Model photodiode response using quantum efficiency and power. Include gain, area, and dark current effects. Get clear current outputs for lab and design work.
| Case | Method | Optical input | Wavelength | QE | Gain | Dark current | Expected photocurrent |
|---|---|---|---|---|---|---|---|
| A | Direct power | 2.0 mW | 850 nm | 70% | 1 | 5 nA | ≈ 0.77 mA |
| B | Irradiance × area | 1.2 mW/cm², 9 mm² | 940 nm | 55% | 10 | 20 nA | ≈ 0.29 mA |
| C | Direct responsivity | 250 µW | — | — | 5 | 2 nA | R=0.45 A/W → ≈ 0.56 mA |
Examples are illustrative and depend on spectral response and bias conditions.
Photocurrent is the light-generated current from a photosensor. For a linear photodiode region, it scales with optical power and responsivity. If your circuit includes transimpedance gain or internal multiplication, the measured current can be higher than the raw diode current.
Many datasheets quote responsivity in A/W, which expects total optical power at the active area. When only irradiance is known, power is computed by P = E × A. The calculator supports W/m² and mW/cm², plus area units m², cm², and mm².
When quantum efficiency is available, responsivity is estimated using R = η q λ /(h c). As a reference, η=70% at 850 nm yields about 0.48 A/W. At 940 nm, η=55% gives roughly 0.40 A/W. These values are typical for silicon near its sensitivity peak.
Real sources are not monochromatic. LEDs can have 20–50 nm spectral width, while lasers are much narrower. If the spectrum shifts, responsivity can change significantly near cutoffs. For silicon, responsivity drops approaching ~1100 nm. Use the wavelength that best matches your dominant optical energy.
Dark current adds an offset, modeled here as Itotal = Iphoto + Idark. Dark current can be a few nA for small diodes at room temperature, but it can rise by orders of magnitude with temperature and reverse bias. Enter your measured dark current for best results.
The gain field can represent amplifier scaling or multiplication devices. For example, a gain of 10 applied to a 30 µA photocurrent produces a 300 µA output. When using a transimpedance stage, convert voltage output to current-equivalent gain if you want a consistent current budget.
Confirm linearity by testing two power levels. If power doubles, the photocurrent should double within measurement error. Watch for saturation from limited bias, amplifier headroom, or high irradiance. Also verify units: 1 mW/cm² equals 10 W/m², a common source of mistakes.
Photocurrents can span pA to mA. Low-light sensing and spectroscopy often operate in pA–nA with careful shielding. Proximity sensing and optical communication receivers commonly sit in µA–mA depending on optics and responsivity. Use the export tools to document inputs and results for lab reports.
If your datasheet provides responsivity at the operating wavelength, enter it directly. Use quantum efficiency when responsivity is missing or you want a wavelength-based estimate from η and λ.
Responsivity depends on how efficiently photons create carriers. The photon energy and sensor material response vary with wavelength, so the same optical power can produce different currents at different wavelengths.
Enter the active photosensitive area that is illuminated. If the beam is smaller than the diode, use the beam spot area. If the beam is larger, use the diode’s active area.
Apply the transmission factor to power. For example, 80% transmission means P = 0.8 × Psource. You can also reduce irradiance by the same factor before calculating.
Possible reasons include internal gain, amplifier scaling, stray light, or responsivity higher than assumed. Check whether you are measuring output current after gain and confirm your power measurement plane matches the detector plane.
Dark current is current that flows without light due to leakage and thermal generation. It contributes offset and noise. Measure it under the same bias and temperature used for your light measurement.
Yes, if you use average optical power for average photocurrent. For peak photocurrent, use peak power and ensure the detector and electronics bandwidth support the pulse width without distortion.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.