I didn't make it obvious enough below that I was interested mostly in infrared, as the current use case is measuring power radiated from warm objects and from the ground. As pointed out in comment by @gerrit (thank you):
You may want to clarify that you're interested in the Earth surface, so about 0.3 – 100 µm will cover everything you need.
In my naivety I first asked a related question over at Earth Science here. Ultimately, I would like to know how much electromagnetic energy power I could detect from an object or a surface.
For a home project, is there a way to measure how much EM power is being radiated from a nearby object? It's fine, preferable even, to ignore energy that radiates away from the detector and only measure what is captured by a nearby detector.
At work we have things like spectrum analyzers and power meters and such that measure power at different frequencies in wired communications, and that's similar to what I'm after, but I'm looking for a way to measure EM radiation, mostly infrared power.
If I could wave my hand and magically have exactly what I want pop into my hand, it would be a 1 meter square flat device with 1 side measuring EM power so I could hold it just above the ground and get a result of "1 watt" (1 watt per meter squared, since the device is 1 meter), or hold it next to a tree or a person and measure power coming off of them.
This is both for my own use (both curiosity and possible future personal projects), and also as a science project for my kids since my oldest one is experimenting with measuring power with a multimeter and solar panel.
If there is a type of device that does this, then that's great and the name of the device would be helpful.
If this kind of measurement requires a process (eg: "use device such-and-such to measure power at lots of points and then use method so-and-so to integrate across a square meter") then that is acceptable as long as it can be done by a hobbyist and doesn't require expensive tools worth thousands of dollars.
How do you measure electromagnetic power over a surface area to find the watts per square meter?