An IR/Visual fusion image is a hybrid image that overlays a thermal display overtop of a visual light image so that certain readings can be highlighted and easily identified.
In its simplest form, a fusion image places the infrared image directly onto the visual image with little or no processing. Mid-range infrared cameras can blend the infrared image (make it partially transparent) so that the visual light image can still be seen behind and on some models you can also scale the thermal image to a custom size.
Some infrared cameras can also display a false color thermal overlay only where temperatures exceed certain user-defined thresholds - this is sometimes called visible alarms.
Fusion is called IR Fusion by Fluke and Thermal Fusion by FLIR.
An example of a pure thermal image, a ‘picture-in-picture’ fusion image, a partially blended fusion image and a visible alarm fusion image. Images courtesy of Fluke Electronics.