# Do the optical power and energy of LED vary with distance?

I have an application with LED and It requires a bigger beam . I could obtain a bigger beam by incresing the distance between LED and surface. But îs there the possibility to decrease the energy delivered ??? Photo attached.

• The energy is absorbed by the medium (air I guess?) – Eugene Sh. Apr 2 at 15:30
• Yes, but it îs about 30 mm. Could the energy loss be more than ..5% ?? – Costas Apr 2 at 15:32
• I think you will need to quantify the nature of the particulates in the air if you are worried about energy being absorbed that way. Also, be sure you aren't saying "energy" when you really mean luminance, luminous intensity, or something else. – Elliot Alderson Apr 2 at 15:37
• Yes, it îs about only energy [Joule] and optical power [W]. – Costas Apr 2 at 15:43
• Is the energy you are talking about energy per unit of surface area (intensity)? Or just energy summed over a much larger surface area? – jonk Apr 2 at 21:17

Ignoring the effects of contamination in the medium between the emitter and the receptor, the energy received when the receptor is up-close to the emitter will be the same as the energy received when the receptor is at a greater distance. This of course assumes that the receptor has a big enough surface area to receive the "spread-out" light.

What goes down is the received energy per square metre (as the distance is increased) but, nevertheless, the same total energy will hit the receptor because as the energy per metre thins-out, it illuminates a bigger surface area hence, energy is the same.

Assuming that you're shining the LED beam through some non-dispersive, non-absorbtive medium (clean air or vacuum, etc.) then the total energy impinging on the surface won't change.

This is basic physics -- the light energy coming out of the LED continues being light, unless it smacks into something that absorbs it, and it goes in a straight line, unless it smacks into something that disperses it.

• Or if it goes too close to a black hole. But then one could argue that the definition of "straight" changes near black holes. Relativity is fun. – Hearth Apr 2 at 16:00
• If the curvature of space-time is significant enough to have an effect over 30mm, you may have bigger problems to worry about than all your light hitting the target. I could edit my question "Assuming that you're on Earth and ..." – TimWescott Apr 2 at 21:09
• Sometimes I just feel like being pedantic when it has slightly amusing results. I should probably try to do that less. – Hearth Apr 2 at 21:15
• I was enjoying the thought of that poor LED getting crushed in a black hole, though. – TimWescott Apr 2 at 21:16
• May as well include boson flocking (bunching) effects since bosons can all have the same state (unlike fermions.) Plus the impact observed in Measurement of quantum back action in the audio band at room temperature. And while we are at it, Experimental rejection of observer-independence in the quantum world. – jonk Apr 2 at 21:29