What do we mean when we speak of invisible light images?

Well, the correct term is false-color images, but it's the same thing. I don't know how much background you have on light, but essentially light is only a very narrow band of electromagnetic radiation. Some other things that are also electromagnetic radiation include radio waves, microwaves, heat (infrared), x-rays, and gamma rays (it's funny that sometimes they call them rays and sometimes they call them waves, but they're all the same thing). The order that I listed them was lowest intensity to highest. You see, light waves don't have a measurable amplitude like other kinds of waves. Instead, their instensity is related to their frequency, or wavelength. Shorter, higher-frequency waves (like x-rays and gamma rays) are more energetic than longer waves like radio waves.

The light we can see is a narrow band between infrared (heat) and x-ray. (UV light is actually between visible and x-ray). But stars, nebula, and other Things That Glow (tm) don't just emit Visible electromagnetic radiation, they also emit radiation on many other wavelengths, all the way from radio waves up to high-power x-rays (and sometimes gamma rays). Since the specific wavelengths these things emit depends on their chemical make up, examining them can tell us much about what we're looking at. When things glow, the colors they emit usually depend on how hot they are. Something that glows in infrared is much cooler than something that glows in X-rays.

Our eyes pick up only red, green, and blue light, and so any mixture of these can appear as any color (eg, red + blue = purple, red + green = yellow)

To make a false-color image, astronomers will usually take 3 pictures of an object from 3 different telescopes (often infrared, visible, and x-ray). Each one of these pictures is grayscale, meaning black-and-white, but when they use each one as a component in a color image, they make the false-color image. For example, a false-color image may have infrared light mapped to red, visible light mapped to green, and x-rays mapped to blue, but how the colors are mapped out is really arbitrary.

Many of the astounding NASA photos of nebulae and other cool things actually look a lot less interesting if just viewed in visible light.

I hope that helped!

X-ray and infrared images are often given false colours to help with interpretation. For example the hottest region of an infrared image will have the shortest wavelength but it is usually colour coded red (a long visible wavelength) and the coldest part of the image will have the longest wavelength but it is usually colour coded blue (a short visible wavelength).

colors in invisible light images mean different things. in x-ray images, colors can indicate differing amounts of x-rays. color in infrared images indicates the amount of heat, generally blue being cooler and red to white being hotter.

In x-rays red colour means very hot purple not so hot and blue cold while gamma rays colours do not show warmness and coldness but quantities and how much it radiates gamma rays

i really cant say. do u mean ghostly colors