While I'm no expert I have played with 8-10 night vision and thermal devices. They are very different in how they work, detection range, and image quality.
Abagail, your OP was asking about thermal scopes. Thermal scopes measure the difference in temperature and aren't very defined in the picture. Two people walking towards you. One is your best friend 100' away and the other is a taller stranger 130' away. with thermal you'd have a very hard time telling who is who.
Thermal scopes can be set to show different colors for different temperatures but lets go with white for hot and black for cold..
Let's say you are looking at a chicken with it's feathers ruffled up 30' away and 40 degrees outside. Most of the bird's body would show up as black the same as the barn behind the chicken because the ruffled feathers are insulating most of the bird but the bird's eyes, feet, and possibly the warm beak from the bird's breath are warmer and will show as white because they are warmer.
Thermal scopes are great for roughly identifying ASSUMING your have a ton of practice learning how different animals (or people) look through in a thermal scope in different weather patterns. This is why I generally don't use a thermal scope when hog hunting. Is it a hog 150 yds away or is it my neighbor's dog? Keep in mind that most decent thermal scopes can detect the heat of a human walking across a pasture at 600 yds so a great early warning device that a problem may be approaching. But usually you won't be able recognize who's coming for a visit until they are within 40-60 yds.
Night Vision is a whole different beast that magnifies low levels of light to sharpen up details. Different types of Night Vision (NV) but all require at least a little light be it moon light or it be a tiny spot light mounted on the scope. But with that tiny bit of moon light or spotlight you can see if it is your neighbors dog or a hog in a bit more detail from further away, often up to 150 yds with gen 3 night vision devise.
But there are also huge drawbacks to any night vision device whether it be a Gen unit or a photo unit. Personally I prefer the photo units for their lower cost.
Topic can be discussed ad-nauseum so bottom line is you get what you pay for but don't buy a car when you need a truck.
Let your neighbor do his homework.
