1) The dispersion angle of the LED. A 90 degree LED spreads its energy over a wider area than a 30 degree LED so it will appear to be less bright.
2) The current. The more current run thru an LED the more output power which all things equal with appear brighter. Of course the LEDs life will always be shorter the more the current you run thru it.
The rest of your question makes no sense in English.
Let me try to convert kosy’s non-English into a coherent pair of questions:
1) What parameters affect an LED's brightness?
2) Is the brightness of a group of LEDs the sum of each LED’s brightness, despite the distance between them?
You cannot change the brightness (W/m^2-sr) of a source by adding additional sources. Since they are separated in space, the constant brightness theorem gives you more total light, but spread out more in angle or area.
Take the example of using a beamsplitter to add two sources. Say the beam splitter is 50/50, the net output is then 50%*L+5-50%*L = L if both LED are the same brightness.
You can change the illuminance at the destination, by adding sources, not unlike stage lighting, but the angle of acceptance is quite large then.
So I suppose the key point here is: what is the reason for the question? Is it entirely academic or are we trying to illuminate an area?
In layman’s terms, you get more illumination on a workbench if you hang up more fluorescent tubes. The result is a “brighter bench”. Now whether this constitutes a “brighter light” clearly gets a bit technical.
If these LEDs were so far away that they both fell within the airy disc, I would have expected the resulting source to look brighter, but IRstuff is the expert here.
English translation aside, I doubt the OP was suggesting the brightness of each LED increases. If you consider a closely nit grouping of LEDs to be the "bulb", then yes, the brightness does increase. If the "bulb" was a 1" cluster of LEDs, the off-axis brightness is going to vary by a very small percentage after only a handful of inches away from the "bulb" (unless, of course, we're talking about LEDs with sub-10 degree viewing angle, then there may be an issue).
After a fifth and sixth reading of the OP's last question, I get the impression the LEDs are spread apart some distance. In that case, the off-axis brightness will need to be taken into account. Time for the OP to come back and give us some more info.
