Looking through the Misty Eyes of the
VM-100 Fog Detector

By Bob Trapani, Jr.

Ringing the fogbell
Harper's Magazine 1874


The days of lightkeepers standing watch and monitoring the slightest mood changes of the seascape are time-honored images forever etched in the minds of lighthouse enthusiasts. Whether tending the light in the middle of a storm or maintaining a grueling vigil at a fog bell for hours on end, the lighthouse keeper knew that no matter what weather conditions arose, he could not neglect his duty for a single moment.

Of all the treacherous sea conditions to contend with, fog is probably the most feared by seafarer. The bewilderment caused by an impenetrable shroud of gray hovering above the sea has long been capable of striking great fear in the heart of the saltiest of mariners. During these anxiety-filled moments, the lightkeeper was always quick to activate the light station’s fog signal in hopes of warning vessels away from dangers in the vicinity of the lighthouse, including the sentinel itself if the station was located offshore.

But as the 1970s gave way to the 80s, the technological advancements of automation removed nearly every vestige of the human element from the world of lighthouses. With the keepers forever gone, it was now up to automatic electronic equipment to “watch” the weather and warn the seafarer.

A view of a VM-100 Fog Detector
Photo by Bob Trapani, Jr.

A view of a VM-100 Fog Detector

The evolution of aids to navigation has always maintained a dynamic spirit for deploying the latest technologies at light stations such as the example of light sources progressing from the classic Fresnel lens to the intriguing 21st century LED beacons. In the same manner, when it comes to coping with the paralysis of fog, the transition has gone from keepers manually ringing a bell to a pair of digital “eyes” that monitor the horizon around the clock. Today, rather than relying on a lightkeeper to switch on a fog signal, the U.S. Coast Guard utilizes a variety of electronic fog detectors to activate fog signals. One such model is the VM-100 Fog Detector.

The VM-100 Fog Detector is a 1990s technology (its predecessor, the Videograph B Fog Detector, was introduced in the 1980s) that interfaces with an audio-visual controller to detect light (from the projector) that is back-scattered from atmospheric conditions such as fog, rain, snow and air pollution. The fog detector is equipped with a complex software package that measures the density of water in the air, and at a preset point activates the automated fog horn during times of reduced visibility. The visibility sensor in the fog detector is a microprocessor controlled meteorological instrument that provides the unit the ability to measure visibility through back-scattered light from particles in the atmosphere.

Inside components of the VM-100 fog detector
Photo by Bob Trapani, Jr.

A look inside at the components
of the VM-100 Fog Detector on the
Cape May Canal West Entrance N.
Jetty Light 11

The dependability of this delicate technology is demonstrated on a daily basis and year after year. Whereas a lightkeeper in bygone years needed to rest periodically from his steadfast watch, the modern, electronic fog detectors need no such break from their redundant vigils. The VM-100 Fog Detector is a reliable instrument, with exceptional accuracy over an extended measuring range. The unit’s compact and durable design consists of a lightweight electronic assembly, hood and pedestal, which are constructed of weatherproof aluminum. Best of all, the unit requires minimal maintenance, with even its calibration completed in an automatic, rapid procedure. Lastly, a good, clear set of “eyes” is required to monitor the ever-subtle changes in atmospheric conditions. To this end, the VM-100 Fog Detector deploys thermostatically controlled heaters to prevent mist and frost from obscuring the windows and lenses of the unit.

Today’s modern electronic navigational equipment will never duplicate the human care and response capabilities once provided by the lighthouse keepers; nonetheless, the digital age continues to evolve and make astounding strides with ensuring the safety of 21st century mariners. That said, when it comes to protecting the seafarer from dangers like fog, one thing is certain – a future technological advance is certain to be waiting just around the corner that will one day relegate the VM-100 Fog Detector to the pages of history just like the fog bells of the golden age of light keeping.

Shark River Inlet Breakwater Light 2
Photo by Bob Trapani, Jr.

The Shark River Inlet
Breakwater Light 2(NJ) sports
a FA/232 foghorn (lower left),
lantern and VM-100
Fog Detector (upper right)

FA/232 foghorn
Photo by Bob Trapani, Jr.

The FA/232 foghorn at
Shark River is automatically
activated during foggy
conditions by the
VM-100 Fog Detector


Other VM-100 Fog Detector Features...

  • The fog detector has a visibility alarm set for three nautical miles of visibility.
  • A self-checking feature enables the fog detector to perform self diagnostics for fail-safe operation. If the fog detector logic detects a failure, the sound signal is activated for continuous operation.
  • Long-life Xenon projector flash bulb.
  • Operates in temperatures ranging from –30 degrees F to +120 degrees F.

Created: December 2001