In evaluating flashlights I'll be using a ten category, twenty point system. Each category will be evaluated and scored between zero and two; zero being absolute failure, one being adequate / average, and two being exceptional. As always, I'll try to explain, as carefully as possible, my rationale for assessing a particular score to a category.
The categories are as follows:
Pretty things, pretty things, pretty things. If the flashlight looks good, sports clean lines and is unadorned with useless crap it will score well here.
Most flashlights I own are constructed of machined aluminum. Aluminum is great. However, there are other materials, possibly better or possibly worse, from which flashlights are constructed. With flashlights, the material from which the lens is constructed is exceptionally important. Durability and weight are the primary considerations here.
Fit of Parts
How well do the parts fit together? Are there wiggles, rattles, or other undesirables?
Just how bright is the flashlight (measured in lumen). Higher outputs aren't necessarily better. Rather, achieving the correct balance of output to other variables (e.g., intended use, flashlight size, battery type, etc.).
Also, having a single well-balanced output isn't determinative either. Having various output levels available, and having those output levels harmonize with one another (i.e., having two output levels, one of 200 lumen and another of 250 would be far from ideal), makes a light far more well-rounded.
How long the flashlight can maintain useful light output. Most LED flashlights have a digitally regulated discharge meaning that the flashlight circuitry maintains the selected brightness as long as it possibly can. This means that the flashlight's brightness won't slowly diminish with use. However, it also means that when the batteries die the flashlight is dead; there isn't any possibility of squeezing a few seconds of useful light out of a practically dead battery as you could try with an old Maglite. Some flashlights have a "warning" system which is indicates when the power is almost depleted.
When it comes to runtime, obviously the longer the better (at a given output). The ability to use juice efficiently is incredibly necessary in a good flashlight.
Beam Pattern and Quality
Beam patterns are typically described in two ways: flood and throw. Flood is dispersing light evenly across a wide area while throw is the ability to focus useful light at a distance. Finding a good metric for flood is rather difficult. Throw, however, is typically measured in meters.
Often, lights have to trade off between flood or throw. Some types of lights are designed to achieve a certain result while others seek to maintain a useful balance between the two. Alternatively, a light can be designed to adjust between various patterns (using aspheric lenses and user-adjustable focus).
The best way for me to evaluate beam pattern is to look at the purpose of the light. If it's designed for everyday carry and general use then a decent balance is wonderful.
Beam quality measures the uniformity of the beam pattern. Any beam has two components: a hotspot (the bright center of the beam) and the spill (the dimmer corona). High quality beams will typically diffuse softly from hotspot to spill. Any inconsistencies in the beam will be noted here.
User interface can make or break a light. If it's difficult to use it's typically not worth using. Most LED lights have multiple discharge levels and they each feature some method of switching between the various options. A great user interface is easy to use and facilitates arriving at the desired output level quickly and efficiently.
How comfortable is the light to hold and use in the hand? I use my flashlights in one of two ways, typically: a hammer grip or a cigar grip. The ability to comfortably use these two grip methods and successfully operate the light is imperative.
Here, I'll also discuss a bit of hands-free use. Sometimes it's necessary to use a flashlight and both your hands. These sorts of tasks require a flashlight which is designed to stay put in a certain position. Often, flashlights have features which prevent rolling and facilitate tail-standing. While lights don't necessarily have to sport these features to be useful, having the option available is always great.
This category will be predominantly scored on grip with hands-free use a secondary discussion not bearing too heavily on the category score.
How is the light designed to be carried? Many lights feature a pocket clip. Some lights are small enough to hang around the neck or sit comfortably in the bottom of a pocket. Depending on how the light retails (e.g., with or without a pocket clip, lanyard, etc.) I'll evaluate and discuss how the carry method suits the light and whether it's functional and effective.
In the event that there's a light that's clearly not intended for carry on one's person I'll evaluate it based on how I would typically carry the light (e.g., in a backpack).
I'm principally looking at carrying a light everyday in a pocket and how comfortable the light is in that role. I'll be sure to explore and explain any other carry methods that might prove useful. If the light can't be comfortably and conveniently carried on my person in a way that I would feel comfortable stepping out the door the flashlight will receive a zero.