my heating fuel supply

[Pacerr]

Remember, you're working in a "light" spectrum that your eye isn't equipped to view without some sort of spectral translation. The "blotches" are variations in the "luminosity/intensity" of a "color?" you can't see just as an xray would reveal things you don't normally see. Sort of like using a 'black light' to see the fluorescence of common items at a disco club.

Using the in-camera IR filter simply tries to replicate what one type of IR sensor/film would produce.