The newer generation and newer night vision devices have build in special electronics that lower the power or reduce image brightness when sensors detect sudden bright light such as flashlights, car headlights and other devices that causes bright light. Otherwise, without bright light cut-off possibility sudden appearance of bright light could damage the image tube and shorten its life span.
Second and third generation of night vision devices have also build in automatic image brightness. This possibility enables automatic adjustment when even smaller amount of sudden bright light occur and also enhances devices lifetime. It also helps to protect of sudden appearance of bright light and prevent damage of night vision device.
Source: Careflight (NV with no BSP and with BSP)