How thin are fiber optic borescopes? Why are they made so thin?

The core secret to the extreme thinness of fiber optic borescopes lies in their image transmission component—the image bundle.

We can understand this through a simple comparison: Traditional rigid borescopes: like a long, thin telescope, with multiple complex lenses inside. Light is repeatedly refracted between these lenses. The lenses themselves have thickness and require precise alignment and fixation, making it difficult to make them very thin.

Fiber optic borescopes: their core is not lenses, but a bundle of extremely flexible glass fibers, thinner than a human hair.

Let's break down in detail how fiber optic borescopes achieve their "thinness":

1. Core Principle: Total Internal Reflection and Pixelated Image Transmission

The fiber optic image bundle is not a single fiber, but rather tens or even hundreds of thousands of extremely thin fibers arranged and bundled together in a regular pattern.

Structure of a single fiber: Each fiber consists of two parts:

1. Core: A high-refractive-index glass center.

2. Cladding: A low-refractive-index glass layer surrounding the core.

Working Principle: Total Internal Reflection: When light travels from the fiber core to the cladding interface, due to the higher refractive index of the core, the light is completely reflected back into the core. Thus, the light travels through the fiber like a "zigzag," undergoing countless total internal reflections from one end to the other with almost no loss.

How is the image transmitted within the fiber? – Pixelation

Tens of thousands of fibers at one end of the image bundle together form an imaging surface.

Each individual fiber can only transmit one point of light (a pixel in the image).

These fibers are arranged in a one-to-one correspondence and according to a regular pattern at both ends.

In this way, the optical image formed on the imaging surface at the front end of the image bundle is decomposed into tens of thousands of independent points of light.

Each point of light is independently transmitted to the other end through its corresponding fiber.

At the other end, all the points of light transmitted from the fibers are recombined to reconstruct the image from the front end.

It can be imagined as an array of tens of thousands of tiny "light guides," each responsible for transmitting light from only one point.

2. Why can this structure be made so very thin?

Based on the above principles, fiber optic borescopes have significant advantages in achieving finer details:

1. Extremely fine material: The diameter of a single fiber can be at the micrometer level (e.g., 10µm, about 1/6 the diameter of a human hair). A bundle containing tens of thousands of such fibers can also have a diameter controlled within 12 millimeters.

2. No need for complex internal lens assemblies: Unlike traditional rigid borescopes that require multiple lenses arranged in physical space, the fiber optic image bundle is simply a regular bundle of fibers. It guides light within the fiber through physical principles (total internal reflection), rather than performing optical imaging in free space, thus saving a significant amount of physical space.

3. High integration:

Image bundle: Responsible for transmitting images, it is the main component.

Guide beam: Surrounding the image bundle is another independent bundle of fibers, specifically responsible for guiding light from an external light source into the body, providing illumination for the observation area.

These fiber bundles can be very compactly integrated, and then wrapped with a flexible sheath and control mechanisms to form a complete borescope.

Why is it necessary to be so fine?

Specifically, the pursuit of "fineness" is primarily based on the following three points:

Adapting to minute spaces: In industry, only sufficiently fine borescopes can smoothly enter tiny channels less than 1 millimeter in diameter and the internal holes of precision parts.

Achieving cost reduction and efficiency improvement: In industrial inspection, it avoids disassembly and damage to equipment or parts, saving time and costs associated with disassembly.

Improving flexibility and accessibility: Fine and flexible fiber optic borescopes can bend and traverse complex and winding paths to reach areas inaccessible to traditional instruments.

In summary, the extreme pursuit of "fineness" drives continuous breakthroughs in borescope technology, aiming to allow human vision to access the previously inaccessible microscopic world, which plays a crucial role in industrial development.