The newer generation and newer analog 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.
Digital night vision devices do not have such a protection, because the light which enters the device comes on a sensor and not into an image intensification tube which amplifies the light input. The sensor in a digital night vision device is not sensible for sudden bright light, even if it's turned on at daytime.
Second and third generation of night vision devices have also built in automatic image brightness. This possibility enables an automatic adjustment when even a smaller amount of sudden bright light occurs and also enhances devices lifetime. It also helps to protect against a sudden appearance of bright light and prevent damage to a night vision device.
Source: Careflight (NV with no BSP and with BSP)