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I noticed that one 10 W LED light bulb was 800 lumens and the other 1000.
Both were warm white, 4000K, 80 CRI.

What exactly makes one brighter than the other?

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  • \$\begingroup\$ Higher efficiency? \$\endgroup\$ Sep 14, 2018 at 8:53
  • \$\begingroup\$ That, or marketing, @Dmitry . \$\endgroup\$ Sep 15, 2018 at 16:23

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The lumen is the unit for the total (and weighted) measure of visible light produced (see also luminous flux). In other words, a particular LED may be better at producing mid-band visible light compared to another. It may also be slightly higher in efficiency i.e. more input power (10 watts) is converted to light.

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  • \$\begingroup\$ A mid power LED is more efficient than a high power LED. The Samsung LM301B (220 lm/W) is the most efficient mid powered (0.5W) LED (today 9/11/18) and the Cree XP-3G (200 lm/W) is the most efficient high power (3W) LED. By "mid-band visible light" I assume you mean spectral distribution, color temperature (kelvins), and CRI factors. \$\endgroup\$ Sep 12, 2018 at 0:12
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The biggest factor in light bulbs is the transmittance coefficient which is the amount of light that gets through the diffused cover.

I asked Klus for the transmittance coefficient of three of their LED diffusers. A clear smooth polycarbonate cover has a transmittance coefficient of 96%.

This is the reply from Klus for various diffusers. The diffuser transmittance coefficient can account for 20% difference from one bulb to another.

(HS)1369- 70% (frosted / opal)
(LIGER)17031-55% (transparent with frosted matte finish)
(HS)17111- 30% (Satin light focusing cover)

Image of Light Focusing Diffuser:
Thickness is a big factor in transmittance.

enter image description here



The loss in efficiency of the LED drivers can range from 5% to 30%.



A high thermal heat factor can reduce the luminosity by about 10%.

The top image is a high power Lumiled Rebel ES and the bottom a mid power Samsung LM301B, both high efficacy LEDs.

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The efficacy (lumens/watt) of the LED can vary significantly from less than 100 lm/W to 220 lm/watt.

A 5700K 70 CRI will have a much higher efficacy (200 lm/W) than a 2700K 90 CRI (150 lm/W) within the same LED product line.

The lower (warmer) the color temperature and the higher the CRI (more like sunlight) both will lower the lumens. This is due to the amount of red.

All white LEDs are a deep blue (450 nm) LED that uses phosphor wavelength converter to convert the blue wavelength to green and red. The phosphor conversion gets less efficient as the wavelength rises from blue to red.

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Photopic Luminous Efficacy (Relative Sensitivity Curve for the C.I.E. Standard Observer) of 555 nm green is 100,000, 525 nm red: 32,300, and 450 nm blue: 3,800.

To get the photopic luminous efficiency the brightness of the 555 nm source was reduced until the observer felt that the two sources were equal in brightness. The fraction by which the 555 nm source was reduced, became the luminous sensitivity with respect to the second observed wavelength.

1 watt of deep blue radiant flux has a luminosity factor (conversion from radiometric (radiant watts) of 555 nm of green has factor of 683 (100%), and 625 nm red has a factor of 220 (32%), and 450 nm blue 26 (3%).

Comparison of irradiance spectral distribution in Lumens and Watts for the same LED:

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    \$\begingroup\$ Great answer. If not for your extremely oddly specialized choice of datasheet graphs, I would never have know that LEDs have been developed specifically for illuminating raw meat. TIL \$\endgroup\$
    – K H
    Sep 12, 2018 at 0:48
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    \$\begingroup\$ @KH I design LED fixtures for horticulture research at the University of Florida. The red meat and marbled meat have a great spectrum for plants. Plants prefer 660 nm deep red photons and some blue. That was the only comparison of photometric and radiometric irradiance I have. Added a new photo for you. \$\endgroup\$ Sep 12, 2018 at 1:10
  • \$\begingroup\$ Really cool. You replied on the wrong post because your answer moved when I upvoted you btw. I was thinking of integrating spheres for measuring total light output regardless of lens/dispersion. I guess you can get a used 18" on ebay for 2 kilodollars, which is still pretty expensive as consumer level spheres go, but not out of the question. \$\endgroup\$
    – K H
    Sep 12, 2018 at 1:27
  • \$\begingroup\$ @KH I evened us out on the votes \$\endgroup\$ Sep 12, 2018 at 1:30
  • \$\begingroup\$ Lol thx but only if you like the answer. I like this question, It got a simple and succinct answer, a full on technical answer and also a practical shopping answer. All separate and valuable =). \$\endgroup\$
    – K H
    Sep 12, 2018 at 1:35
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I was just looking to buy a new led light bulb and noticed that one was 800 lumens and the other 1000 lumens and both were rated for 10 watts.

What exactly makes one brighter than the other?

There are a massive array of factors affecting LED brightness including quality of LED, color, human perception of that color, CRI, drive current and voltage, thermal dissipation and driver efficiency.

In addition to these factors that actually affect results, LED lumen measurements differ to some degree between companies, and some will even add up the lumen ratings of the LEDs without measuring, giving higher than actual ratings. Some people really appreciate high CRI LEDs and almost everyone will not like extremely low CRI arrangements. This means if it is high CRI, you might like an 800 lumen bulb better than a low CRI 1000 lumen one, even with the same color temperature and assuming the labelled brightness is accurate.

LEDs in general are more energy efficient than CFL and incandescent and in some cases there are factors that can even mitigate incandescent (If a house is entirely electric heated at wintertime, it makes no difference if heat comes from a lightbulb or a radiator.). Between electrical and thermal design, LED die improvements and advances in switching technology, manufacturers have still left themselves plenty of headroom for improvement even on more expensive bulbs.

Where I live there is also no strong correlation between price and quality for many bulb types due to market factors. A19 bulbs can cost anywhere for $1.50 to $5 and the cheap ones can be quite good and the more expensive ones nothing to write home about.

The best way to find a good bulb is to either evaluate it(if you're at a hardware store and they have a row of lightbulbs, turn them all on and take a picture.), find out what's in it(A good bulb will have good quality/ high efficiency LEDS, more LEDS running at lower power as opposed to few at higher power, and a better driver in it(more efficient). Or read reviews to get an idea of your best options. I'll sometimes buy $2 bulbs on sale just to test them out, but for more expensive socket types where bulbs are still $20 minimum, It could definitely be worth some effort to compare before you get a set of 11 new bulbs for the designer track lighting in your kitchen or something.

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    \$\begingroup\$ You usually don't need to quote the question, because it is right up there. \$\endgroup\$
    – Arsenal
    Sep 10, 2018 at 12:40
  • \$\begingroup\$ @Arsenal "You usually don't need to quote the question, because it's right up there." - Good point. \$\endgroup\$
    – K H
    Sep 10, 2018 at 14:27
  • \$\begingroup\$ In the US the Dept. of Energy sets the lumen measurement method for all light bulbs so there should be no difference between any 800 lm to any other 800 lm bulb. Emphasis on should. The best method for comparing light bulbs is by comparing their luminous efficacy (lumens/wall watt). For example Home Depot almost always specs the lm/W for every light bulb. \$\endgroup\$ Sep 11, 2018 at 23:21
  • \$\begingroup\$ @Misunderstood We likely have the same or similar standards in Canada, but I can still buy multiple 800 lumen bulbs with noticable differences in brightness. Really far out numbers/results require going online and buying from non-mainstream manufacturers, so there is relative consistency between store bought products, but in my experience, not quite fine enough to be certain that the 8 watt bulb is better than the 9 watt bulbs with the same marked lumens. \$\endgroup\$
    – K H
    Sep 12, 2018 at 0:38
  • \$\begingroup\$ For A19 bulbs in Canada which the government subsidizes, but I don't know how much, the subsidies completely screw up any assumption of correlation between quality and price. I certainly don't mind $1.50-$2 bulbs but it makes discussions of LED shopping difficult. \$\endgroup\$
    – K H
    Sep 12, 2018 at 0:42

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