You are here: Home / News / Blog / How Do Door Peepholes Work?

How Do Door Peepholes Work?

Views: 589     Author: Site Editor     Publish Time: 2019-10-29      Origin: Site

How Do Door Peepholes Work?

Every door has a small round hole, which is called the doorpeephole, such as the security steel door, the double swing door with armored door peepholes and so on. It is a small optical instrument installed on the door of a residential building, which is convenient for residents to observe the outdoor situation. It has become a necessity for home safety.

How Do Door Peepholes Work?


1. Composition of Door Peepholes

Door peephole, like telescopes and microscopes, is composed of objective and objective lenses. The objective lens is concave lens and the eyepiece is convex lens, but the focal length of the objective lens is short and the focal distance of the eyepiece is long. It makes the outdoor person become a positive and narrow virtual image, which falls just within the focus of the eyepiece, and finally gets a narrow positive virtual image, which is just near the eye's visual distance, so that the situation outside the door can be clearly seen.


2. Function of Door Peepholes

Looking out through the door peephole, you can see all the scenes within 120 degrees of field of view outside the door, but you can't see anything inside through the door mirror. If this mirror is installed on the door of public houses or private apartments, it can play a certain role in preventing theft and security of families.


3. Principle of Door Peepholes

The door peephole is made up of two lenses. When we look out from inside the door, the objective lens is a concave lens and the eyepiece is a convex lens. The focal length of the objective lens is extremely short, which makes the outdoor person into a small erect virtual image. This image falls within the first focus of the eyepiece. The eyepiece acts as a magnifying glass, and finally gets a more enlarged erect virtual image. This image is just in the vicinity of the human eye's bright distance. For the situation outside the door, it can be seen clearly.


However, through this door peephole why can't you see anything when you look inside from outside?

When looking down, the concave lens becomes the eyepiece, and the convex lens becomes the objective lens. In the indoor scene, the refracted beam passing through the objective lens should generate an inverted real image, but it falls on the eyepiece before it is imaged. Because the focal length of the eyepiece is extremely short, the resulting erect virtual image is very close to the eyepiece, only 2 to 3cm. Because the aperture of the door mirror is very small, the outdoor person has to look close to the eyepiece. So the distance between the human eye and the image is only 2 to 3 cm, which is much smaller than the near point of the normal human eye. Therefore, for the outdoor peeper, everything in the room will be invisible. (The near point is that the human eye can see the minimum distance of the object. The near point distance increases with age, the young person's near point is about 10cm, but when the person is 50 years old, the near point is about 40cm.)


In a word, if the door peephole is installed on the door of public house or private apartment, it can play a certain role in guarding against theft and security of the family. Door peephole is still a great help to us.