Submarine Periscopes & Photonics Masts - Eyes Above the Waves

The first submarine periscopes were simple tubes with angled mirrors at each end, providing a crude view of the surface. Simon Lake is credited with installing one of the first practical periscopes on a submarine in 1902. Early periscopes had poor image quality, limited magnification, narrow fields of view, and fogged easily. They were typically fixed and could not rotate - the entire submarine had to turn to scan different bearings. Despite these limitations, the periscope gave submarine commanders their first ability to observe the surface while remaining submerged.

Periscope technology matured rapidly during the World Wars. Prisms replaced simple mirrors, dramatically improving image quality. Periscopes became rotatable through 360 degrees and retractable into the hull. Magnification systems with multiple power settings (typically 1.5x and 6x) were added. Anti-fog coatings, trainable elevation, and stadimeters (range-finding scales superimposed on the image) made the periscope a true combat instrument. By WWII, attack periscopes incorporated split-image rangefinders and could estimate target speed, range, and bearing simultaneously - essential for calculating torpedo firing solutions.

Cold War periscopes incorporated advanced optics, night vision capability (image intensification), and increasingly sophisticated electronics. The US Navy standardized on the Type 2 (search) and Type 18 (attack) periscope designations. Periscopes grew larger to accommodate additional sensors - some included radar warning receivers, communication antennas, and even small radar sets built into the head. Electronic image intensifiers allowed operations in near-total darkness. However, the fundamental limitation remained: the periscope was a hull-penetrating tube requiring the control room to be positioned directly below it.

The transition from optical periscopes to photonics masts began in the 1990s. The Royal Navy's Astute-class and the US Navy's Virginia-class were among the first designed from the start with non-hull-penetrating photonics masts. The Kollmorgen AN/BVS-1 on the Virginia-class replaced the traditional periscope eyepiece with high-definition cameras feeding digital displays in the control room. This revolutionary change freed submarine designers from placing the control room directly beneath the sail and eliminated a major structural weakness in the pressure hull.

Current-generation photonics masts are sophisticated multi-sensor platforms. They combine visible-light cameras (often 4K resolution), thermal infrared imagers, laser rangefinders, ESM suites, GPS receivers, and communications antennas in a single mast. Advanced image processing software provides automatic target detection, tracking, and classification. Panoramic imaging captures a complete 360-degree view in seconds. Some masts incorporate laser designators for precision-guided munitions. The digital nature of the system enables augmented reality overlays showing target data, navigation information, and tactical displays superimposed on the live camera feed.

What is the difference between a periscope and a photonics mast?

A traditional periscope is an optical tube that physically penetrates the submarine's pressure hull, using mirrors and prisms to relay an image from above the waterline to the operator below. The operator looks through an eyepiece and physically rotates the tube. A photonics mast replaces the optical tube with a telescoping mast containing cameras, infrared sensors, laser rangefinders, and other electronic sensors. It does not penetrate the hull - instead, it is mounted externally and feeds digital imagery via cables through a hull penetration. This means the control room can be located anywhere inside the submarine, not necessarily directly below the mast. The digital imagery can be displayed on multiple screens, recorded, enhanced, and shared with the entire crew.

Why did navies replace periscopes with photonics masts?

Photonics masts offer numerous advantages over traditional periscopes. First, they eliminate the large hull penetration required by a periscope tube, which is a structural weakness and limits how deep the control room can be placed. Second, digital sensors provide far more capability than the human eye alone - infrared cameras work at night, laser rangefinders give precise distances, electronic support measures detect radar emissions. Third, multiple crew members can view the imagery simultaneously on screens, and it can be recorded for analysis. Fourth, image processing software can automatically detect and track contacts. Fifth, the non-hull-penetrating design gives submarine designers more freedom in internal layout. The US Navy's Virginia-class was the first US submarine designed from the start with photonics masts instead of periscopes.

How far can a submarine see through a periscope?

The visual range through a periscope or photonics mast depends on the height of the mast above water (which determines the optical horizon), atmospheric conditions (visibility), and the magnification used. With the mast raised about 1-2 meters above the surface, the optical horizon is approximately 3-5 nautical miles. With high-power magnification, a submarine can identify ships at that range in good visibility. However, in practice, periscope exposure is kept to an absolute minimum - typically just seconds at a time - to avoid detection by radar or visual lookouts. Modern photonics masts with their electronic sensors and fast 360-degree rotation can capture a complete panoramic image in seconds, minimizing exposure time.

Can enemy radar detect a submarine periscope?

Yes, modern radar can potentially detect a periscope or photonics mast, though it is very challenging. The mast presents an extremely small radar cross-section - typically less than 0.1 square meters - and is only exposed for brief periods. However, modern search radars with pulse compression and sophisticated signal processing can detect such small targets at ranges of several nautical miles, especially in calm seas where there is less wave clutter. This is why submarine commanders minimize "scope time" and why modern photonics masts are designed with radar-absorbing materials and shapes that reduce their radar cross-section. Electronic Support Measures (ESM) sensors on the mast can detect incoming radar, warning the submarine to lower the mast before it is detected.

How many periscopes or masts does a submarine have?

Most military submarines carry two periscopes or masts. Traditionally, these were designated the Type 18 (search periscope - slimmer, harder to detect, used for initial observations) and Type 8 (attack periscope - larger, with more optical power and a stadimeter for range measurement, used during torpedo attacks). Modern submarines like the Virginia-class carry two photonics masts: the AN/BVS-1 (primary imaging mast with visible and infrared cameras, laser rangefinder, and ESM) and a secondary mast. Having two masts provides redundancy and allows different sensor configurations. Some submarines also have separate communications masts and ESM masts that can be raised independently.

What sensors are on a modern photonics mast?

A modern photonics mast like the AN/BVS-1 on the Virginia-class contains a comprehensive sensor suite: a high-definition color CCD camera for daytime visual imaging, a thermal infrared camera (FLIR - Forward Looking Infrared) for night operations and detecting heat signatures, a laser rangefinder for precise distance measurement to targets, an Electronic Support Measures (ESM) antenna for detecting and classifying radar emissions from other ships and aircraft, and a GPS antenna for position fixing. Some masts also include a laser designator for guiding precision munitions, a communications antenna, and an Identification Friend or Foe (IFF) interrogator. All sensor data is digitized and can be displayed, recorded, enhanced, and distributed throughout the submarine.