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What Is an Electric Eye?

By Alan Rankin
Updated May 16, 2024
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“Electric eye” is an old-fashioned term for a photodetector, an electronic circuit activated by light. First developed in the 1890s, this concept has been adapted to new devices and technology and is still in use in the 21st century. Common applications include remote controls, automatic doors, and motion detectors. This particular term has gradually faded from use at the end of the 20th century, although it was revived in 2009 to describe advances in artificial sight.

A photodetector operates by detecting the presence or absence of light or radiation of a similar wavelength, such as infrared (IR) radiation. The earliest photodetectors were designed in the late 19th and early 20th century. They used cathode tubes that released electrons in the presence of light, activating or deactivating a connected electrical circuit. The term “electric eye” was used to describe the device by comparing it to an eye, which also responds to light. Other terms included “magic lamp” and “magic eye.”

By the 1930s, the photodetector had multiple uses throughout the developed world. Safety devices use them to stop machinery if a worker crossed into a hazardous area. They detected intruders and counterfeit bills, security functions that are still in use today. Set to a certain color, which is determined by the wavelength of light, photodetectors could be used for sorting and quality control in industry. This same property allowed them to work with IR light, which is invisible to the human eye, making the device itself undetectable.

The once-novel electric eye had become a commonplace device by the last third of the 20th century. Familiar uses included automated doors in supermarkets and other public buildings and remote controls for garage doors and electronic devices. The concept was so useful that it was adapted to new technologies as the cathode tube became obsolete. Light-sensitive computer chips called photovoltaic cells eventually fulfilled the same function. Another consequence of the device’s ubiquity was the gradual disuse of the term.

In 2009, researchers into human sight announced a new prosthetic device for blind and sight-impaired people. This consisted of a microchip attached to the eyeball, where it could interface with the optic nerve. The chip detected signals sent from a face-mounted camera and transmitted them to the brain. The result did not fully restore sight, but at least allowed the user to detect shapes and faces. Media reports on the device naturally described it as an “electric eye.”

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Discussion Comments
By anon354041 — On Nov 05, 2013

What's the cost of an electric eye? Are they available now? Which company or country introduced this idea, and when? When was this invention completed?

By everetra — On Sep 23, 2011

@allenJo - It’s the more current usage of the term “electric eye” that has me excited. That would be its use as a vision aid.

Imagine being able to implant a chip that would virtually give people the ability to achieve near vision! This used to be the stuff of science fiction.

I remember Geordi LaForge from the TV series, Start Trek: The Next Generation. He was blind but he wore a visor that gave him vision. Considering that Star Trek is set around the year 2400 and we are implementing the first phases of that technology now, we are well ahead of schedule in keeping pace with science fiction.

By allenJo — On Sep 23, 2011

@NathanG - That's hilarious. I do remember those electronic kits from a few years back.

What surprises me is how long ago the technology has been in use. I had no idea that these types of circuits were built and used back in the 1930s.

I thought that they had only come on the scene in the late 1950s, around the time that the transistor went into mass production. But I guess it would make sense, if you could build the same kind of thing using cathode tubes like the article talks about.

By NathanG — On Sep 22, 2011

I used to build photovoltaic circuits back when I played with electronic circuits as a hobby during my teenage years. They were really easy to put together; the photovoltaic cell was a standalone component.

It could act as a switch to trip a relay when light was present (or not present). I had a couple of teenage friends of mine who were into building these and other kinds of circuits too.

One unique application my friend built was a “parent detector.” He built a circuit that received light from a low level beam, which was aimed at the base of the top of the stairs.

Whenever his parents came up the stairs and tripped the light, it would alert him and he would suddenly behave. It’s strange the motivations that get you into electronics.

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