Photographic films are more efficient at measuring more energetic photons.
A faint blue star, since blue light has a short wavelength and is hotter, with a higher energy than red or infrared light. Emission lines are produced when an electron emits a photon and goes to a lower energy level. The ground state is the lowest energy level, so no photon can be emitted. This IR light is not visible to us, that is, this wavelength of radiation is not in a range that we can detect with our eyes. When we read a book, we are seeing the reflected visible light from the sun or the lights in the room.