A well-designed optical filter enables efficient transmission of light at the relevant wavelengths for your machine vision-based device. By creating the best possible conditions for the underlying sensor, the optical filter optimizes the device’s performance and helps protect your device as an integrated part of the enclosure.
Are machine vision, embedded vision, and computer vision the same thing? Not sure about the differences between these definitions? Read more in this article.
In the following, we will dive a little deeper into some of the key factors you should consider when designing and choosing the optical filter for your application.
It’s much easier and cost-effective to replace a cover glass than the expensive equipment behind it! Providing simple mechanical protection of components inside the enclosure is the first important task of your optical filter. The cover glass is an integral part of the enclosure.
Will your product be used in humid or dusty environments? If so, you’re designing the enclosure to withstand those environments and properly protect the electronics inside. Naturally, the optical filter and bonding used to mount it must provide the same resistance to external factors while retaining transmission throughout the device’s lifetime.
Furthermore, physical impacts can also reduce the service life and reliability of your product unless it is designed to handle them.
Therefore, you should consider the environment in which the sensor is placed when designing the optical filter for the sensor. In regards to these considerations, impact resistance, scratch resistance, and abrasion resistance are some of the factors you might want to consider.
Most devices are designed to only pick up light in narrow wavelength ranges. Achieving optimal transmission for such devices means filtering out unwanted light wavelengths that could create interference and lead to reading errors or lower reading speeds. It’s a matter of optimizing device functionality through filtering, and this is the second important task of your optical filter.
Define exactly which wavelengths your device needs and which wavelengths you need to filter out. This way, you can specify the filtration properties of your optical filter accordingly. If you’re working with a sensor that relies on light in the red spectrum, for example, get an optical filter that absorbs all other wavelengths than wavelengths in the red spectrum.
Prioritizing certain wavelengths over others is also known as ensuring the highest possible signal-to-noise ratio (SNR). Here, noise is defined as light at unwanted wavelengths. Read this article to learn more about SNR and why it is important for optical filters.
The filtration properties of an optical filter are given by its transmission curve. This is a graph that shows which wavelengths are filtered out and which are transmitted to the device. Learn more about transmission curves in this article.
You’ve designed your enclosure to provide the necessary protection, and you’ve filtered out unwanted light – but you may need even more transmission than what a simple PMMA solution can provide – enter AR coatings.
Reflections from the cover glass reduce light transmission. If you’re working with high-performance sensors, this may create inefficiencies and reading errors. To solve this problem, you need to maximize signal strength beyond what you can do with an optical filter with no surface treatment. Adding an AR coating is a highly effective way of increasing the transmission of your optical filter by significantly reducing surface reflections.
Reflections typically absorb around 8% of the incident light that hits the cover glass (4% for each side of the cover glass), meaning that less light is transmitted to the sensor underneath. An AR coating ensures that a lot less light is reflected: The normal 8% loss can be reduced to under 1% reflection, giving you a light transmission of over 99%.
Not sure what the differences are between anti-glare and anti-reflection? Click here to read our article about it
Reflections in depth
To explain the physics involved in a little more detail: At every acrylic-to-air interface, approximately 4% of the light is lost to refraction while passing the interface (because air and acrylic have different refractive indexes). This happens in two instances (once on each side of the cover glass), resulting in a loss of around 8% light. An AR coating reduces the reflections without scattering the light in all directions along the surface of the glass, and more light is transmitted through the material.
If there is more than one sensor or an integrated light source behind the cover glass, a shadow mask can also improve performance. Light from integrated sources has the same effect as incident light: it bounces off the cover glass and can disturb the sensor(s). A shadow mask helps absorb these internal reflections, improving the device’s performance.
Protection, filtering, and AR coating are the three key factors you need to consider when selecting an optical filter, but you may also need to consider other parameters:
The design of the filter might be important in some applications. If you are designing surveillance camera systems, for example, you may not want the presence of a camera to be obvious to the casual observer. Or maybe you just have strong preferences for how the device looks. In such applications, you may want an optical filter with a black-panel effect in front of your vision system while still adhering to the three abovementioned factors when designing an optical filter.
We hope this article has given you some insights into what you should consider when selecting an optical filter solution. Whether these considerations are relevant naturally depends on your specific application, but we recommend that you evaluate their importance when designing your device to maximize the durability and performance of your sensor.
If you have a project coming up where you need an optical filter, we would love to help you find the right solution. You may rest assured that we think of everything when designing the optical filter for your exact project – including the factors covered above. We help you strike the right balance between cost and performance so you’re sure that your device meets certain reading speeds and accuracy requirements.
You can always contact us if you want to draw on our expertise. We’re ready to help you find a customized solution for your project.