Lighting for Machine Vision - Parts Analysis

The first step in doing a part analysis, is to give the part the A-T-E-S-T?

As a part is evaluated you must look at a part and see if it matches one or more of the following characteristics.

  • Absorption (Is there color involved?)
  • Texture (How Light Bounces off a part)
  • Elevation (Maybe I can come in at certain angles to make features light or dark)
  • Shape
  • Translucency (Back Lighting Techniques)

The top 5 Lighting Techniques used in general are:

  • Backlighting (transmitted) - Opposite of Camera
  • Ringlights and Arrays - Bright Field Lighting
  • Diffused On Axis Lighting (DOAL) - Light introduced on same axis of camera
  • Dome Lights (SCDI and CDI's) - Diffused On-Axis Light
  • Dark Field - Low angle lighting

What to Look for when evaluating absorption of a part

Look for change in light absorption, transmission, or reflection. Examples may include:

  • Fuses in block
  • Wire Colors
  • Ink Printed matter
  • Plastic Cups
  • UV emission
  • IR thru plastic

What to Look for when evaluating Texture of a part

Look for change in surface texture or finish. For example a textured surface is darker than polished surface, so with a DOAL (Diffused On Axis Light) shining straight down on a part light will hit a textured surface and scatter the light.  After it scatters it will go away.  With a shiny surface it will bounce right back up to the camera.

When we go to dark-field we get an inverse image. The textured surface is actually brighter than the shiny surface. As light comes in at a lower angle and hits a textured image it scatters and comes up to the camera.

  • Polished Surface
  • Laser annealed
  • Sandpaper Grit
  • Scratched Surface
  • Material Change

What to Look for when evaluating Elevation of a part?

Look for change in height from surface to camera:

  • Notched Part
  • Dot-peen mark
  • Embossing
  • Engraving
  • Angled/Beveled
  • Foreign debris

What to Look for when evaluating shape of a part?

Look for change in shape along the X-Y axis. Examples may include:

  • Parts on conveyer
  • Coins
  • Edge dimensions
  • Short Shot
  • Injection molding

What to Look for when evaluating translucency of a part?

Look for change in density- related light transmission Examples may include:

  • Drilled Hole
  • Thin area in plastic
  • Plastic Lens ID
  • Multilayer Material
  • Debris in Liquid
  • LCD Inspection

In using the A-Test we can consider the following solutions for each characteristic:


  Problem Solution
Absorption Color Involved Use the Color Wheel
Texture Change in Surface Using Scattering Properties Techniques
Elevation Change in Z Bounce Light off of Part in a Direction
Shape Change in Shape Bending of Light
Translucency Change in Density Bending of Light

Atest Quick Reference Chart

The Color Wheel is Required for Controlling Color

The color wheel is the most important tool for controlling color. The way we use this is to always use the opposing color to make it dark. The 3 opposing colors opposite to the initial part color make the color dark. You cannot use colors in the color wheel which are near to the actual color because it many cases those colors are which are near to the actual color make up that color, therefore if we hit that color with one of those colors it will reflect the same color back to the camera. We always want to use the part of the spectrum that the color doesn't have to make it dark.

Infrared Light (IR) Can help us make parts look colorless

If we are working with parts which have a variety of colors and we want all colors to look the same we use IR

It converts all colors into having the same grey scale.

Light Sources - Consider it's efficiencies and aging characteristics

In setting up a vision system with the best camera, software etc. and we don't choose a light source that will be very stable over time and will be constantly changing, in short periods of time we will not be successful in maintaining the tolerances that we have setup for our system.

150 watt Halogen Fiber Optic Light Sources will only get 200-500 Hours

If you crank the 150 watt source all the way up you will get 200 to 500 hours at best to reach a 50% point of its initial intensity. Fluorescents will give you approximately 7000 hours.

Red LEDs will give you 60,000 hours running 24/7

In the harshest of conditions, the LED lights are rated at over 60,000 hours at 40 degrees Celsius running 24 hours a day and 7 days a week.

Unstable Light Environments will change over time

Many operators on the factory floor find themselves tinkering with light sources because there environment is changing which may include ambient light or unstable light sources. Not Optimizing lighting will result in time and money spent maintaining the system

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