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I have two lamps

6W LED(OSRAM 50R): equiv 40W or 170 lm

15W WW827 Tornado Philips, equiv 85W or 950 lumen

Why the 2x power difference and 6x luminosity difference?

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FYI the spec sheet you link to for that Philips CFL shows a Wattage Equivalent of 70, not 85. – Henry Jackson Dec 27 '12 at 21:19
Do you know about Turbo Energy Saver, 802163xx? – Val Dec 28 '12 at 0:47
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The key is that a lumen is not a unit of power, it's a measure of power /weighted by the response of the eye/. For instance, incandescent bulbs produce tons of light you can't see, which is why they produce so few lumens per watt. – ArgentoSapiens Dec 28 '12 at 2:56

3 Answers

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There are a couple terms involved that all have specific meanings:

  • Watts: the amount of electricity used by the bulb (a.k.a. "power")
  • Lumens: the amount of visible light the bulb produces (a.k.a. "luminosity")
  • Equivalent watts: an approximate incandescent bulb wattage that outputs a similar amount of light.
  • Luminous efficiency: the amount of light put out by the bulb per watt. Higher means better efficiency.

There's not a direct relationship between watts and lumens, but you can look at the luminous efficiency to compare bulbs. Wikipedia lists values for common household bulb types:

  • Incandescent bulb: 12-18 lumens / watt
  • Halogen incandescent: 24 lumens / watt
  • Compact fluorescent: 45-75 lumens / watt
  • LED: 60-100+ lumens / watt

The LED you list has an efficiency of 28 lumens / watt, which is pretty poor for an LED. LEDs have been increasing in their efficiency; maybe it's an older model? Your CFL is 63 lumens / watt, which is pretty good for a CFL.

Compare those to the current gold standard for mass-market LED bulbs, Philips' L-prize LED, which puts out 940 lumens using 10 watts, or 94 lumens / watt.

You're also seeing the problem with "equivalent watts", which is kind of an unhelpful metric. I don't know if there's any regulation on what manufacturers can claim as the incandescent equivalent, but I've seen bulbs as low as 700 lumens and as high as 950 lumens all claim to be "60 watt equivalent". Even among incandescents there's a range of light output for a specific wattage. The most reliable way to look at brightness is the "lumens" rating. If you're trying to match the brightness of an existing bulb, try to look it up online to find the lumens.

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That is interesting also. The same LED family has 140 lm/3W and 170lm/6w. But, equivalent efficiency grows as 25W -> 40 W (almost doubled). I do not understand anything. – Val Dec 28 '12 at 0:55
Incandescent bulbs tend to be more efficient at higher wattages. E.g. a 100 watt bulb puts out more light than two 50 watt bulbs. That's one of the reasons that wattage is a bad way to measure light output, even within a single bulb technology. – Henry Jackson Dec 28 '12 at 1:04
Yet, we see that 25 W has 5.6 lm/W whereas 40W bulb only 4.25 lm/W. The power efficiency of Incandescent lamps drops with wattages. This adds to my frustration rather than understanding the specs! – Val Dec 28 '12 at 1:39
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Yes, it's true that the LEDs you picked go the other way. I'm not sure why, perhaps there were other manufacturing considerations. LEDs work differently from incandescents and are more complicated. I encourage you to completely ignore the "25 watt" and "40 watt" equivalent ratings. They're just marketing info that has no real relationship to the bulbs. It's like putting "family size" on a box of cereal — what does that even mean? – Henry Jackson Dec 28 '12 at 2:06
I do not agree. We understand what the brightness of incandescent lamp means. We event know that its brightness grows superlinearly w.r.t. consumed power. If we don't, then, we might say that luminosity in lumens is meaningless either. – Val Dec 28 '12 at 11:47
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As was noted, the luminiocity of the reference (incandescent) lamp is not linearly proprtional to its power. The more powerful incandescent lamp is, the higher is its luminous efficacy (lumen/watt):

incandescent lamp 100 W     13.8 lm/W   2.0 %
incandescent lamp 200 W     15.2 lm/W   2.2 %
     halogen lamp 100 W     16.7 lm/W   2.4 %
     halogen lamp 200 W     17.6 lm/W   2.6 %
     halogen lamp 500 W     19.8 lm/W   2.9 %

So, when we rise power two times, luminosity increases 6 times. This is my speculation. But it cannot be a good answer because, as we see for powers > 100 W, 2x power increase adds only 1% to the efficiency, not 300% as needed for 6x.

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There are considerations for light output other than just the wattage. E.g. that table shows that two different 100 watt incandescent bulbs have different light outputs. – Henry Jackson Dec 28 '12 at 2:09
Why to blame the table if yourself you say that "Incandescent bulbs tend to be more efficient at higher wattages". Look your comments above. I have fixed the table to show that it is true for both incandescent and halogen. – Val Dec 29 '12 at 12:07
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Lumens is the absolute amount of light that a bulb puts out. You can use this to accurately compare how bright two bulbs will be. The one with the higher lumens value is the brighter light.

However, most people have no idea how bright a 170 lumen or a 950 lumen bulb is. What they do have is experience of how bright a 40W or 85W incandescent light is.

By using the "equivalent watts" value bulb manufacturers are making it easy for the typical shopper to have the information they require in order to make an informed decision over which bulb to buy. However, as Henry Jackson points out this is usually a figure calculated (or even made up) by the manufacturer and should always be treated with some caution.

I would assume that each manufacturer has measured the light output from a "standard" incandescent bulb (or a number of bulbs) and used that lumen value to calculate the "equivalent watts" of their bulbs.

Once incandescent bulbs have been completed phased out and/or more different bulb types are on the market you might see a switch to using the lumens value exclusively as this becomes more widespread and accepted.

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That is right. But, I wanted to ask how incandescent equivalent is computed and related to it. I have given concrete numbers to explain. – Val Dec 29 '12 at 12:33

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