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Tech Focus: Using an IR cut filter to improve color accuracy

May 8, 2024

Using a single camera to capture video and images in different lighting conditions requires specialist technology. An invaluable addition to autofocus-zoom cameras is an IR cut filter, also referred to as an IR blocking filter, Hot Mirror or Day and Night filter.

The primary function of an IR cut filter is to block infrared (IR) light while allowing visible light to pass through.

Digital camera sensors are sensitive to both visible and near-infrared light. In regular imaging, infrared light can interfere with image quality by causing color shifts, especially when using color filters or capturing images in bright light. The IR cut filter helps mitigate this issue by preventing infrared light from reaching the camera sensor.

How an IR cut filter works

The filter typically sits in front of the camera sensor, either as a separate mechanical fixing or as a coating on the lens itself. It works by selectively blocking light wavelengths above a certain threshold, usually around 700 to 750 nanometers, which is the beginning of the infrared spectrum.

By filtering out infrared light, the camera can produce more accurate color reproduction and reduce color casts, resulting in clearer and more natural-looking images, especially in daylight conditions. However, in specialized applications such as infrared photography, where capturing only infrared light is desired, camera operators may need to remove or replace the IR cut filter to achieve the required effect or achieve night vision. Removing the filter allows all IR light to be captured, even if conditions are dark to the human eye. IR LEDS can be used to illuminate the scene further.

Spectral sensitivity chartAn IR cut filter is typically made of a material that is transparent to visible light but opaque to infrared radiation. The material used for IR cut filters is carefully selected to achieve this property, often involving specialized glass compositions or thin-film coatings. The filter can be tuned to match the specific sensor (see diagram).

The mechanism by which IR cut filters block infrared light varies depending on the material used. Some filters operate through interference effects, where layers of materials are precisely stacked to create destructive interference for infrared wavelengths. Others rely on absorption properties, where the material absorbs infrared radiation, preventing it from passing through.

In digital imaging systems, the IR cut filter is typically positioned in front of the camera sensor or integrated directly onto the sensor surface.

Advantages of an IR cut filter

By blocking infrared light, the IR cut filter helps prevent color shifts, reduces noise, and improves contrast in captured images. It may also remove bright spots caused by equipment that uses IR LEDs/light to sense position/distance.

In machine vision applications, an IR cut filter may be necessary to ensure accurate color representation. Without an IR cut filter, infrared light can interfere with color detection, leading to inaccurate color readings or false color representation. This is crucial, for example, in quality control applications where precise color matching is required, such as paint manufacturing.

In industrial settings where machine vision systems are deployed, consistency is key. Infrared light can cause glare or reflections that obscure details in the image. By removing infrared wavelengths, the IR cut filter can improve contrast and clarity, making it easier for machine vision algorithms to detect and analyze objects or features of interest.

In surveillance and security camera systems, especially those designed for outdoor use, the IR cut filter may be called a “day/night filter” because it automatically switches between modes based on ambient light conditions, allowing the camera to capture both daytime and night time images effectively. When the light level gets low, the camera removes the filter and switches to a black and white image only.

Factors to be aware of when using an IR cut filter

While IR cut filters are important components in digital imaging systems for maintaining color accuracy and image quality, they do have some challenges and limitations.

Despite their effectiveness in blocking infrared light, IR cut filters may introduce subtle color shifts or alterations to the image. This can occur due to variations in the spectral transmission characteristics of the filter or interactions with specific lighting conditions. While modern filters are designed to minimize color shifts, some degree of adjustment or correction may be necessary.

In low light conditions, the effectiveness of IR cut filters diminishes, leading to compromised image quality or reduced visibility. This is because the filter’s blocking of infrared light can also impact the overall sensitivity of the camera system, particularly in situations where infrared illumination is used for night vision or surveillance purposes. Cameras used for imaging in varying light should have a switchable IR cut filter to allow the admittance of infrared light in dim or dark conditions. The IR filter and sensor colour adjustments should be tuned to give accurate colour representation in day light. The IR filter does absorb some light and there is also a lot of light in the near IR band even at night so removing the filter increases the visible light but most importantly enables the sensor to detect the IR light and produce a much brighter image.

When using additional optical filters or interchangeable lenses with digital cameras, compatibility issues may arise with IR cut filters. Some filters or lenses may introduce optical aberrations or vignetting that affect the performance of the IR cut filter, leading to undesirable artifacts or image degradation. Ensuring proper compatibility and alignment between components is essential to maintain optimal image quality.

Alternatives to an IR cut filter

If any of the above factors are problematic, there are alternative methods and technologies that can be employed to achieve similar outcomes in digital imaging systems.

Image processing algorithms can be used to digitally remove or suppress infrared light from captured images. This approach involves analyzing the spectral characteristics of the image data and selectively filtering out infrared wavelengths during post-processing. While software-based solutions offer flexibility and convenience, they may not be as effective as physical filters in certain scenarios, particularly in high-contrast or low-light conditions. As seen in the spectral response curves, this is complex and difficult to compensate for.

The Bayer filter array, which is commonly used in digital camera sensors to capture color information, can be modified to include additional layers or patterns optimized for IR filtering. By incorporating specific spectral filters directly into the sensor design, camera manufacturers can achieve IR blocking without the need for separate physical filters. However, modifying the Bayer filter array may require significant changes to sensor manufacturing processes and can impact overall sensor performance.

Autofocus-zoom cameras with an IR cut filter

All our Harrier and Oriole cameras have IR cut filters to optimize image capture in scenes with low lighting levels. In well-lit conditions, the IR filter remains in place in front of the lens to block infrared light from reaching the sensor, this delivers more accurate color reproduction. In low-light conditions, the camera switches to Night mode and retracts the IR filter, allowing both visible light and near infrared light to reach the image sensor, enhancing the resulting image.

Unusually for board-level cameras, our Oriole HD-SDI Board Camera includes this feature, delivering excellent night vision compared to other cameras of this size.

All the models within our Harrier camera range feature Day and Night mode capability using an IR cut filter or similar technology, allowing users to select a camera with up to 55x zoom and rolling or global shutter which will enable accurate color representation in different lighting conditions. Our AF-Zoom cameras can be further enhanced by the addition of a Harrier Camera Interface board to enable a wider range of output options including IP, USB, HDMI and 3G-SDI.

Take a look through the range or get in touch to find a model that will perfectly suit your application.