I have n bulbs of luminosity k in fixture a. How many bulbs of luminosity l do I need in fixture b for the brightness of a to match the brightness of b?

Specifically, I'm wondering how many standard LED components I would need to approximately replicate the light of a 60W incandescent bulb of 80lm. The LED fixture would be designed to mimic the shape of an incandescent bulb to ensure comparable relative distribution of light.

I'm confused because luminosity is calculated in relation to surface area, but I'm not sure how that applies here. Additionally, I'm referring to standard through-hole LEDs of 2.1lm--most search results only show information about consumer fixture bulbs.


1 Answer 1


Lumens do add up. So if you have one n lumen flashlight and another m lumen flashlight the total luminous flux will be n+m.

However interestingly our eyes (and ears) don't operate on a linear scale. 2x the lumens will not be perceived as 2 times the brightness because our eyes and ears work on a logarithmic scale.

In your case however lumens might not be the best measure because lumens is the unit which indicates the total amount of useful light emitted by the light source not the light reaching a spot.

The best (most accurate way) to estimate how many LED's you require would be to take a unit area and to measure the illumination using a lux meter. Do it with your test incandescent and then with an LED in different shapes. There is no definitive answer because an LED array in a different shape will yield a different answer.

By simply mimicking the shape of the incandescent bulb i doubt you will achieve your desired results because the distribution of light will be fundamentally different.

  • \$\begingroup\$ I'm trying to make a programmable LED lightbulb approximately as powerful as a standard consumer incandescent bulb. If I can't add lumens because of unequal distributions, how many LEDs would I need? \$\endgroup\$
    – twinlakes
    May 31, 2015 at 20:37

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