How Photonics Masts Will Work

By: Kevin Bonsor
The photonic mast, seen here in testing, is one of the revolutionary design changes incorporated into a new type of U.S. submarine. See our collection of submarine pictures.
Photo courtesy Naval Sea Systems Command

Crews onboard a submarine can spend months at sea, submerged, with no way to catch even a single glimpse of sunlight -- the only window to the outside world is the eyepiece of the periscope in the control room. The periscope is a fundamental piece of submarine equipment, and provides valuable visual data during battle and in determining the ship's position.

Despite its valued service for more than 80 years, the U.S. Navy will soon say "so long" to the conventional periscope. In 1999, construction began on a new breed of attack submarines that won't have a periscope. Instead, these new Virginia-class submarines will use non-penetrating imaging devices called photonics masts to perform surveillance tasks. Each new submarine will be equipped with two photonics masts, which are basically arrays of high-resolution cameras that capture and send visual images to flat-panel displays in the control room.


In this edition of How Stuff Will Work, you will learn the basics of a conventional periscope and how photonics masts will revolutionize the way submarines see the world around them.


Dancing with the Gray Lady

A petty officer conducts a periscope sweep prior to surfacing. Photonics masts will replace conventional periscopes.
Photo courtesy U.S. Navy

The orders "down periscope" and "up periscope" are, for most of us, completely tied in with our notion of submarines. These are phrases that we've heard countless times in submarine movies, in which there's usually a dramatic scene of a submarine captain hanging onto the handles of a periscope, looking out at the enemy above the water. Standing watch at the periscope like this is called "dancing with the gray lady."

A periscope's basic purpose is to allow submarine crews to see objects above the water while the ship remains submerged. A simple periscope can be constructed out of a vertical tube with mirrors placed at a 45-degree angle at the top and bottom of the tube. These devices basically collect light from an image and direct that light from one mirror at the top of the periscope to the mirror at the bottom of the periscope. Of course, modern submarine periscopes are more sophisticated than that.


Periscopes on submarines may be as long as 60 feet (18 m). When a submarine is submerged to a depth that is equal to the length of the periscope tube, it is considered to be at periscope depth. Because of the length of these periscopes, having two mirrors attached inside the ends of the periscope is not a sufficient method for carrying an image from the top window to the eyepiece.

In a submarine periscope, prisms are used in place of mirrors at the top and bottom of the periscope tube, aligned parallel to each other. The top prism collects light from an image and bounces that light, through as series of lenses and two telescopes that run the length of the periscope tube, down to a second prism. This prism at the base of the tube reflects the light into a secondary tube, which consists of two lenses, and then through the eyepiece.

Periscopes are contained inside a periscope well within the ship's sail. On submarines, "sail" is a term often used to describe the conning tower. This tower is the cylindrical chamber attached to the top of the submarine. It is traditionally located directly above the control room. The well runs from the top of the sail to the bottom of the ship. A periscope can rotate to give the operator a 360-degree view of the ocean surface. If there is an object (or enemy ship) that needs to be avoided, the submarine will dive. Once below periscope depth, sonar typically is used for navigation.

The photonics mast will allow the control room to be located on the roomier second deck.

Newer submarines are equipped with two types of periscopes, one on the right (starboard) side and one on the left (port) side. For example, the USS Springfield has a Type 2 attack scope on the starboard side and a Type 18 search scope on the port side. The Type 18 scope is limited to operations in the daylight. It takes photographs with a 70-mm digital camera and then displays those images on a television monitor. Some periscopes also have night vision, a still camera, a video camera and can magnify images being viewed.

Virginia's New Look

A computer-generated image of what the new Virginia-class attack submarines will look like.
Photo courtesy U.S. Navy

There are two problems with conventional optical periscopes. First, a periscope well runs the entire height of the ship to house the periscope, and its size restricts the arrangement of the sail and interior compartments. The second problem is that periscopes can accommodate only one person at a time. The Navy has developed a new AN/BVS-1 photonics mast to solve these two problems. The Virginia-class attack submarine, scheduled for debut in 2004, will be the first submarine equipped with photonics masts.

The photonics mast provides the imaging, navigation, electronic warfare and communications functions of a conventional optical periscope, according to U.S. Navy documents. Each Virginia-class submarine will have two photonics masts, which do not penetrate (retract into) the ship's hull. The mast will rise like a car antenna, in a telescopic motion.


Electronic imaging equipment will replace the prisms and lenses of the old optical periscopes. The heart of the system is the sensor unit that will protrude through the water. This multiple electro-optical sensor is located in a rotating head. The masts are equipped with three cameras, including a color camera, a high-resolution black-and-white camera and an infrared camera, to provide imaging for the submarine. There is also a mission critical control camera in a separate, pressure-proof and shock-hardened housing, and an eyesafe laser range finder that provides accurate target ranges and aids in navigation.

Components of the AN/BVS-1 photonics mast
Photo courtesy Naval Sea Systems Command

The periscope well that houses these masts will be contained only in the ship's sail. The smaller size of the periscope well allows for more freedom in determining the location of the ship's control room. With conventional periscopes, the control room had to be placed in the cramped upper deck. In the new Virginia-class submarine, the control room will be located on the wider second deck and will have a more open layout.

The photonics mast will allow the control room to be located on the roomier second deck.

Images from the photonics masts are sent via fiber optics to two workstations and a commander's control console. The two photonics masts are controlled via joystick from any of these stations. Each station contains two flat-panel displays, a standard keyboard and a trackball interface. Images are recorded on both video cassette and CD-ROM.

The photonics mast is the latest tool to be added to the United States' electronic-warfare arsenal. This new technology will make the Virginia-class submarines the most advanced and automated submarines in the naval fleet.