You can gently stick the cardboard on a vertical plane, and the camera is mounted on a tripod.

Set a lamp at 45 degrees to the left and right of the cardboard, and the output of the lamp should be exactly the same. If there is a soft mask, the volume of the soft mask should be similar. Anyway, the light source is made exactly the same.

Reflection is because the emergent angle of light is just caught by the lens angle, which can be avoided by adjusting the angle of the lamp, but both sides must be adjusted together.