How can I prevent microwave radiation absorption by a camera lens/electronics inside the microwave (behind the shield)?
This is a very dangerous idea. Your questions and your diagram show the potential for you or someone to get badly hurt.
The only safe way is to place the camera outside the unmodified microwave oven. Make a hood to screen out ambient light. Buy a microwave with a black coloured screen instead of a white one. But don't go modifying the microwave.
Some more detail if you are not convinced:

Your diagram shows the camera poking through a hole. Don't do this! A quarter wave, or 3 cm of wire, will form an effective antenna and transfer most of the power of the microwave, to the cable! This will not just damage the camera and the phone/computer, it could injure you if you're holding it, or standing very close.

You will notice that there's a gap between the punched metal window and the clear plastic window. This is because there are evanescent waves extending about one hole diameter, out of each hole. The plastic stops you getting too close. Don't put the camera touching the surface of the metal.

Microwave ovens are full of dangerous voltages. What happens when your glue comes loose or a child trips on the USB cable and it falls and touches the high voltage terminals inside? The whole phone or computer could be charged to a dangerous voltage, you could kill yourself (Darwin award) or someone else (manslaughter). Don't go adding wires to the inside of a microwave oven.

Metal doesn't magically shield the camera from microwaves. It needs to be continuously welded or cleaned and bolted down with an RF gasket. You can't make a shielded box for the camera, inside the cavity, without some way of welding it to the wall of the oven.


If you are looking for a way to design an oven with a camera port, for manufacture (after full safety testing) then there are ways of doing it safely. The 12 cm waves cannot pass through a waveguide smaller than cutoff so you could install a port or tube, perhaps 40 mm diameter and 150 mm long down which the camera could safely peer. There are ways of making the hole larger, even bigger than a wavelength, using specially designed chokes. These are used for conveyor belts running through microwave ovens for continuous cooking.