# Energy delivered by a LED in Joule

A 500mW laser with a beam area of 0.25Cm2 used for 20 seconds delivers 40 J/cm² A 200mW laser with a beam area of 0.1Cm2 used for 20 seconds delivers 40 J/cm² A 30mW laser with a beam area of 0.015Cm2 used for 20 seconds also delivers 40 J/cm² Each of these probes apparently apply the same "dosage". However, the total energy delivered is clearly different [10 Joules, 4 Joules and 0.6 Joules respectively].

--------So what it is the formula for energy delivered by laser or LEd ?? many thanks

• Energy = Power * Time, if you capture all the energy in the beam. – The Photon Mar 25 at 1:40
• Just take note of the fact that the problem presents different affected areas. Since the energy per unit area is the same, the only thing affecting the total energy delivered is the differences in area. Or think of this another way. Suppose I had two laser pulse weapons on a space-based orbital platform. One can deliver $10^{20}\:\text{J}$ over the area of exactly one building in a city and the other one can deliver $10^{20}\:\text{J}$ over the area of one building in a city but do it for all of the buildings in the entire city at once. Which delivers more total energy? – jonk Mar 25 at 2:32
• Some of the confusion for the calculation is directly related to the article's number two issue: "Dosage expressed as J/cm² is inadequate." – Nedd Mar 25 at 7:50

With the input you're given, you can calculate

$$\rm Energy = Power \times Time$$

What you're calling dosage is apparently

$$\rm Dosage = \frac{Energy}{Area}$$

which is why you can get equal dosages for different energy absorbed, if you just scale the area accordingly.

• In other words a larger dosage is delivered (in equal time) if the beam size is larger. Even when the specified energy per unit area is listed as being equal. – Nedd Mar 25 at 7:47
• @Nedd, not according to these definitions. A larger beam with the same energy per unit area would provide more total energy, but the same dosage as defined in OP's article. – The Photon Mar 25 at 16:02
• Your answer help me a lot.. now. – Costas Mar 26 at 9:56
• Sorry about voting.I do not know exactly to use.Please give me a LINK how to use vote section of answer.Thanks to all for answers – Costas Mar 26 at 11:05