An optical detector circuit uses a photodiode connected to an operational amplifier with a feedback resistor.

The responsivity $R_{\lambda }$ of the photodiode is defined as:

$$R_{\lambda }=\frac{I_p}{P}$$

where $I_p$ is the photocurrent and $P$ is the incident light power.

At a wavelength of $\lambda =850nm$, the responsivity of the photodiode is 0.45 A W^-1. Monochromatic light at the same wavelength with an incident power of $8.0\mu W$ is directed onto the photodiode. The operational amplifier has a feedback resistor of $390k\Omega$.