FusionSight® & PeriSight® are shutterless, so the IR image will never be shut down.
To give the most precise information on temperature, and so to be the most sensitive, an IR sensor must be calibrated so one temperature will be one level of grey. An IR sensor equipped with a shutter means that the IR sensor is disabled from time to time while being calibrated. This is a major inconvenient for the user because he will lose IR image when observing targets.
