"Flame proving by rectification". This is a great subject, how do you tell for sure that you have a flame on your pilot ?
Flame detection using scanners is quite costly just for the pilot, and there is another issue. Many of the last generation scanners would struggle to tell the difference (or discriminate) between the radiation from the pilot and the radiation from the main flame.
Things get more complicated with a gas ignitor and an oil main flame. Many Last Gen Scanners were either IR sensitive or UV sensitive. As you can see, this is getting complicated and expensive. One simple solution is to use "rectification".
Some smart chap discovered that the ionized portion of a flame actually conducts electricity. From there it was a simple step to prove beyond doubt if you had a flame. You see, if you pass any current from a small surface area to a large one, quite a lot of it gets through. When you pass it from a larger surface area to a smaller one, not as much "hits the target" and the current flow is diminished. Now the spark rod changes its role after the ignition sequence, and becomes a source of a very low current AC.
The BMS or DCS is "listening at the pilot body (earth) for a current. Because the surface area of the spark rod is much less than the surface area of the inside of the pilot tip, the flow of current through a flame arrives at the earth side with a large DC content.
It has been partially "rectified" to a DC current.
This is very clever, because, say the torch overheats and the spark rod droops and touches the earthed tube of the pilot. This will just pass AC to the BMS, and it will not "pass" the flame. If there is a break in the circuit anywhere, it will see no current and will not "pass" the flame.
The only way a DC component can exist is if the circuit is operational, and, we have a flame.
Tip: Because the proving current is very small, the tip and rod may need cleaning to maintain reliable flame proving.