Transmission curves – and why they’re important for optical filters

Optical filters are all around us – whether you realize it or not.

Most of these filters have one thing in common: they selectively allow certain wavelengths to pass through while blocking others. The only exceptions are those filters that let through all visible light, making them transparent such as this one:

Transmission curves are a valuable tool for showing the characteristics of different kinds of optical filters. Transmission curves are line graphs with transmittance (in percentage) on the vertical axis and wavelength (in nm) on the horizontal axis. Transmission curves, therefore, show how much light at specific wavelengths passes through the optical filter.

Disclaimer: In this article, we are going over different types of transmission curves to give you an overview of how they look, but a transmission curve might not necessarily tell the whole story of an optical filter’s characteristics. Therefore, this article aims to explain how different types of transmission curves look on a graph rather than how the transmission curves of our filters look (which depends on the context and how the transmission curve is set up – more on that later!).

There are a few different types of optical filters that are designed to transmit or reflect specific ranges of wavelengths. Let’s go through them one by one.

Long-pass Filters

First up, we have long-pass filters. These are also known as cut-on filt ers, and they transmit wavelengths that are longer than a certain threshold while blocking shorter wavelengths. These are com monly used to remove shorter wavelengths that might be scattered or absorbed by the sample being observed. A long-pass filter’s transmission curve could look something like this:

Short-pass Filters

Next, we have short-pass filters. These are also known as cut-off filters, and they transmit wavelengths that are shorter than a certain threshold while blocking longer wavelengths. These are often used to remove longer wavelengths that might interfere with the measurement of shorter wavelengths.

Therefore, short-pass filters do the opposite of long-pass filters, and their transmission curve looks something along the lines (no pun intended) of this:

Band-pass Filters

Then we have band-pass filters. These transmit a specific range of wavelengths while blocking wavelengths outside of that range. These are used to isolate a particular spectral line or range of interest:

Notch Filters

Next up, we have notch filters. These are also known as band-stop filters since they block a specific range of wavelengths while transmitting wavelengths outside of that range.

These are used to eliminate specific interference patterns or unwanted spectral lines. The transmission curve of a notch filter kind of looks like a band-pass filter flipped upside down:

Other kinds of filters

There are also some more specialized filters that are designed for specific applications.

For example, color filters are used to transmit or reflect certain colors of light while blocking others.

Lastly, we have interference filters, which rely on the constructive and destructive interference of multiple thin-film layers to transmit or reflect specific wavelengths. These are super precise and are often used in spectroscopy and other fields where high precision is a must.

It’s important to remember that a transmission curve only shows a range of light wavelengths. What happens outside of the graph is not necessarily given.
With a long-pass filter such as the following figure, for example, logic says that the filter might continue with approximately the same transmission beyond the wavelengths shown in the transmission curve:

However, that is not always the case. The transmission curve shown above is our Solaris™ S306, which is a filter designed for NIR applications that open to wavelengths above approximately 800 nm. However, the transmission curve beyond 850nm for this filter looks like this:

Therefore, a transmission curve might not always tell the whole story. As we mentioned at the beginning of this article, the context and application for which the optical filter is needed are always crucial to understand before settling on an optical filter.

The reason that the transmission curve of acrylic optical filters looks like this in the longer wavelength ranges (1500nm and up) is because of the physical properties of the material. This means that light at this wavelength range is more or less closed off – or opaque, regardless of which color or type of filter it is.

Therefore, it is no longer relevant to talk about the percentage of transmission above a wavelength of approximately 1500nm when talking about acrylic optical filters. These types of filters are not made for these wavelengths.

So, there you have it!

Optical filters are a really useful tool that allows us to selectively transmit or reflect specific wavelengths of light in order to study or manipulate it. Transmission curves help us visualize this transmission at different wavelengths so that we can choose the perfect optical filter for different applications.

Whether you’re a product developer, designer, or something completely different, the choice of optical filter significantly impacts your application’s design and performance.

That’s why we’re always open for a talk about the requirements for cover glass in your specific application. Want to get in touch with us? Then click right here.

… And thanks a lot for reading along! If you’d like to learn more about optical filters, you can read more articles by clicking right here.