Hello,
Im working on a project where I want check from which voltage / current the LED´s beginning to emit photons.
Im a professional electronic technician and this is a private project, but Im thinking about making a public study paper, so that others can use the results.
A LED light up from already 2-15mA what the human eyes can see. But already before the eyes can see the light, the LED is emitting photons. And this is my target, I want to measure as exactly as possible.
My problem is, that I can only measure voltages and currents. But I dont have the knowledge about photonic to calculate the (theoretic) emitted photons.
To measure the photon emission, I would use 4 types of sensors, LDR, photodiode, phototransistor and a reverse poled LED.
I were glad if you can tell me how to calculate the emitted photons.
For the LEDs which I will use in the experiments, I will use blue and red LEDs.
How for a soon answer,
thanks
Petr
You can use the traditional way of calculating the number of photons emitted.
The human eye is actually a quite formidable light sensor (some have reported observing down to three photons). And measuring very faint amounts of light is tricky. But basically you multiply the quantum efficiency with the current, to get the number of photons. But you might have trouble finding the quantum efficiency of LED at very low currents.
http://www.nature.com/news/quantum-technology-probes-ultimate-limits-of-vision-1.17731
Hello, thanks for your answers.
Unfortunately Physic is not major, I dont have studied it, Im only a professional electronic technician. Its my curiosity about LED light and I made few experiments with Light depending resistors. And than I thought about it to test when a sensor, depending which type silicon diode or phototransistor or LDR will detect LED light, and so I want to test when the first photons which are emitted react on the surface of the sensor.
May you can tell me about how to calculate the photons?
@Ji Xiaoli
Can you explain me this more detailed?
I know only about wavelenghs, but the other is totally new for me.
Can you help?
thanks
Sorry for the delay. Have you work out the problem? I don't know how to explain it clearly to you. Maybe you can get it from the general way of measuring the photons number of LEDs.
So how to to measure the photons emitted from LEDs?
You can put the LED chip into an integrated sphere and light the LED under electrical current injection, then measure the light-output power (P) and spectral distribution. If it emits single-color light with a narrow-spectral width, the number of emitted photons will be P/hv (hv is the energy of a single photon). If the spectral width cannot be neglected, a more acccurate should be given by integration (since the photon energy is correspongding to its wavelength).
Dear Petr,
welcome,
You may already solved the problem but as you did not yet get a satisfactory answer, i will give an answer which may solve the problem.
The solution is to use a sensor ( PHOTODIODE, PHOTOTRANSISTOR) with known spectral response. Such spectral response which is a curve of the photocurrent Iph per watt of the incident light power in a specific wavelength versus wavelength . To see such curve please follow the link: https://en.wikipedia.org/wiki/Photodiode
So you subject the diode to the LED source it will give you a current according to the incident photopower. Since the LED is more or less a monochromatic source so it can be emitting at a specific waveform lambda. You can get the emission wavelength from the data sheet of the LED.
Then you can calculate the photopower Pph incident on the LED by dividing the current reading Iph of the detector by the its sensitivity S at this wavelength. So, Pph= Iph/S watt, then we can convert pph to number of photons/sec = Pph/ Eph, where Eph is the photon energy = hc/lambda, where h Planck constant, c speed of light in space and lambda is the light wavelength.
Best wishes
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