TTL detector, through-the-lens detector

The through-the-lens (TTL) detector refers to a secondary electron detector that is used in a combination with a strongly- excited objective lens, such as an inlens objective lens or a semi-inlens objective lens.
In the strongly-excited objective lens, the secondary electrons emitted from a specimen are caught by the stray magnetic field of the objective lens. These electrons are guided upward along the optical axis of the objective lens. For efficient collection of the secondary electrons, the detector is placed above the objective lens. The detector of such an arrangement is called TTL (through the lens) detector irrespective of type of the detector.
In addition, the TTL detector (placed above the objective lens) can also be used to detect the backscattered electrons emitted from the specimen, being called the TTL type backscattered electron detector. Figure schematically shows an example of the arrangement of TTL type secondary-electron and backscattered-electron detectors in combination with a strongly-excited objective lens.

Fig. Schematic of the arrangement of a secondary electron detector and a backscattered electron detector in a combination with a semi-inlens, strongly-excited objective lens.
The specimen is placed in the magnetic field of the objective lens.