# How many photons per second is one Lumen?

Also the side question is how many Joules is one photon (any between 450-660nm). Thank you

P.S. I am asking because I want to estimate how much thermal energy should be dissipated by LED when part of known energy is emitted as light.

P.S. Got an answer from Robert. Thank you. So those 6500 Lumen 100Watt white LED arrays emit about 9 watt of energy in photons alone and rest 91W goes into heat. Not bad.

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Depends on the wavelength. See goo.gl/8vnpG (Box 3.2, for 555 nm) or wolframalpha.com/input/… (for 540 nm). For your side question, I think you meant wolframalpha.com/input/?i=energy+of+a+photon – Robert Smith Nov 16 '10 at 4:27
Fantastic. Thank you Robert. – user299 Nov 16 '10 at 4:40
Honestly, a question that can be answered by literally copying and pasting it into Wolfram Alpha doesn't strike me as particularly valuable. – David Z Nov 16 '10 at 5:05
A very LMGTFY or wikipedia question, I would simply apply the agreed policy meta.physics.stackexchange.com/q/14/62. Don't see any particular reason for downvote. @Robert I think you should have answered in the answer field, not in the comment. – Bernardo Kyotoku Nov 16 '10 at 10:47
I really think Robert should write a proper answer and RocketSurgeon select it as the correct answer to remove this question from Unanswered list. – Cem Nov 17 '10 at 21:34

Approximately $10^{15}$.

See this:

Photon flux of 540 nm light from the mechanical equivalent of light and the integrated spectral sensitivity of the human eye:

$3.8×10^{15}\ photons/s$ (photons per second)

$6.3×10^{-9}\ mol/s$ (moles of photons per second)

Also see this reference.

Note: this summarises Robert's answer in the question comments and is set to CW.

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