What to compare
Sensor resolution
Thermal sensors come in three main tiers: 256x192, 384x288 or 400x300, and 640x480 or 640x512, roughly 49,000, 110,000 to 120,000 and 307,000 to 327,000 pixels. More pixels show more detail at distance, which helps you tell one animal from another.
NETD and refresh rate
NETD is the smallest temperature difference the sensor can show, in millikelvins, and lower is better. Refresh rate is how many times a second the sensor captures a new image, so a higher rate keeps a moving animal smooth.
Rangefinder and battery
The rangefinder's maximum range should comfortably exceed the distances you actually use. Battery life varies more than most specs, from about 4 hours on some scopes to 15 hours or more on others: Alpen rates its Apex LRF scopes at 15 hours.
Frequently Asked Questions
Should I choose a thermal scope or a clip-on?
A thermal scope replaces your day optic. A clip-on mounts in front of the day scope you already have, so you keep it for daylight.
Is thermal hunting legal?
Rules differ by state and by species. Check your state wildlife agency before you hunt with thermal.
Do I need a rangefinder on a thermal scope?
Not always. A scope without one costs less, and if you mostly scan, a handheld thermal monocular may suit you better. A built-in rangefinder earns its place when you want the distance without a second device.
For the specs in depth, read Thermal Imaging Specs Explained, and to choose between the technologies, Thermal vs Night Vision for Hunting.