Handheld and weapon mounted lights from Surefire®, BlackHawk®, Insight®, Streamlight® and others are all quality lighting instruments designed/intended for hard street use and are worthy of your consideration. All make standard incandescent lamp lights as well as LED (Light Emitting Diode) lamps. LEDs have the advantage that they last for thousands of hours and will gradually fade versus an incandescent which can blow with little to no warning. Additionally, there are advantages and disadvantages to both rechargeable and lithium batteries. Rechargeable lights are more economical, but with the low cost of three volt lithium batteries nowadays, they should be considered for duty as well, since they offer high lumen output in a small package. Consider the Navy SEAL saying, "One equals none and two equals one," when equipping yourself. Your primary light could be a rechargeable and your backup (and you should carry one) can be a lithium battery light carried unobtrusively on your duty belt. Lights should be carried by all officers on all calls at all times, regardless of the shift (darkened basements and attics abound and most dope houses I've been in usually have one light bulb in the whole house...in the kitchen).
Whatever light you carry, remember that you do not want to leave the light on, but rather you should "paint the area" with light, as Ken Good says. Use two to three seconds of light, then turn the light off and move. Once a suspect is located, you can leave the light on as you challenge (hopefully from behind cover).
Lasers such as those by Lasermax and Crimson Trace® can increase hit potential in low or subdued lighting as well. Both make quality products that can be carried affixed to your duty pistol while on duty. No longer a novelty, these laser devices have proven themselves in actual street combat.
The vital link in success in adverse lighting is training. Sadly, this is the area where most LE agencies fail. While most agencies in my state conduct little additional firearms work each year outside of the state qualification, the state course of fire does include a low light event. Unfortunately, agencies are allowed to conduct this event in daylight while wearing welding goggles. Whatever value this event held is gone with this allowance. That said, one event does not a prepared officer make. We need to train our officers to be effective in these low light conditions. If an agency does not have access to an indoor range or cannot use their outdoor range at night, why not conduct training using Airsoft weapons? I've conducted excellent force on force training programs using Airsoft, as well as with paintball guns. These low cost training modalities can be used just about anywhere. Why not set up a judgmental course of fire with cardboard good guy/bad guy targets at night, in a parking lot or in the city equipment garage after hours? These training opportunities are vital for officers to learn to properly use their lights to locate and identify targets, as well as practicing their shooting techniques. Force on force is the ultimate in low light training, showing the importance of proper use of light and cover.
Own the Night
Nothing comes easy in skill development. You must advance your knowledge base of low light encounter training, as well as practice your tactics and techniques. Muhammad Ali said, "The fight is won or lost far away from witnesses--behind the lines, in the gym, and out there on the road, long before I dance under those lights." Before the fight, prepare, equip and train and you'll own the night!