If your red lensed fashlight has an incandescent bulb, it may not hurt your night vision, but it will light up everything to someone using NVDs. The incandescent bulbs produce a good bit of heat which is IR radiation, which can be seen by the NVD's and IR sensors used on today's battlefields. If you want to have a high-speed red light, or any other color, look into lights that are based on LED(Light Emitting Diode) technology. They produce very little, or no IR signature, and can last a very long time on very little battery power. The rub is that they are pretty pricey, and they are not very bright. they are bright enough to read by if you are close to the object you are trying to read or fiddle with, but if you are looking for something on the ground, the red LEDs are not powerful enough to illuminate the area. There are some LED lights that use various color of LEDs in an array, that when combine produce a white light which can be brite enough to see the ground in front of you if walking, but that kind of defeats the purpose of getting past the NVD's because you then become visible to the naked eye.