To get the Infrarang, you need to collect around 45 Golden Cogs throughout the stages (each stage has 10), after that, go to Julius' lab and interact with his machine to get the Infrarang.
Note: The Infrarang is extremely useful for getting the Picture Frames, whenever you're close to one, it will start beeping and you can see in your aiming view where they're located, make sure to keep the Infrarang in hand at all times.