Selecting the Correct Lens and Lighting

The key steps to successfully installing a vision system

  1. Selecting the devices required for inspection
    Choose the correct devices that meet the inspection requirements.
    Camera / Controller / Lighting / Lens / Monitor
  2. Sensing and judgment
    Perform testing on the actual target with the vision system.
    Reference parts for OK and NG products
    Inspection cycle time
    Variety of inspection items
  3. Selecting the installation location and procedure
    Review the specific installation locations.
    Target in motion/stationary
    Environmental conditions, including ambient light and vibration
  4. Controls for automation
    Review the I/O controls for the vision system.
    Image capture timing / Judgment output / PLC control / Data output
  5. On-site testing
    Test the vision system on the actual production line.
    Fine setup adjustment
    Statistics
    I/O control check
  6. Understanding basic operations
    Basic setup procedures to maintain stable inspection.
    Setting tolerances / Sensitivity adjustment / Changing the inspection settings / Item registration

LED lights are commonly used in machine vision image processing. There are three steps in the process of selecting the best lighting for the inspection requirements.

  1. Determine the direction of emission
  2. Determine the lighting type and shape
  3. Determine the lighting color (wavelength)

Standard lighting types (for LED lights)

  • Coaxial
    CA-DX

  • Low angle
    CA-DL

  • Direct ring
    CA-DR

  • Backlight
    CA-DS

  • Dome
    CA-DD

  • Bar
    CA-DB

Selecting instruments required for inspection [Light/lens selection]

Step 1:
Determining the direction of emission(specular reflection, diffuse reflection, transmission)

Basics of Light

When incident light (brown arrow) hits the surface of the part, light broadly separates and travels in four different directions.

  1. Specular reflection reflecting off at the same angle
  2. Diffuse reflected light that diffuses on the surface of the part
  3. Transmitted light that passes straight through the part
  4. Diffuse transmitted light that transmits scattering across the surface of the part

Using the incident light as 100%, the ratio of light specularly reflected is called specular reflectance, the ratio of other reflected light is called diffuse reflectance, and the sum of the two is called total reflectance.

There are three main types of lighting directions used with machine vision.

  • Specular reflection type
    Lens receives light that specularly reflected off the target, which works effectively with parts with reflective properties, such as a glass substrate.
  • Diffuse reflection type
    Lens receives uniformly distributed light while releasing the specularly reflected light. This type is effective for inspections done through transparent tapes in order to ignore reflective parts.
  • Transmission type
    Light is emitted from behind the target to detect the silhouette with the transmitted light. This type is useful for parts that easily pass light, such as non-woven fabric.
(1)Example of specular reflection type : Glass substrate flaw inspection / (2)Example of diffuse reflection type : Microchip print inspection over film / (3)Example of transmission type : Foreign particle detection in non-woven fabric
Point 1:

Determine the direction of the light based on the material/shape of the target and the inspection purpose. Select from three options: specular reflection, diffuse reflection, and backlighting.

Step 2: Determining the lighting type and shape

After determining the lighting direction, select the lighting type (model) based on the inspection requirements, background, and ambient environment. From the lighting options for each lighting direction, select the suitable lighting type as illustrated below.

Typical lighting type for each lighting direction
Specular reflection Coaxial light Ring light Bar light
Diffuse reflection Low angle light Ring light Bar light
Transmission Light source Bar light -
(1)Example of detection by specular reflection
Select the lighting based on the part and inspection requirements > Light reflects onto the surface of the glass > Need to clarify the difference between the glass and background > Emission perpendicular to the part is optimal > Space can be provided above the part
(2)Example of detection by transmission
Select the lighting based on the part and inspection requirements > Metal part has an uneven surface and does not allow for uniform specular reflection > Shaping defects can be detected by silhouette using transmitted light > Lighting can be set up behind the part > Backlight is the best selection

Step 3: Determining the lighting color/wavelength

The final step is to decide the color of the lighting that works the best for the part and background. White is generally selected when using a color camera. However, when using a black-and-white camera, the following knowledge becomes more important.

Complementary colors: Pairs of colors opposite each other on the complementary color circle are called complementary colors. When complementary color light is shined on a color, the color becomes close to black. / Wavelength: Light appears as a different color depending on the difference in wavelength. For example, light with longer wavelength (red) passes through films easily while light with shorter wavelength (blue) is more prone to diffusion with unevenness in the surface, such as scratches.

Point 2:

Instead of trying every light without due consideration, you can select the correct one efficiently by following the procedure below:

  • Decide which type is appropriate: specular reflection, diffuse reflection, or backlighting.
  • Decide on the shape (model) and size of the light.
  • Decide on the color (wavelength) of the light.

Step 4: How to calculate the focal distance and field of view

One of the lens specifications is focal distance. Typical lens sizes used in factory automation include 8 mm 0.31", 16 mm 0.63", 25 mm 0.98", and 50 mm 1.97". You can calculate the working distance (WD), which is the point at which the camera comes into focus, from the field of view (FOV) and focal distance necessary for the target to be captured.

The WD and FOV values depend on the lens’ focal distance and image sensor size. For WDs greater than the minimum WD where a close-up ring is unnecessary, the WD can be estimated using the following proportion.

WD: Viewing angle = Focal distance: sensor size

Example 1: When using lens with a focal distance of 16 mm (0.63") and the image sensor size is 3.6 mm (0.14"), WD should be 200 mm (7.87") in order to have a field of view of 45 mm (1.77").

Step 5:
How to increase the depth-of-field (range of heights at which the lens remains in focus)

  • Lens with shorter focal distance has a greater range of depth.
  • As the distance to the target gets farther, the deeper the range of depth becomes.
    →Note that the depth-of-field is smaller when using a close-up ring or macro lens.
  • The smaller the opening of the aperture, the deeper the range of depth is.
    →It is easier to obtain focus using aperture zoom and bright light.

This example compares the image captures obtained with a smaller aperture and max. aperture when capturing a target that has a tape attached to indicate the height on a slope as shown in the figure below.

Image captured using smaller aperture (CA-LH25) / Image captured with max. aperture (CA-LH25)

Step 6: Contrast ratio based on lens performance

The images on the right show the same target, captured using KEYENCE’s high-resolution lens CA-LH16 and standard lens CV-L16. The difference in the images derives from the differences in lens material and structure. Selecting a high-resolution lens depending on the application can provide images with higher contrast.

Lens CA-LH16/CV-L16
Inspection target Copying paper
Field of view 60 mm 2.36" / Stain size: approx. 0.3 mm 0.01"
CA-LH16 High-resolution lens: Stain level 54 / CV-L16 Standard lens: Stain level 38

Point 3:

Select a lens by following the procedure below:

  1. Decide the focal distance based on the image capturing size and possible installation distance.
  2. Estimate the necessary depth of field based on the shape of the target.
  3. Select whether to use a high-resolution lens or a standard lens based on the inspection accuracy.

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