Selecting the Correct Lens and Lighting

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

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.

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.

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.

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.

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.

CA-LH16

CV-L16