Hi folks,
How can I know the output power of a LED as a function of the forward current in it?
How can I know the output power of a LED as a function of the forward current in it?
I couldn't find the characteristic curves on the datasheet. It only gives out the corresponding output power at 10mA forward current. I need to know the output power in some other cases, then I can know the power receivered by a corresponding photodiode. could you give me some hints?The datasheet is the best way. Without it it takes some specialized equipment, or an educated guess.
the transmitter I'm going to use is: SFH756v from Avago Technology;That, and give us the specs you do have, including part numbers and where you're buying them from.
the signal I want to transmit is an AC signal around 1mV. but when this signal comes to the LED, the current through the LED may be around 25mA. But these values are only some testing ones, I have to modify the circuits to get better performance. That's why I need the output power as a function of the current, not only a few values.What range do you want?
Do you understand the difference between electrical power and optical power and why this quantity may be difficult to obtain?the signal I want to transmit is an AC signal around 1mV. but when this signal comes to the LED, the current through the LED may be around 25mA. But these values are only some testing ones, I have to modify the circuits to get better performance. That's why I need the output power as a function of the current, not only a few values.
Yes, that guides me much. Thank you so much. Now the problem is that I could not find luminous intensity as a function of input power on the data sheet. I am so confused. I downloaded the datasheet on internet and I also checked on other datasheets, they were the same. I could not find a datasheet which contains the luminous intensity information. I don't know what's wrong. Could you give me some hints again?The simple answer to your original question is that since you are converting electrical energy to light and some heat there will be some loss in the process. As a starting point I would hypothesize that this process was 80% efficient. Then I would compute the input power as the product of the forward voltage and the forward current. If you take 80% of that number that would be your first approximation. Now you have to scan the datasheet or find a way to measure the luminous intensity for a given power input to confirm or refute the 80% efficiency hypothesis. Rinse and repeat until you are satisfied.
The radiometric unit for light output is the lumen. It is the amount of light produced by a 1 candela point source into a solid angle of 1 steradian. The scaling factor between watts, a unit of power, and lumens is complicated by the dependence of the frequency(wavelength) content of the light. More info can be found at:
http://en.wikipedia.org/wiki/Photometry_(optics)
http://en.wikipedia.org/wiki/Luminous_flux
http://en.wikipedia.org/wiki/Power_(physics)
Is that what you were looking for?
about 10 metersRange as in distance.
PS LEDs & Phototransistors are not necessarily linear...
"between 100 and 200 μWatts at 660 nm",These devices are fiber optic transmitter/receiver pairs. The stated value of output power delivered to the fiber is between 100 and 200 μWatts at 660 nm with a fairly narrow bandwidth. You should be able to go back to the wikipedia article and find the equivalent in lumens. This efficiency is just over 1% if we assume Vf=1.85V @ 10 mA. I guess my earlier estimate was off by a considerable margin. Oh well.
I repeat, they are completely unsuitable for the transmission of analog signals.
On the datasheet from Infineon was 200 μWatt @ 10 mA number and below that was the notation >100. In terms of specmanship I interpret this to mean that the mean of the normal distribution of values occurs at 200 μWatts and that the -3σ point occurs at 100 μWatts. No maximum value is specified. In the absence of better information I'd be tempted to infer that the +3σ point would be at 300 μWatts."between 100 and 200 μWatts at 660 nm",
how do you get these values? On the datasheet, it only says 200uW@10mA.
"they are completely unsuitable for the transmission of analog",
Do you mean, the SFH756v and SFH250v not suitable for analog signals? My superviser told me they would like to detect the variation in DC source, and asked me to do some simulation about small AC signal to imitate that variation. I don't know which devices are suitable for transmit the AC signal. Could you give me some hints?
no, a PHD student.Does supervisor mean teacher?