Technical Parameters of Telecentric Lenses

Technical Parameters of Telecentric Lenses

Telecentric lenses offer unique advantages in optical and machine vision applications, providing high-resolution images with minimal distortion and perspective errors. Below are the key technical parameters and considerations when selecting and using telecentric lenses:

1. High Image Resolution

   • Image resolution is measured in terms of the Contrast Transfer Function (CTF), indicating the spatial frequency contrast ratio in line pairs per millimeter (lp/mm). Most machine vision integrators use low-cost, low-resolution lenses, resulting in blurry images. Telecentric lenses from AFT, even with smaller pixel sensors (such as 5.5-megapixel, 2/3"), can deliver high-resolution images.

2. Near-Zero Distortion

   • Distortion coefficient indicates the percentage difference between the actual size of an object and its image size on the sensor. Standard lenses can have over 1-2% distortion, affecting measurement accuracy. AFT telecentric lenses, with strict manufacturing and quality control, limit distortion to below 0.1%.

3. No Perspective Error

   • For precision linear measurement, it's crucial to observe objects from the exact front view without including the side perspective. Telecentric lenses achieve this by positioning the entrance pupil at infinity, capturing only parallel rays along the optical axis, reducing perspective errors.

4. Telecentric Design with Ultra-Wide Depth of Field

   • AFT's double telecentric lenses enhance natural depth of field through aperture and magnification adjustments. This allows for consistent imaging across a range of object distances, without significant changes in magnification.

How to Choose a Telecentric Lens

While machine vision has rapidly developed in China, many still find telecentric lens selection challenging. Here are some guidelines for choosing the right telecentric lens:

• When to Choose Telecentric Lenses

  • Given their unique characteristics, telecentric lenses are ideal for applications involving:
    1. Objects with significant thickness (thickness > 1/10 of the Field of View (FOV) diameter).
    2. Objects not in the same plane.
    3. Uncertain object-to-lens distances.
    4. Objects with apertures or three-dimensional characteristics.
    5. Low-distortion and consistent brightness requirements.
    6. Defects detected only under parallel lighting conditions.

• Key Parameters to Consider

  • Object-Side Size: The shooting range.
  • Image-Side Size: The sensor size of the CCD.
  • Working Distance: The distance between the front surface of the lens and the object.
  • Resolution: The pixel size of the CCD sensor.
  • Depth of Field: The range over which the lens can produce a clear image. Larger magnification results in smaller depth of field.
  • Interface: Camera interface type, often C or T.

Choose a lens with an object-side size suitable for your shooting range, and ensure the image-side size aligns with the sensor size of your CCD or CMOS camera. This will help you calculate the magnification and choose the appropriate telecentric lens. Be mindful of depth of field, as higher magnification reduces it, which may require you to adjust your lens selection.