Where do you being when zeroing in on one? First, consider the main use of the headlamp. Camping, hiking, industrial setting, sub-freezing environment, caving, tropical setting, searching, plumbing, etc. Once you decide on your main use, selection is easier.
Headlamp Lumen Output
"Lumens" are the measure of how much light your headlamp will produce. Headlamps are available from 0.02 lumens (moonlight mode) up to 1000+ lumens. When selecting the lumen output, you will want to consider the intensity and cd rating. Also, LED color is something to consider. A neutral LED will make the surroundings appear as your eyes see. Other LED colors are cool white and warm white.
When it comes to lumens, no need for "overkill." A headlamp used in your house or when performing jobs close up doesn't require very high lumens. A general rule in lighting is operate your light to meet the use. High lumens tend to drain your battery faster. So buy a headlamp with lumens to match your use.
Headlamp Maximum Beam Distance
What is the maximum distance you need to illuminate? Since you will be operating the light a different modes, it is important to know maximum beam distance at each output mode.
Headlamp Run Time
Don't consider just lumens. Think about how long you need the headlamp to operate when "in the field." Are you going to be hiking for several days and not have access to power to recharge your rechargeable headlamp? Do you use throw-away batteries? How may do you want to carry for backup? Are you going to be in the workplace on an 8-hour shift? You want a headlamp that will have the longest run time on one set of batteries or one charge. Operating the headlamp on the lowest lumen setting for your environment conserves battery power. If you are going to need a high lumen setting, then you want to check how long the headlamp will run at that high output. Different LEDs and headlamp electronics produce different run times. Be sure to see what battery source was used in the stated run times. Different battery types give you different run times in many cases.
Going on a 2-day hiking trip? Check the headlamp run time chart and plan what it would take to make sure you have lighting for the full trip.
Headlamp Output Mode Settings
Headlamps vary from two output settings to 5 or more. Do you need an SOS mode for emergencies? Do you need a moonlight mode for night reading or for extended run times?
A popular setting in today's headlamp is the "burst" mode. The burst mode allows for a temporary high-lumen beam. Burst is accessed by continually pressing a button. Once you quit pressing the button, the light drops down to the next lowest setting. Always check to see if the highest lumen stated for the headlamp is a burst mode or true mode. True output modes can be access without having to hold a button down.
It is important to look at the run time at each mode. That will give you an idea how often you will have to replace the batteries or recharge the batteries.
Headlamp Beam Pattern
Do you need a beam that has a flood (wide beam) or more of a spotlight (more forward-focused beam)? Do you want a headlamp with a nice combination of flood and beam?
Hiking, searching, and hunting normally require a spotlight beam. Camping, caving, and industrial uses generally use a flood beam.
Some headlamps have both flood and spotlight beams. Always check the product images to see samples of the beam pattern the headlamp produces.
Weight is important when you consider the headlamp will sit on your head for and extended time. Things to consider: the more batteries, weight is added. Are the batteries in the head of the headlamp? Is there a separate battery compartment? Is the battery compartment attached to the headband or does it have a wire so it can be carried in your pocket? Does the headband have the strap across the top of your head to offer support?
Battery type affects weight. If you want high lumens, it will take multiple throw-away alkaline batteries. That increases weight. High-lumen headlamps normally take the more powerful rechargeable li-ion batteries. One 18650 battery can produce many more lumens than one alkaline battery. So if you need a lightweight headlamp with high lumens, you will need to look at the 18650 battery headlamps or the CR123A battery headlamps.