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Telecentric Lens for Image Processing

Telecentric Lens for Image Processing

Telecentric optical system is an optical design that where the principal ray in parallel to the optical axis. It eliminates distortion problems by collimating the light entering the lens and suitable for imaging 3D objects. Co-axial illumination is suitable for recognizing object with high reflectance such as wafer, glass, and metal.

Non-telecentric lens

  • Smaller size
  • The number of lenses is fewer.
  • Object size changes as the object goes up and down.

Double side telecentric lens

  • Primary lays of object and image side are parallel to optical axis.
  • Object size does not change when object goes up and down.
  • Large size and high cost

Object side telecentric lens

  • Principal ray of object side is parallel to optical axis.
  • Required for co-axial illumination
  • Object size does not change when object goes up and down.
  • Small size, compared to double side telecentric lens

Image side telecentric lens

  • Principal ray of image side is parallel to optical axis.
  • Object size changes when object size goes up and down.
  • A lens for video camera should be this optical system to correct color aberration.

Standard Lens

Telecentric lens

Size of 3D object changes when it goes up and down when non-telecentric lens is used.

Telecentric lens is suitable for accurate measurement of 3D object.

光学倍率の求め方

Most of lenses in this catalogue are designed for finite distan ce. Image format (sensor size) divided by object size equal to optical magnification. It is th e most important to select a lens.

Image Format

Area Sensor

Image Size(inch)  1/4 1/3 1/2 1/1.8 2/3

2/3 (5Mega)

1 1.1 1.2
Vertical(mm)  2.7 3.6 4.8 5.35 6.6 7.1 9.6 12.2 15.15
Horizontal(mm)  3.6 4.8 6.4 7.14 8.8 8.47 12.8 12 15.15
Diagonal(mm) 4.5 6 8 8.93 11 11 16 17.4 21.4

Examples of area sensor used for machine vision. Different size of sensor will be expected to be available for various applications.

Line Sensor

Sensor Size(mm)  10.24 14.34 20.48 28.67 28.67 35 36 57.34 61.44
Pixel size(μm) 10 14 10 14 7 4.7 7 7 5
Resolution(pixel) 1024 1024 2048 2048 4096 7450 5150 8192 12288

Length of line sensor is formed, depended on pixel size and resolution. As the sensor size is larger, design and
manufacture of a lens for line sensor are more difficult and complicated.

Formula of optical magnification

Field of View (FOV)

The actual size of a viewed object that can be taken when a len s is mounted to a camera.
Ex). Magnification: 0.5x Image format: 1/2

 

Vertical FOV=4.8÷0.5=9.6mm  Horizontal FOV=6.4÷0.5=12.8mm

Magnification

Magnification (M) = Image format/FOV

Electronic magnification and monitor magnification

Electronic magnification

Magnification of an image on a CCD camera when it is displayed on a monitor screen.

Monitor magnification

Magnification of an object displayed on a monitor screen through a lens.
Ex). Magnification: 0.5x Image format: 1/2 Monitor size: 15 inch (1 inch = 25.4mm)

Electronic magnification 15 x 25.4 ÷ 8 = 47.6x     Monitor magnification 0.5 x 47.65 = 23.8x

レンズの焦点距離と撮影範囲の求め方

Formula of photographic range

Ex.
Object distance: 100m Focal length: 50mm  CCD: 2/3

Formula of Focal length

Ex.
Object distance: 20m  Height: 6.6m  CCD: 2/3

Formula of conjugation relationship

Basics formula Horizontal magnification Object point distance Image point distance

F No./NA Formula

Relationship of object side NA and
image side NA (NA’)
Relationship of F No. and Effective F no.(Fe) Relationship of NA and Effective F No.