There have been other threads like this in the past and are probably search-able.
Basically we'll assume that you are trying to signal people who might not be looking for you since the situation is often that an emergency has cropped up right then and there, and there has not been enough time for a formal search to be mounted. Example: you are back country skiing and you've hurt your knee and cannot move and it's getting toward dusk. You see people moving about 3 miles across the valley.
First thing is that the light has to be powerful enough to be seen. Filters and diffusers lower the output so you probably want to avoid them in the early stages.
Second, a steady beam does not attract as much attention as a change in the light as others see it, either a sweeping beam or flashing whether an SOS or just on and off. There has been a lot of debate about the SOS feature on some flashlights as there are a lot of people who don't know anything about Morse code even including a simple SOS. The upshot of that debate was that any change in the light as seen by others got noticed a lot more than a steady beam. Using up a lot of energy by whipping the light back and forth even if you are not certain there is anyone in the area will waste your energy. I would think that letting a light that flashes do the work for you is smarter. Conserving your energy is an important survival skill.
Third, if the light does not work, you are in deep, deep do-do. Spare batteries or a spare light are light weight enough to make sense in almost any situation.
After you have been missing awhile, if you have let people know where you were going and when they should have expected to hear from you (you did perform that simple and basic task, didn't you? If not you should now be hearing Fred Sandford saying, "You big dummy!"), a search should be underway. Helicopters and the like are probably going to be coming later rather than sooner, but even so, a bright, sweeping or flashing light is again going to attract attention sooner and easier than a steady beam. The crews on the ground might not be able to see as large an area as someone in the air, but the folks in the air have a lot more to look at and that makes you a smaller needle in a bigger haystack. The guys on the ground have a smaller area to examine and your light should stand out better. However, a light shining into the air will have a different interpretation than one seen as being shined on the ground, right?
