones with both operating position restrictions and requirement to be used with the M137 pulse start ballast can achieve about 93 lumens/watt after being broken in but without any aging beyond that.

High Pressure Sodium - The extreme low is about 36 lm/W for a 35 watt one with improved color properties, and about 64 lm/W for a standard 35 watt one. The extreme high is 140 lumens/watt for 1000 watt ones. This is without accounting for ballast losses and is after a breaking in period of 100 hours but no other aging.

150 watt ones with the immediately above qualifications achieve about 106 lumens per watt, slightly less for mercury retrofit versions compatible with H39 ballasts.

Keep in mind that the low color rendering index and the fact that the spectrum disfavors visibility to night vision can make sodium vapor lamps appear to illuminate less than other lamps of the same lumen output and same light distribution pattern.

Low Pressure Sodium - The extreme low is 100 lumens/watt for the 18 watt one and the extreme high is 180 lumens/watt for the 180 watt one. This is without accounting for ballast losses. Aging usually does little damage to light output, but instead causes power consumption to increase slightly.

Keep in mind that low pressure sodium has low stimulation of night vision and an extreme of bad color rendering properties because its spectrum is essentially monochromatic. Only essentially - details are visible in the simulated low pressure sodium vapor spectrum that I show in my spectra page. The secondary wavelengths are enhanced for easy visibility, and show up unrealistically bright on many LCD monitors.

LED - The most efficient white ones to have newly hit the market achieve typically about 85-115 lumens/watt with favorable heatsinking and before accounting for losses in the ballasts that are typically required. This is also for high power LEDs rated for 700-1500 milliamps max being operated at 350 milliamps, while they are often operated at higher currents at which they are less efficient (often achieving 70 lumens/watt for top grade LEDs). One 3 amp model (a Cree XM-L one) operated at 350 milliamps achieves 150-160 lumens/watt at 350 mA and good heatsinking.

A laboratory prototype has been announced as achieving 250 lumens/watt. Slightly higher overall luminous efficacy can be achieved by adding a fluorescent material that converts blue light to yellowish green light, and a white LED with such an addition will appear noticeably greenish-yellowish.

As for an extreme low - gallium phosphide "pure green" (yellowish green but less yellowish than usual) LEDs have been marketed around 1980 and achieve only in the general ballpark of .1 lumen per watt. Rare old yellow silicon carbides can be this inefficient or even worse. Of course, ones that produce nominally infrared wavelengths but that are actually slightly visible achieve even lower overall luminous efficacy.

As for a typical of white LEDs in devices sold in the first half of 2008 - I would say mostly 15 to 45 lumens per watt.

Written by Don Klipstein.

Copyright (C) 2006, 2008, 2010 Donald L. Klipstein ( - Please read my Copyright and authorship info.
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