Everything you need to know about IR bandpass filters and lenses

While choosing the right camera is a key factor for embedded vision applications, selecting the right lens plays just as important of a role in many of them. For instance, some applications require bandpass lenses to block a certain wavelength of light and allow the required wavelength of light to fall on the image sensor. As several bandpass lenses are available in the market, picking the right one depends on the unique requirements of your application.

For example, one of e-con Systems’ customers was developing a device that projects elements onto a surface using both visible and IR light. However, the camera monitoring the projected display should capture the IR data alone for the system’s algorithm to process. It proved to be a challenging decision to select the best-fit IR bandpass filter for their application. So, e-con Systems stepped in and closely worked with the customer before suggesting the absorption type IR filter for See3CAM_CU30 – low light USB camera from e-con Systems.

Now, let’s go in-depth and better understand the world of IR bandpass filters and lenses that use them.

What is an IR bandpass filter and lens?

IR bandpass lens allows only the IR light and blocks the remaining spectrum. If your application needs to capture only the IR light for processing, then you must choose the bandpass lens – depending on the IR wavelength used. And this is done by using what is called an IR bandpass filter. An IR bandpass filter uses a special optical glass coating, which allows only IR wavelength (usually NIR range from 780-1500nm) to pass through and blocks the visible wavelength (380nm to 700nm).

There are two types of IR filters, namely:

1. Reflection type
2. Absorption type

Reflection type IR filter

The reflection type infrared filter, also known as an optical cold mirror, is made on the optical white glass by vacuum coating. It reflects the visible light and allows infrared light to pass through. This infrared filter’s exterior looks similar to a mirror.

Figure 1 – a lens with reflection type IR filter

Absorption type IR filter

The absorption type IR filter usually has a black coating or is made up of black-colored glass. It absorbs the visible light and allows only the infrared light to fall on the sensor. It is important to note that the IR sensitivity of the absorption type infrared filter is higher than the reflection type.

Figure 2 – a lens with absorption type IR filter

Transmission percentage of absorption & reflection type IR filters

In this section, let us look at the comparison of the transmission percentages (or percent transmission) of reflection type and absorption type IR filters. Transmission percentage is nothing but the ratio of the transmitted light intensity and the incident light intensity. The two images given below illustrate these values for both the types of IR filters:

Figure 3 – transmission percetange of reflection type IR filterFigure 4 – transmission percentage of absorption type IR filter

Upon comparing the data, you can see that the absorption type IR filter has higher IR sensitivity and blocks most of the visible light when compared to the reflection type IR filter. However, both of them are advantageous – with the absorption type IR filter used for use cases that need higher transmittance of IR and reflection type IR filter for lower transmittance.

Below are the comparison images captured using cameras with reflection type and absorption type lenses under visible light source alone:

Figure 5 – comparison of images taken with reflection and absorption IR filter lenses under a visible light source

From the above images, you can understand that the reflection type IR filter allows some range of visible light wavelengths, which explains why the images seem to be colored. But in the case of the absorption type IR filter, the visible light transmittance is very low and the IR transmittance very high. So, it blocks most of the visible light.

Now let us look at the same comparison when the light source is a combination of visible and IR light.

Figure 6 – comparison of images taken with reflection and absorption IR filter lenses under a visible+IR light source

In the above image, you can see that the IR light illumination is a lot more prominent in comparison with the visible light illumination – the images from both the IR filters look similar.

See3CAM_CU30 camera – a UVC-compliant low-light USB camera board solution

e-con Systems, backed by 19 years of experience as an OEM manufacturer, has customized numerous lens types, including IR bandpass lenses, for our clients. We also provide an array of camera solutions that allow granular lens customization like See3CAM_CU30 – a 3.4 MP UVC-compliant low-light USB camera board based on the AR0330 sensor from onsemi®. It comes equipped with a sensor that ensures exceptional low-light performance and a dedicated ISP chip to perform auto functions such as auto white balance and auto exposure control.

We hope this blog helped you understand what IR bandpass filters and lenses are, how they work, and whether they could be exactly what your application needs to achieve superior low-light imaging. If you are looking for any help in integrating cameras into your product, please write to us at camerasolutions@e-consystems.com. Visit the Camera Selector page on our website to see the complete portfolio of e-con’s cameras.