"The designers of non-penetrating optronic masts for submarines have made substantial strides since their initial ground-breaking experiments in the 1970s and 80s. Sensor quality has also advanced considerably since that time, and the potential advantages of optronic masts, in terms of improved discretion, reduced ship impact and greater hull integrity/safety compared with penetrating designs, are now well recognized. In consequence, among the major navies there are now submarines in regular service whose crews are wholly dependent on non-penetrating solutions, no longer having a conventional direct-view penetrating periscope capability to fall back on."