How To Make a Reverse Peephole Viewer

Statistics show that most private property violations occur when a house or hotel room is empty.

In order to prevent  “surprises”, the use of Reverse Peephole Viewers (RPV) could help us in these risky situations.

Shomer-Tec Reverse Peephole Viewer Grey

What is a Peephole Viewer (PV)?

A Peephole has been used as a security measure to confirm who is there and what happens outside, almost as old as when doors began to be installed in homes.

With a simple Peephole, the visual angle is limited by hole dimensions and door thickness. The thicker the door, the viewing angle is proportionally reduced.

The PV is a small device, generally installed in the central part, at the height of a person, in the external doors as a security device.  Due to its particular design, it provides users with an angular view (around 120-180 °) to allow them to get a wide view of the space outside.

Another important feature of the design is that if someone from the outside wants to see the inside through the PV, they will see a very small and distorted image.

To obtain these functions, the PV uses a Fisheye lens at the external surface of the door. (1)

These lenses catch a very wide range of lights rays directions from outside space. Some additional lenses will modify the image and concentrate it for the human eye, on the inner surface of the door. The image provided has a unique perspective, allowing us to see what is outside the door.

Peephole Viewer History

The  PV was a revolutionary invention of George Winningham in 1932.  That was quickly adopted in almost all the external doors of houses and hotel rooms from that moment on.

How Peephole Viewers Work?

To obtain this wide view and “one-way vision”, the PV has a special combination of lens types. They have been placed at the proper distance, using the principles of diffraction of rays of light to obtain this particular effect.

Usually, the lenses are located in a cylinder metal structure with a plate at the end of both sides of the cylinder. These plates are the view portion of PV on each door side.

What is a Reverse Peephole Viewer?

RPV is a small device that lets you see from outside to the inside of the room through the installed PV on doors, but what is the average cost to install a storm door

You have to put the RPV next to the outside portion of PV and see it with the RPV viewer. It provides a clear vision of the room next to the door. You may check here how to install storm doors.

How Does a Reverse Peephole Viewer Work?

The RPV uses the same principles of light ray diffraction of PV to reverse its image effects.

Therefore, within the RPV there is a special arrangement of lenses, with distance adjustment between them, to obtain focus.

Basic Components of a Reverse People Viewer?

The main component within an RPV is the lenses. These will be in charge of modifying the light rays.

Lenses are transparent components of glass, crystal, or plastic, limited by two surfaces, at least one of them being curved. Basically, there are two types of lenses :

Convergent: they are thicker in the center than at the edge, and they concentrate (converge) the light rays that pass through them, at a point called the focus.

Divergents: they are thicker at the edges than in the center and they disperse light rays that pass through them.

RPV device is assembled by the following:

  1. Lens A: it is a converging lens that takes the divergent light rays from the PV fisheye lens and prepares it for the next lens. It can be a convex face and the other concave. (2)
  2. Lens B: There is a group of lenses to convert the image received from Len A into the correct position and dimension to be usable by the human eye.
  3. Structure: Typically, a metal cylinder is used as a frame to locate each lens. Due to different PV designs, some of these lenses must be moved through the axis of the cylinder to adjust the focus.
  4. Theory Calculation: The dimensions, distances, and lens shapes have to be supported by the theory of difracción and lens calculations, to obtain appropriate results. 


You have to take into count some points to obtain appropriate usage for RPV:

  1. Light: If the room that we would like to see is totally dark or with poor light, the RPV will not be useful. Therefore, we must consider maintaining at least a minimum lighting condition before leaving the room.
  2. Unblocked: Some of the PV has a mechanism to block the hole. This prevents outside-in vision but must be open for RPV to work.
  3. Handling: The RPV has to be portable. Should be with us at the moment we return to our place, or easy to located in a secret place near to our door.

Actual Security Alternatives

The PV was an excellent invention for security matters at that time. From that moment on, the technology improvements have been huge. Now exist peepholes integrated with WiFi, with infrared visions, capable of sending messages if something is detected outside or inside, and here’s how to build a spy camera with the use of old-phone camera devices, proximity sensors, and many others.. Anyhow the original peephole with the fisheye lens still is used in many places.


RPV is one of the tools as a fence for personal security. Available to be used indoors with PV device. Although PV was invented at the beginning of the previous century, still it’s used in many homes and hotels.

In the market, you can find some options for this kind of device.


(1) Fisheye –
(2) lens –

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About John Fox

ebbb364ee14268bd3b77496cab3d1d78?s=90&d=mm&r=gCertifications: Certified Alarm Technician (CAT)
Education: Denver Security Academy
Lives In: Denver Colorado

John Fox has worked as a security consultant in Denver for over 20 years.
With Safe Now, he's taken those two decades of experience and decided to share it to help people online make the right security decisions for their businesses and families.

John writes security tips and guides, product recommendations, and prevention guides.

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