Thanks for your help chemisttree. However, I'm specifically looking for something that could act like invisible ink. I have modified a camera to see only infrared, and I want to find something that I could use to write something that is only visible through the camera.
Does anyone know which, if any, liquids or chemicals absorb infrared light but not visible light? In other words, something that looks clear but appears dark or black when viewed in infrared.
Help is greatly appreciated!