I went through so many different designs of lights over the years in the military I lost track. Some were excellent, some were ridiculous, and many were somewhere in between. When I look for a light to put on a weapon, there are 3 primary criteria: compact, reliable, functional for the intended use. Obviously the intended use is to see the target in the dark- but what does that mean, exactly? To me, it means that it will provide sufficient illumination at the places and conditions it will be needed. For example, if it is to be used for defense of a 2,500 square foot suburban home and the areas immediately around that structure, I don't need the type of light a SWAT member would need during a clearing operation in a darkened shopping mall, or a SEAL might desire during a raid on the grand mosque in ramadi. I will confess, I have no idea what an illum is, and I don't really care. I have always taken a light that I was considering using and shined it in darkened areas to decide if I like it. For most civilian applications, a good light that will illuminate down a hallway and inside of the average room, closet, crawl space, etc. for the user to be able to see what is in front of him/her and achieve target ID is what is needed. For this application, something like the insight M3 checks all of the blocks. It can also be used on a handgun, and they can be had for less than $100. Another good choice is a simple surefire nitrolon G2X with LED ($60 amazon), mounted in the excellent Viking Tactics light mount ($25 amazon). My FAVORITE light is the surefire scout. They are about the size of an old mini mag light, but use the 123 batteries (like every light should) and have a very bright LED bulb. The downside is they start in the $200+ range and go up to often exceed the value of many weapons that they are mounted to. Of course, the ones I used I didn't have to pay for, and I had no idea they cost this much, but I sure liked using them. I don't mess with lasers on rifles, unless the laser is IR and night vision is in play. For sighting under white light, I either use the irons or red dot mounted to the weapon. If you are firing a rifle using a white light, it should be mounted to your shoulder exactly like it is in the daylight- your irons/red dot are what you use then- why would you change this in the dark? I also use a white light on my AR I use when hunting hogs at night after I shoot one, when I move to the carcass to "dead check" it.