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?
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.
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.
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.
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.
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:
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.