The image intensity drops in part of the image
In your images, is there a clear area with lower intensity with respect to its surroundings?
Have you checked the alignment of your illumination, is it centered and uniform?
Is the spot visible when you look through the eyepiece?
There can be dust on the entrance window of the FLIM camera. To clean the camera entrance window, make sure you use (compressed) air and that the camera and control unit are turned off. Please make sure that no direct sunlight or other high intensity light entering the camera when unscrewing the camera from the microscope.
If you turn the camera when connected, is the area turning with the image or is it staying on the same place? If it turns, the area is part of the image that is being projected to the camera. If it stays at the same position, the camera sensitivity may be reduced. Please contact Lambert Instruments.