What are the differences between telecentric lenses and ordinary lenses?

In machine vision inspection, lenses are used alongside machine vision lighting to capture images. Commonly used lenses include telecentric lenses and regular lenses. What's the difference between them, and how do their outcomes differ?  FALenses Technology explains the differences between telecentric lenses and regular lenses.

In industrial applications, regular industrial lenses and telecentric industrial lenses are commonly used. The key difference between them is that telecentric lenses are designed with parallel light paths, specifically to correct the parallax issues of traditional industrial lenses. This design maintains a consistent magnification across a range of object distances, preventing the effect of "bigger closer, smaller further." This characteristic is crucial when the measured objects are not on the same plane. In contrast, regular industrial lenses vary in magnification based on the distance between the lens and the object (or CCD sensor). The closer the object is to the lens (shorter working distance), the larger the resulting image. When using regular lenses for measurement, the following issues can arise:

1. Different magnification due to varied measurement planes: The measured object may not be on the same measurement plane, leading to inconsistent magnification.

2. High lens distortion: Regular industrial lenses tend to have more significant distortion.

3. Parallax: As the object distance changes, the magnification can also change.

4. Low resolution: Regular lenses may have lower resolution.

5. Uncertainty at image edges due to geometric properties of lighting: The geometric characteristics of visual light sources can cause uncertainty at the edges of images.

Telecentric lenses effectively solve these problems, with minimal parallax error, making them suitable for high-precision measurement and metrology. At specific working distances, the magnification remains constant after refocusing, as the telecentric lens's maximum field of view is directly related to the proximity of the lens to the aperture stop—the larger the lens, the larger the required field of view. Telecentric lenses are high-end industrial lenses with outstanding image quality and minimal distortion, particularly suited for dimensional measurement applications, and can be used with software for precise measurement.

Telecentric Lenses

I. Comparison of Telecentric Lenses and Regular Industrial Lenses for Shooting Small Holes

Under identical shooting conditions, regular lenses have visual focus errors, resulting in shadows in the inner walls of small holes, affecting the extraction of hole edges and reducing image precision.

Using a variable zoom telecentric lens, the characteristic parallel light feature of the telecentric lens ensures clear outlines of small holes, with accurate data, without needing extensive image correction.

The comparison shows that telecentric lenses are better suited for detecting the diameter of thick small holes and for measurement applications that require precise edge extraction.

II. Comparison of Telecentric Lenses and Regular Industrial Lenses for Shooting Gears

Regular industrial lenses, with their non-parallel light design, have visual errors. Images taken with regular industrial lenses show the inner walls of gear holes, with the entire gear appearing convex along the center, and data on hole diameters and outer edges are inaccurate, making edge reading difficult, requiring complex algorithmic correction.

With telecentric lenses' parallel light design, gear images have clear inner and outer edges, with accurate hole diameter data, without visible inner walls or outer distortion, negating the need for image correction.

III. Comparison of Telecentric Lenses and Regular Industrial Lenses for Shooting Molds

With regular lenses, visual errors and focus inaccuracies result in visible inner walls and unclear edge extraction when shooting plastic mold ventilation holes (circled in red), causing shadows, affecting recognition, and yielding inaccurate data.

Using telecentric lenses, with their telecentric characteristic of parallel light, these visual errors don't occur. When shooting ventilation holes on plastic molds (circled in red), inner wall edges are clear, with accurate data. When photographing round holes (circled in green), edges are clear, allowing for burr detection and accurate hole diameter measurement.