Responsivity and quantum efficiency converter (photodiodes and image sensors)
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KEY POINTS
Responsivity is R = QE × λ[nm] / 1240 (A/W), so at equal quantum efficiency it grows in proportion to wavelength. At 100% quantum efficiency and 550 nm, responsivity is about 0.44 A/W. Image sensor quality is set by read noise and dark current in low light and by photon shot noise in bright light.
Responsivity and photocurrent
Image sensor pixel signal and noise (during exposure)
Signal electrons exceed the full well and the pixel saturates. Shorten the exposure or reduce the light level.
The incident optical power above is used as the power falling on one pixel.
Ideal responsivity (QE = 100%)
Responsivity at common wavelengths
| Wavelength | Photon energy | QE = 100% | QE = 70% |
|---|---|---|---|
| 405 nm | 3.061 eV | 0.327 A/W | 0.229 A/W |
| 450 nm | 2.755 eV | 0.363 A/W | 0.254 A/W |
| 532 nm | 2.331 eV | 0.429 A/W | 0.300 A/W |
| 633 nm | 1.959 eV | 0.511 A/W | 0.357 A/W |
| 850 nm | 1.459 eV | 0.686 A/W | 0.480 A/W |
| 940 nm | 1.319 eV | 0.758 A/W | 0.531 A/W |
| 1064 nm | 1.165 eV | 0.858 A/W | 0.601 A/W |
Quantum efficiency and responsivity
Quantum efficiency is the number of electrons collected as current per incident photon; responsivity is the current in amperes produced by one watt of light. Longer-wavelength photons carry less energy each, so the same power contains more photons, and at equal quantum efficiency responsivity grows in proportion to wavelength.
Silicon photodiodes barely absorb wavelengths beyond the band gap (1.12 eV, about 1100 nm), and absorption is weak in the near infrared, so quantum efficiency falls there. Short wavelengths are absorbed near the surface and easily lost to recombination, which backside illumination (BSI) and surface passivation improve. Low-light image quality is set by read noise and dark current; bright-light quality is set by photon shot noise.
FAQ
How do I convert QE to A/W?
Multiply QE (as a fraction) by the wavelength in nm and divide by about 1240. For example, 70% at 550 nm gives about 0.31 A/W.
What limits image quality in low light?
Read noise and dark current. In bright light, photon shot noise, the square root of the signal, dominates.
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Last updated: October 23, 2026