The Art of Contrast

As many of us who work in the expansive field of Machine Vision understand, achieving optimal contrast is paramount. We are also well aware that various lighting techniques available to us aid in achieving the best possible Machine Vision Contrast. To refresh the memory of some, let’s briefly review the three main lighting techniques:

Darkfield Lighting:

In darkfield lighting, the light is directed in such a way that it does not enter the camera lens directly. Instead, light is angled toward the object, ensuring that only the light scattered or reflected by the object is captured by the camera. This results in a bright image of the object set against a dark background. The advantage of this technique is its ability to reveal extremely small defects or features on the object’s surface, such as scratches, particles, or other irregularities that might be overlooked with other lighting methods. This makes it particularly suitable for quality control in industries where precision is crucial.

Bright Field Lighting:

This method involves light falling directly on the object and reflecting back into the camera lens, producing a bright image with well-defined contrast between the object and its background. Bright field lighting is excellent for inspecting objects with a uniform surface, focusing on capturing the shape, dimensions, or specific markings on the object. It is the most direct and intuitive form of lighting in Machine Vision, ideal for general inspection tasks without the need for special lighting to reveal complex surface features.

Coaxial Lighting:

Unique in its approach, coaxial lighting emits light along the same optical axis as the camera lens. This is often achieved by passing light through a semi-transparent mirror, which then directs it straight down onto the object. The light reflects back through the same mirror to the camera lens. This technique is especially useful for inspecting highly smooth or reflective surfaces, where other lighting techniques might create unwanted shadows or reflections. It ensures a uniformly lit image free from direct reflections, making defects or features on the surface clearly visible.

On a nearly weekly basis, I receive field inquiries about achieving the best Machine Vision Contrast. As many of you know, I have access to a well-equipped vision lab, thanks to the collaboration between our company, Photon Mission, and my former employer, Vision Light Tech. Our companies are often sought after for less obvious solutions.

A Case Study:

The challenge was to identify damages on various high-gloss plastic parts. The aim was for an OK-NOT OK analysis that differentiates the quality of your product from the competition. The high-gloss parts varied in color from black to light cream and everything fashionable in between. The request was for a one-size-fits-all solution with a light source capable of revealing everything on the different products. Additionally, there was a completely transparent product with a challenging shape.

There I was in the Vision Lab, all the manufacturer’s samples laid out before me. The first impression of a product is often decisive. However, in this case, the explicit wish of the client to have a checkpoint in their production line occupied much of my thoughts. As usual, I started with what seemed easiest – in this case, the glossy red cover of a kitchen appliance. My first thought was coaxial. A large coaxial lamp of 200 x 200 mm was a hit. The red LEDs did their job perfectly, making the scratches more than visible and easily detectable by vision software. By adjusting the lamp’s intensity, the other colors could also be analyzed, some better than others, always keeping the customer’s wish in mind. Even the dreaded black high-gloss cover revealed its secrets. The trinity of light-lens-filter (red light with a BP635 filter for the lens) yielded good results.

Encouraged, I moved on to the other plastic products. The transparent, oddly shaped one. No matter what I tried, coaxial light was not successful. I had to switch to a different type of lighting. After much experimentation with other techniques, I achieved success with a 300 x 300 mm backlight with a polarization sheet on the lamp and a polarizer on the lens, opting for a red 635 nm backlight in this case.

Conclusion: Basic knowledge (often gained from learning from mistakes) and simply testing with different lighting usually yield the desired result. I am fully aware that having access to a well-equipped vision lab was key to success in this case. Unfortunately, due to the diversity of products to be inspected, the desired one-size-fits-all solution was not feasible.

Struggling to achieve good contrast in your vision setup? Contact me. Together, we’ll find a solution!

#enlightenedbyPM #Machinevision #Contrast #QC