Does the machine vision camera angle affect your filter's performance?

Introduction

If you are setting up a machine vision system, you might wonder if tilting the camera or looking at an object from an angle changes how your optical filter works. The short answer is: Yes, it absolutely does. In fact, the angle of the camera is one of the most common reasons why machine vision systems fail to capture the right colors or contrast. Let's break down why this happens and what you can do about it.

What is the "Angle of Incidence"?

In optics, the camera angle is referred to as the Angle of Incidence (AOI).

Imagine a laser pointer pointing perfectly straight down at a piece of flat glass. That straight-on angle is exactly 0 degrees. If you tilt the laser pointer to the side, that new angle (measured from the straight-on line) is your Angle of Incidence. When a camera is looking at an object at an angle, the light bouncing off the object is hitting the camera's filter at an angle, too.

The Main Problem: The "Blue Shift"

Optical filters are designed to let specific colors (wavelengths of light) pass through to the camera while blocking the rest.

When light hits the filter straight on (0 degrees), the filter behaves exactly as advertised. However, as the angle increases, the colors the filter lets through start to change. Specifically, the filter starts letting in shorter wavelengths of light. In the light spectrum, shorter wavelengths are closer to the color blue.

Because of this, this phenomenon is called the "Blue Shift." If you have a red filter and you tilt the camera too much, the filter might stop letting red light through and start letting in orange or yellow light instead. This will ruin the contrast your machine vision camera needs to read barcodes, inspect parts, or sort items.

Which Filters are Most Affected?

Not all filters react to angles the same way:

• Interference Filters (Highly Sensitive): These are the most common filters used in machine vision (like narrow bandpass filters). They are incredibly precise, but they are very sensitive to camera angles. Even a 10-degree tilt can cause a noticeable Blue Shift.
• Colored Glass Filters (Less Sensitive): These are also known as absorption filters. They are essentially just glass dyed a specific color. They are not very precise, but they do not suffer from the Blue Shift when tilted.

Why Does This Happen? (The Layer Cake Effect)

Why do interference filters shift their colors? It helps to understand how they are made.

An interference filter isn't just one piece of glass; it is made of dozens or even hundreds of microscopic, ultra-thin layers of material stacked on top of each other. Think of it like a transparent layer cake.

When light hits the filter perfectly straight on, it travels through the layers via the shortest possible path. The layers are perfectly spaced to let your desired color pass through. However, when light hits the filter at an angle, the light has to travel a slightly different, longer path through those micro-layers. This change in the physical path alters how the light waves bounce around inside the filter, tricking the filter into passing different colors.

How to Solve Angle Problems in Machine Vision

If your machine vision setup requires looking at things from an angle, here is how you can fix the Blue Shift:

1. Keep it Straight: The best solution is always to mount the camera, so it is looking as close to straight-on (0 degrees) as possible.
2. Use a Wider Bandpass Filter: If you need to let in red light at exactly 650 nanometers, but your angled camera keeps shifting the filter to 640 nanometers, you can buy a "wider" filter that lets in everything from 630 to 670 nanometers. This gives you a buffer for the shift.
3. Custom Angle Filters: Filter manufacturers can design a filter specifically for your angle. If you know your camera is permanently tilted at 30 degrees, they can build a filter that perfectly hits your desired color only when tilted at 30 degrees.

Conclusion

Camera angle is a crucial factor in machine vision. Even slight tilts can cause a "Blue Shift," changing what your camera sees and potentially causing inspection failures. By understanding how light travels through your filters, you can adjust your setup or choose the right equipment to ensure your machine vision system works perfectly every time.