High temperature sources emit most power at high frequencies or short wavelengths. The following diagram shows the distribution with wavelength of the energy emitted by black-body sources at temperatures of 6000 K, approximating to that of the sun, and 290 K, a typical temperature for the earth's surface or atmosphere. The solar irradiance is calculated at the distance of the earth from the sun. Notice that the two curves are almost entirely separate with solar radiation dominating for wavelengths less than about 4 microns and emitted earth radiation dominating for longer wavelengths. This region is used to record images of the earth's surface and of clouds by reflected sunlight.
Images are by emitted thermal radiation.