Telecentric vs. Non-Telecentric Lenses in Machine Vision: Precision, Trade-offs, and Selection Strategy

In industrial machine vision systems, lenses function much like the crystalline lens of the human eye—they determine how light is projected onto the image sensor, directly impacting image clarity, distortion, and the reliability of measurement accuracy. Among the many lens types available, telecentric lenses and non-telecentric lenses (conventional lenses) are the most widely used, yet fundamentally different in performance and application. The former, with its “no parallax and constant magnification” characteristics, is essential for high-precision measurement, while the latter dominates general imaging scenarios due to its cost efficiency and flexibility.

I. Telecentric Lenses: The Optical Benchmark for High-Precision Measurement

Telecentric lenses were developed to address a critical limitation of conventional optics: measurement errors caused by object distance variation. Their unique optical design ensures that light enters the lens in parallel rays, eliminating perspective errors at the source and making them indispensable in precision inspection.

1. Core Optical Characteristics: Precision Enabled by Parallel Light Paths

The defining feature of a telecentric lens is its telecentric optical design. Whether object-space telecentric, image-space telecentric, or bi-telecentric, the principle remains the same: specialized optical groups ensure that incoming light rays remain parallel.

• Object-space telecentric: Ensures that movement of the object along the optical axis does not shift the image position, only affecting focus (typically used with collimated lighting).

• Image-space telecentric: Prevents measurement errors caused by slight misalignment of the sensor.

• Bi-telecentric: Combines both advantages and is ideal for high-precision applications, especially for 3D object inspection.

This design delivers two key benefits:

• Constant magnification: Within the depth of field, image size remains unchanged regardless of object distance, eliminating scaling errors.

• No parallax: Features at different heights (e.g., stepped parts or objects with holes) are imaged without positional shift, ensuring measurement consistency.

2. Advantages and Application Scenarios

Telecentric lenses excel in applications requiring high accuracy and low distortion, including:

• 3D structure measurement: Such as step height, hole depth, and gear tooth thickness. For example, in engine valve inspection, a bi-telecentric lens ensures consistent measurement between the valve head and stem, with errors controlled within ±0.001 mm.

• Multi-plane inspection: Ensures consistent magnification across different planes, such as multilayer PCBs or assembled components.

• Variable working distance environments: Maintains stable imaging despite small positional variations (e.g., ±5 mm conveyor fluctuation).

• Low-distortion requirements: With distortion typically below 0.1% (as low as 0.05% in high-end models), suitable for semiconductor and precision mold inspection.

Additionally, when paired with collimated lighting, telecentric lenses suppress reflections effectively—making them ideal for detecting surface defects like scratches on metallic parts.

3. Limitations: Cost and Practical Constraints

Despite their advantages, telecentric lenses come with trade-offs:

• High cost: Complex multi-element designs (often 10+ elements) make them 3–5 times more expensive than conventional lenses.

• Limited field of view (FOV): Large FOV requires custom designs, increasing both cost and physical size (some exceed 300 mm in length).

• Lower light throughput: Parallel light design reduces brightness by 20–30%, requiring stronger illumination.

II. Non-Telecentric Lenses: The Cost-Effective Choice for General Imaging

Non-telecentric lenses, including fixed focal and zoom lenses, are the most commonly used optics in industrial vision. They follow traditional optical principles, where light enters in diverging rays, making them suitable for general-purpose imaging.

1. Core Characteristics: Flexibility and Affordability

The defining trait of non-telecentric lenses is that magnification varies with object distance:

• Objects closer to the lens appear larger.

• Objects farther away appear smaller.

This makes them more suitable for qualitative inspection rather than precise measurement. Their main advantages include:

• Low cost: Simple optical structure (typically 3–5 elements), costing a fraction of telecentric lenses.

• Wide field of view: Easily supports large-area imaging (e.g., wide-angle lenses for large objects).

• Compact and easy integration: Small size, lightweight, and standardized interfaces (C/CS mount).

2. Limitations and Boundaries

However, their drawbacks limit their use in precision applications:

• Magnification instability: Small distance variations can cause 5–8% measurement deviation.

• Parallax errors: Height differences lead to positional inaccuracies (e.g., >0.1 mm error).

• Higher distortion: Typically 1–3%, especially at image edges.

Therefore, non-telecentric lenses are best suited for:

• Barcode and QR code reading

• Presence/absence detection

• Packaging inspection

• General appearance inspection

III. Key Comparison and Lens Selection Strategy

1. Parameter Comparison

2. Selection Decision Framework

Step 1: Define the task

• Measurement → Telecentric lens

• Detection/recognition → Non-telecentric lens

Step 2: Evaluate working distance stability

• Variable → Telecentric

• Fixed → Non-telecentric possible

Step 3: Determine accuracy requirement

• ≤0.01 mm → Telecentric

• ≥0.1 mm → Non-telecentric

Step 4: Consider budget

• High precision budget → Telecentric

• Cost-sensitive → Non-telecentric

IV. Conclusion: Application-Driven Lens Selection

Telecentric and non-telecentric lenses are not competitors, but complementary solutions tailored to different needs.

• Telecentric lenses provide precision at a higher cost.

• Non-telecentric lenses offer flexibility and affordability for general tasks.

The key to selection lies in application fit: neither over-specifying nor compromising critical requirements. Choosing the right lens ensures optimal performance, cost efficiency, and reliability in industrial vision systems.

You may contact us at chenguo@mindvision.com.cn to gain more in-depth technical insights and practical applications in the fields of machine vision and optical imaging.