The key to deploying a robust machine vision application in a factory automation setting is ensuring that you create the necessary environment for a stable image. The three areas you must focus on to ensure image stability are: lighting, lensing and material handling. For this blog, I will focus on the seven main lighting techniques that are used in machine vision applications.
On-Axis Ring Lighting
On-axis ring lighting is the most common type of lighting because in many cases it is integrated on the camera and available as one part number. When using this type of lighting you almost always want to be a few degrees off perpendicular (Image 1A). If you are perpendicular to the object you will get hot spots in the image (Image 1B), which is not desirable. When the camera with its ring light is tilted slightly off perpendicular you achieve the desired image (Image 1C).
Off-Axes Bright Field Lighting
Off-axes bright field lighting works by having a separate LED source mounted at about 15 degrees off perpendicular and having the camera mounted perpendicular to the surface (Image 2A). This lighting technique works best on mostly flat surfaces. The main surface or field will be bright, and the holes or indentations will be dark (Image 2B).
Dark Field Lighting
Dark field lighting is required to be very close to the part, usually within an inch. The mounting angle of the dark field LEDs needs to be at least 45 degrees or more to create the desired effect (Image 3A). In short, it has the opposite effect of Bright Field lighting, meaning the surface or field is dark and the indentations or bumps will be much brighter (Image 3B).
Back Lighting
Back lighting works by having the camera pointed directly at the back light in a perpendicular mount. The object you are inspecting is positioned in between the camera and the back light (Image 4A). This lighting technique is the most robust that you can use because it creates a black target on a white background (Image 4B).
Diffused Dome Lighting
Diffused dome lighting, aka the salad bowl light, works by having a hole at the top of the salad bowl where the camera is mounted and the LEDs are mounted down at the rim of the salad bowl, pointing straight up which causes the light to reflect off of the curved surface of the salad bowl and it creates very uniform reflection (Image 5A). Diffused dome lighting is used when the object you are inspecting is curved or non-uniform (Image 5B). After applying this lighting technique to an uneven surface or texture, hotspots and other sharp details are deemphasized, and it creates a sort of matte finish to the image (Image 5C).
Diffused On-Axis Lighting
Diffused on-axis lighting, or DOAL, works by having a LED light source pointed at a beam splitter and the reflected light is then parallel with the direction that the camera is mounted (Image 6A). DOAL lighting should only be used on flat surfaces where you are trying to diminish very shiny parts of the surface to create a uniformed image. Applications like DVD, CD, or silicon wafer inspection are some of the most common uses for this type of lighting.
Structured Laser Line Lighting
Structured laser line lighting works by projecting a laser line onto a three-dimensional object (Image 7A), resulting in an image that gives you information on the height of the object. Depending on the mounting angle of the camera and laser line transmitter, the resulting laser line shift will be larger or smaller as you change the angle of the devices (Image 7B). When there is no object the laser line will be flat (Image 7C).
Real Life Applications
The images below, (Image 8A) and (Image 8B) were used for an application that requires the pins of a connector to be counted. As you can see, the bright field lighting on the left does not produce a clear image but the dark field lighting on the right does.
This next example (Image 9A) and (Image 9B) was for an application that requires a bar code to be read through a cellophane wrapper. The unclear image (Image 9A) was acquired by using an on-axis ring light, while the use of dome lighting (Image 9B) resulted in a clear, easy-to-read image of the bar code.
This example (Image 10A), (Image 10B) and (Image 10C) highlights different lighting techniques on the same object. In the (Image 10A) image, backlighting is being used to measure the smaller hole diameter. In image (Image 10B) dome lighting is being used for inspecting the taper of the upper hole in reference to the lower hole. In (Image 10C) dark field lighting is being used to do optical character recognition “OCR” on the object. Each of these could be viewed as a positive or negative depending on what you are trying to accomplish.
