Flashlights

 Operation 

You're going to want a flashlight that has memory mode, or a separate control of modes or simply on/off. A light where you have to cycle through different modes to get to the one you want will be very difficult to manage on set. Imagine an actor needing to turn on a light and it starts in strobe mode.

 PWM Dimming 

Many flashlights dim by using Pulse Width Modulation, or PWM. This dims the light by essentially pulsing it at a frequency that our eyes can't detect. The issue with this is that it can lead to flicker with certain camera shutter speeds. There are flashlights that don't use PWM for dimming, but it's always good to get the lights to the camera department for testing. The lighting department can dim with ND filters if needed. Here's some more information on PWM and how to test for it.

 Colour Temperature and CRI 

Colour temperature is measured in Kelvins. When we think of it in film terms we think of daylight or tungsten. LEDs often go by "warm white" or "cool white". Many LED flashlights are going to be on the colder spectrum, but there are warm white and neutral white options available. Talk with the DOP about what kind of look is wanted from the scene. CRI stands for "Colour Rendering Index" and is a scale of how well a light source shows the colour of something compared to a natural light source. A higher number is better.

 Batteries 

If the flashlight you have does not have an integrated rechargeable battery, there are a few choices of batteries available.

Alkaline batteries are disposable and prone to leaking if left in the light for too long. They are a good choice for lights that you've rented though, in case you accidentally return a light with the batteries. Remember to remove them before storing the flashlights. AA batteries, for example run at 1.5v and will lessen as they are used resulting in a flashlight that will get dimmer.

Lithium (Non-Rechargeable) are a good choice for flashlights as they are less prone to leaking, work well in both high and low temperatures, and maintain their voltage output until near the end (in other words, full brightness until they die, not a gradual dimming). These are much more expensive than alkaline batteries and are also disposable. Many flashlights use CR123 batteries which are lithiums.

NiMH Rechargeables are a great alternative to alkalines. Modern NiMH batteries can last as long as 1000 charges. This is a great value and much better for the environment. A potential drawback of NiMH is that they run at 1.2volts (compared to 1.5v of alkalines). This means that a light may appear slightly dimmer, although often it's not noticeable. A good NiMH will often last just as long as an alkaline. Look for higher numbers like 2,000 mAh which reflects the capacity of the battery. NiMH batteries are also not prone to leaking. Good brands include Eneloop or anything made in Japan.

Rechargeable Lithium batteries come in many shapes and sizes. A common one for larger modern flashlights is the 18650. You can also get lithium rechargeables that can replace CR123 for lights that use those, and sometimes a light will take an 18650 instead of two CR123s, but be careful of the voltage if you're going the other way around (2 CR123s are 6v, 1 18650 is 3.8v) Rechargeable lithium batteries really are the best way to go for flashlights, but it means you will need another battery ready to swap out if one dies. New to the market now are AA and AAA sized lithiums that recharge with a USB cable-- these are more expensive, but they provide 1.5 volts and a lot of convenience. Do not buy cheap rechargeable batteries. Look for name brand ones that are made in Japan. Keep your rechargeable batteries in the device, and any spares in a case and in a fireproof bag. Carefully recycle any batteries that are missing part of their plastic wrapper.

Here's a good site that explains a bit more