The conversion gain of an imager or imaging system is linking the output to the input. In the good old days of the CCDs it was simply the ratio of the voltage variation at the source follower output versus the amount of electrons supplied to the floating diffusion. The output voltage variation could be measured by means of an oscilloscope (good old days !). And the amount of charge supplied to the floating diffusion could be characterized by measuring the reset drain current. A simple, but efficient method, because in most CCDs the reset drain voltage has/had a separate connection and is/was not connected to the supply of the source follower.
With today’s CMOS devices that is different. The in-pixel source follower and in-pixel reset transistor have a common connection, and separate measurement of the reset drain current is no longer possible. But there is still the Photon Transfer Curve (PTC) that can help to characterize the conversion gain of the complete CMOS imaging chain : how many volts or how many digital bits do we get out for every electron generated and transferred to the in-pixel floating diffusion ? Of course I do realize that we have spent a lot of time and blogs on the PTC which can be used to measure the conversion gain. I will not repeat all that great stuff over here. The various PTC options to obtain the conversion gain are the shot noise method (= noise versus effective signal) and the mean-variance method.
Besides the CCD reset-drain method and the PTC method, a third possibility exists to characterize the conversion gain. Although with today’s safety rules, this method is no longer that popular. But using a radio-active Fe55 isotope in front of the sensor can do the job. Fe55 has an energy of 5.9 keV and is generating in silicon about 1620 electrons. In the case the sensor has large pixels and the radiation source is kept “far away” from the sensor, the chance is pretty large that some pixels are hit by a single X-ray photon and most of them are not hit at all by the incoming X-rays. In this way some (large !) pixels will nicely collect all and just all 1620 electrons generated by a single incoming X-ray photon. Simple and efficient !