Communication is vital for sharing information and connecting with others. One of the most universal and enduring forms of communication has been through light signals. Using light and patterns of illumination provides a way to transmit messages visually over both short and long distances.
The use of light signals and optical communication dates back thousands of years. Ancient civilizations like the Greeks, Romans, Chinese, and Egyptians all had systems of signaling using torches, lamps, or mirrors that could relay predetermined messages. The most basic light communication involved simple signals like a lit torch in a tower warning of danger. More complex coded signaling systems also emerged, especially for military operations.
Over time, light signaling became more advanced with the development of specialized optical telegraphy systems in the 18th and 19th centuries. These optical telegraphs used semaphore arms or shutters to convey information between stations within a line of sight. The advent of Morse code and electric telegraph lines eventually made these optical telegraphs obsolete. However, light continues to be used for communication in numerous ways today. From smoke signals to fiber optic internet, light facilitates the transfer of information across distances in a wide variety of applications.
This article will provide an overview of some of the different ways light can be harnessed for communication purposes, both in the past and present day. We’ll explore some of the early pioneering optical telegraph systems as well as modern technologies like lasers and fiber optics. While languages and communication methods have evolved, light remains one of the most universal and efficient mediums for transmitting messages.
Morse code is one of the earliest and most enduring forms of using light for communication. It transmits messages by turning an optical or audio signal on and off in patterns that correspond to the letters, numbers, and punctuation of a message.
The most common system of Morse code uses short and long pulses of light or sound to represent the dots and dashes that make up individual letters and numbers. For example, the letter 'A' is denoted by a short pulse (dot), a slightly longer silence, and then a long pulse (dash). The numbers 1 through 5 are signified by:
- 1 = .----
- 2 = ..---
- 3 = ...--
- 4 = ....-
- 5 = .....
With combinations of dots, dashes, and spaces between each character, full sentences can be transmitted across long distances using Morse code. It was originally developed in the 1830s for use with the electrical telegraph, which sent pulses of electric current to control an electromagnet and produce the clicks that operators could interpret.
Morse code was later adapted for other methods of transmitting messages over distances using blinker lights (called signal lamps), audio tones, and flashing ship or lighthouse beacons. It remains in use today by amateur radio operators, as well as for some automated distress signals like an SOS.
Learning Morse code takes practice but can be picked up easier with the help of charts, apps, games, and audio repetition of the most common letters. With proficiency, messages can be sent and received at faster speeds approaching or exceeding 20 words per minute. Though invented nearly 200 years ago, Morse code remains an effective way to communicate over a distance using simple on-off pulses of light or sound.
Signal lamps are a visual way to communicate over distances using light. They use a focused beam of light to send messages in flashes of Morse code that can be seen and interpreted by someone with a telescope or other viewing device.
Signal lamps work by using a bright light source, historically an oil or kerosene lamp, and a reflector to concentrate the light into a focused beam. The beam is interrupted with a shutter to produce short and long flashes representing Morse code.
Signal lamps came into widespread use in the 19th century for naval communication between ships. One of the most common was the Aldis lamp, invented in the 1860s by British Royal Navy Captain Arthur St. Vincent Aldis. The Aldis lamp used a kerosene lamp as the light source and a convex lens with perforated shutters. An operator would aim the lamp toward the receiving ship and flash Morse code messages by opening and closing the shutters with a trigger.
With ideal conditions, signal lamps could transmit messages accurately over distances up to around 20 miles during daylight hours. The focused nature of the beam helped prevent interception of messages by other ships not aligned with the lamp. However, fog, smoke, and inclement weather could impair visibility.
The use of signal lamps for ship-to-ship communication declined in the 20th century with the advent of wireless telegraphy and radio. However, they are still used for some ceremonial and formal messaging between naval ships today. Signal lamps represent an early ingenious use of light to enable long distance visual messaging before electronic communication.
Semaphore is a system for conveying information at a distance by means of visual signals with hand-held flags, rods, disks, paddles, or occasionally bare or gloved hands. Information is encoded by the position of the flags; it is read when the flag is in a fixed position. Semaphores were adopted and widely used (with hand-held flags replacing the mechanical arms of shutter semaphores) in the maritime world in the 19th century. Semaphores are still used by maritime and naval services.
The flag semaphore system uses two short poles with square flags, which are held one in each hand. The poles are held vertically, and the flags are held diagonally, pointing downwards towards the ground. Each flag represents a letter of the alphabet. By holding the flags in different positions, words can be visually spelled out and communicated over distances.
Semaphore is a simple and fairly robust way to communicate visually, as it does not require electricity or electronics. However, it has some limitations. Only one word can be conveyed at a time, and words must be spelled out letter-by-letter, making it a slow system compared to Morse code. The people communicating must be within line of sight of each other to see the signals. Inclement weather such as rain, snow, fog, or high winds can obstruct signals. And a modest amount of training is required to learn the system. Semaphore saw widespread use globally in maritime communication prior to the advent of radio, but today it serves a more niche purpose.
Native American tribes have used smoke signals as a form of long distance communication for thousands of years. The signals allowed tribes to send messages across large distances by creating a coded language with smoke.
Smoke signals work by releasing large puffs of smoke through a fire. The key is creating smoke that is thick and dark enough to be seen from far away. Using wet vegetation and leaves creates white smoke, while burning oil or rubber produces dark black smoke that rises high into the air.
Different configurations, quantities, and colors of smoke have specific meanings that others can interpret if they know the code. For example, three short puffs typically mean "attention" or "look here." A single puff could mean "all is well" while two puffs may signal "come here." More elaborate smoke signal codes use combinations and sequences to convey complex messages and information over distances.
The range of smoke signals depends on the landscape and weather conditions. On a clear day with good visibility, smoke signals can transmit messages 10-20 miles away or even further if elevated on high ground. However, rain, fog, or trees and terrain can limit the effective range substantially. Native tribes would strategically position signal fire locations to maximize visibility.
While obsolete as a major long distance communication method, smoke signals still hold cultural significance for many Native American tribes. The signaling techniques and coded languages passed down through generations represent important cultural heritage and traditions. While technology has enabled new forms of communication, smoke signals provide a window into the resourceful ways tribes maintained vital contact and transmitted information across vast distances.
Lighthouses have been used for centuries to warn and guide ships at sea. Lighthouses use light signals that have a distinctive flash pattern, sequence, color, and interval between flashes to help mariners identify their location along dangerous coastlines.
Lighthouse keepers developed complex signaling systems to communicate information to ships beyond just warning of hazards. By observing the flashing light pattern, sailors could determine the lighthouse's unique identity and their relative bearing from the coast.
Each lighthouse had its own characteristic flash pattern and frequency that acted as an optical "fingerprint" for mariners to recognize. These light sequences were published on nautical charts so captains could decode the lighthouse signals. Some patterns were simple like one long flash every 5 seconds. Others were more complex with combinations of short, long, and varying intervals of darkness between flashes.
During the day, lighthouses also utilized unique shapes, colors, and markings to signify daytime identification. These day markers helped ships visually spot the lighthouse and confirm their position while navigating along the coastline. The day markers corresponded to the nighttime flashing light sequence for that lighthouse.
Lighthouse keepers had to maintain the lamps and rotating optics that shaped the light beams. They ensured the foghorns, lights, and day markers were functioning properly to communicate essential signals to mariners day and night. The unique light flash patterns, colors, day markers, and fog signals of lighthouses played a vital role in communication and navigation for maritime safety.
Aircraft use lights for navigation, anti-collision warning, and landing guidance. These lights allow aircraft to signal their presence and actions to others.
Aircraft navigation lights include red and green sidelights on the wingtips and a white light on the tail. These colored lights indicate the plane's direction and orientation at night or in poor visibility. For example, if an aircraft is approaching head-on, one would see the red and green sidelights. But if it is flying away, only the white tail light is visible.
Strobe lights or anti-collision lights flash rapidly to increase an aircraft's visibility. They are usually installed on the plane's wingtips, tail, and belly. The high-intensity white xenon flashes make the aircraft stand out against the background. All commercial airliners have strobe lights to warn others of their presence.
Landing lights help illuminate the runway and taxiways during nighttime operations. They are mounted on the airplane's nose or wings. Pilots switch them on during the final approach, landing, and taxiing phases. Landing lights improve the pilot's vision and allow safe navigation on the ground. Some aircraft even have retractable lights that can extend downward for better lighting.
So whether it's for navigation, collision avoidance, or landing, aircraft lights play a crucial role in air safety. Their specialized colors, positions, and brightness help indicate the airplane's course and actions through visual signals. Aircraft lights enable safer flight in low visibility and darkness.
Traffic lights are one of the most familiar uses of light for communication in everyday life. The first electric traffic light was invented in 1912 in Salt Lake City, Utah. It was manually operated and had red and green lights. In the 1920s, traffic lights started being interconnected into systems to coordinate the flow of traffic.
The first three-color traffic light was created in Detroit, Michigan in 1920. It had red, amber, and green lights. The amber light warned drivers to prepare to stop as the light was about to turn red. This helped improve safety at intersections.
Modern traffic lights use a universal color code:
- Red means stop
- Amber means prepare to stop
- Green means proceed with caution
Traffic lights are programmed with set light cycles that alternate which direction gets the green light. The cycles are optimized by traffic engineers to keep traffic flowing efficiently. Sensors embedded in the road detect waiting vehicles and can adjust light cycles accordingly.
Pedestrian crossing signals are integrated into many traffic light systems. They use symbols and lights to indicate when it's safe for pedestrians to cross. Walk signals illuminate a white walking person icon. Don't walk signals illuminate a red raised hand icon telling pedestrians to wait. Countdown timers show how many seconds are left to cross. Accessible pedestrian signals use audible tones, verbal messages, and vibrating surfaces to assist visually impaired pedestrians.
Traffic lights are a ubiquitous example of using colored lights for communication on our roads. The standardized light sequences, durations, and meanings facilitate the safe and efficient movement of both vehicles and pedestrians through intersections. Traffic engineers continue to optimize traffic light systems as transportation needs evolve.
Fiber optic communication utilizes pulses of light sent through ultra-pure glass fibers to transmit information. The light forms an electromagnetic wave that can carry signals over long distances with less loss compared to electrical cables.
At the transmitting end, a laser or LED emits light pulses that represent the data being sent. The light travels along the glass fiber core, reflecting off the cladding that surrounds the core. This principle of total internal reflection enables the light to travel long distances with minimal loss of strength.
Fiber optic communication has many applications, notably in telecommunications where it is used for telephone, internet and cable TV transmission. Its high bandwidth capacity enables it to carry vast amounts of data. Fiber optic networks form the backbone of the internet.
Other uses include computer networks, closed circuit TV, and for sending medical images and remote surgery procedures via video. The aerospace and defense industries also rely on fiber optics for navigation systems and remote sensing.
The advantages of using fiber optic communication include:
- Higher bandwidth and faster speeds - fiber can carry much more data than copper cable
- Lower attenuation so signals can travel further without degradation
- Immunity to electromagnetic interference from other cables or appliances
- Smaller size and lighter weight per capacity
- Greater security as the glass does not allow tapping or voltage leaks
- Non-conductive so no risk of spark hazards
By harnessing the speed and efficiency of light waves, fiber optic communication has revolutionized telecommunications and enables the broadband services we rely on today. From telephone calls to streaming movies, fiber optics make our modern digital world possible.
Laser communication, also known as free-space optical communication, refers to the wireless transmission of data through free space using light from lasers. This emerging technology has several advantages over traditional radio frequency communication:
- Extremely high bandwidth - Laser communication can achieve bandwidths of 10 Gbit/s and beyond. This allows the transmission of vast amounts of data very quickly.
- Highly secure - The laser beams are very narrow and focused, making interception difficult. The data is hard to intercept or jam.
- No licensing required - Laser communication uses light wavelengths that do not require governmental licensing like radio frequencies do. This reduces regulatory barriers.
- Small size and weight - The laser communication devices are much smaller and lighter than radio equipment. This is crucial for spacecraft and aerial applications.
- Low power - Laser transmitters and receivers require less power than radios. This is important for portable and battery-powered applications.
Some key current and future applications of laser communication include:
- Spacecraft - NASA and other space agencies are developing laser communication for near-Earth spacecraft as well as deep space missions. Data rates 10-100X greater than radio are possible.
- Aircraft - Airlines and militaries are looking at laser communication to replace radio and reduce weight on aircraft.
- Autonomous vehicles - Self-driving cars and drones can use laser links to share real-time data with ultra-low latency.
- Quantum communication - Quantum entanglement for absolute security may one day be possible using lasers across global distances.
- Disaster relief - Laser links allow rapid network deployment when infrastructure on the ground is compromised.
- High altitude platforms - Balloons, airships and drones in the stratosphere can create an aerial laser mesh network for broadband internet access.
The future looks bright for laser communication technology. As costs come down and technology improves, high bandwidth laser links may become common both on the ground and through the skies. This could transform aviation, telecommunications and space exploration.