Effective QE | Strolls with my Dog

Effective Quantum Efficiency as I calculate it is an estimate of the probability that a visible photon – from a ‘Daylight’ blackbody radiating source at a temperature of 5300K impinging on the sensor in question after making it through its IR filter, UV filter, AA low pass filter, microlenses, average Color Filter – will produce a photoelectron upon hitting silicon:

(1) $\begin{equation*} EQE = \frac{n_{e^-} \text{ produced by average pixel}}{n_{ph} \text{ incident on average pixel}} \end{equation*}$

with $n_{e^-}$ the signal in photoelectrons and $n_{ph}$ the number of photons incident on the sensor at the given Exposure as shown below. Continue reading The Difference between Peak and Effective Quantum Efficiency →

Now that we know how to determine how many photons impinge on a sensor we can estimate its Effective Quantum Efficiency, that is the efficiency with which it turns such a photon flux ( $n_{ph}$ ) into photoelectrons ( $n_{e^-}$ ), which will then be converted to raw data to be stored in the capture’s raw file:

(1) $\begin{equation*} EQE = \frac{n_{e^-} \text{ produced by average pixel}}{n_{ph} \text{ incident on average pixel}} \end{equation*}$

I call it ‘Effective’, as opposed to ‘Absolute’, because it represents the probability that a photon arriving on the sensing plane from the scene will be converted to a photoelectron by a given pixel in a digital camera sensor. It therefore includes the effect of microlenses, fill factor, CFA and other filters on top of silicon in the pixel. Whether Effective or Absolute, QE is usually expressed as a percentage, as seen below in the specification sheet of the KAF-8300 by On Semiconductor, without IR/UV filters:

For instance if an average of 100 photons per pixel were incident on a uniformly lit spot on the sensor and on average each pixel produced a signal of 20 photoelectrons we would say that the Effective Quantum Efficiency of the sensor is 20%. Clearly the higher the EQE the better for Image Quality parameters such as SNR. Continue reading What is the Effective Quantum Efficiency of my Sensor? →

Musings about Photography