In embedded vision, the camera's "eyes" capture far more than just the visible spectrum. From ultraviolet to infrared, the non-visible light domain holds a wealth of visual information, enabling machines to accomplish tasks beyond the reach of humans. However, effectively utilizing this information requires precise control of the light entering the image sensor. This is where infrared pass filters come in.
As a consultant specializing in camera modules, this article will provide an in-depth analysis of the technical principles and applications of infrared pass filters and infrared bandpass filters. We will explore the differences between different wavelength filters and discuss why choosing the right IR lenses and filters is crucial in demanding embedded vision environments.
What is an IR bandpass filter and lens? A Deeper Understanding of Infrared Lenses Filters
A complete infrared imaging system typically consists of three core components: a light source, an infrared lens filter, and an image sensor. An IR bandpass filter is an optical component designed to allow only infrared light within a specific wavelength range to pass through, while blocking visible light and other unwanted wavelengths. This is the opposite of the common IR-cut filter, which blocks infrared light to ensure accurate image color.
Infrared lenses, on the other hand, are optical lenses designed specifically for infrared light. Conventional lenses are typically optimized only for visible light (approximately 400-700 nm), which causes chromatic aberration when focusing infrared light, resulting in blurred images. Therefore, in infrared vision systems that require precise imaging, filters must be used in conjunction with specialized IR lenses.
Combining these two forms a complete infrared optical solution: an infrared lens filter. Understanding the principles of this combination is the first step in building an efficient infrared vision system.
How does an infrared filter work?
The operating principle of an infrared filter is primarily based on optical interference and absorption effects. A filter typically consists of a glass substrate and multiple thin-film coatings. The thickness and refractive index of these coatings are precisely calculated to create constructive or destructive interference with specific wavelengths of light. In this way, the filter selectively transmits certain wavelengths while reflecting or absorbing others.
Infrared pass filters can generally be categorized into two main types: IR longpass filters and IR bandpass filters. IR longpass filters allow all light above a specific cutoff wavelength to pass. For example, a 720nm longpass filter blocks light between 400 and 720nm, while allowing all light above 720nm to pass. In contrast, IR bandpass filters only allow a very narrow spectral band to pass, such as 850nm ± 10nm. Each type of filter has its own application scenarios and technical challenges.
Key Features of IR Bandpass Filters
When selecting IR bandpass filters, engineers should consider the following key characteristics, which are crucial to determining their performance and suitability:
- Center Wavelength: This is the wavelength at which the filter has the highest transmittance, typically matching the peak wavelength of the light source (such as an infrared LED), such as 850nm or 940nm.
- Full Width at Half Maximum (FWHM): This is the spectral width at which the filter's transmittance reaches half its peak. The narrower the FWHM, the greater the filter's wavelength selectivity, but the less light it passes.
- Peak Transmittance: The filter's maximum transmittance at its center wavelength. High transmittance (e.g., >90%) is a sign of excellent performance, ensuring more effective light signals reach the sensor.
- Blocking Band: The wavelength range over which a filter blocks unwanted light. A good IR bandpass filter should completely block visible light, and its cutoff depth is typically measured by optical density (OD).
- Angle of Incidence Effect: Filter performance varies with the angle of incidence, a significant technical concern for wide-angle lenses and non-telecentric optical systems.
What is the difference between 720nm and 950nm infrared filters?
In practical applications, 720nm and 950nm are two common infrared pass filters. The difference between them is a key consideration when selecting a filter.
720nm Infrared Filter:
- Characteristics: Belongs to the near-infrared (Near-IR) range. It allows all infrared light above 720nm to pass through, while blocking most visible light.
- Advantages: It transmits a wider spectral range, resulting in higher light flux and brighter images. It is suitable for scenarios that require a certain amount of visible light information (such as the contrast between near-infrared and visible light).
- Pain Points: Due to its lower cutoff wavelength, a small amount of red visible light may still pass through, which is a problem in applications that require color purity.
950nm Infrared Filter:
- Characteristics: Belongs to the deep infrared range. It only allows light above 950nm to pass through, almost completely blocking all visible light.
- Advantages: It enables "pure" infrared imaging without interference from visible light. It is often used for night vision surveillance that requires complete concealment, as the light emitted by 940nm infrared LEDs is completely invisible to the human eye.
- Pain Points: The light flux is much lower than that of the 720nm filter, requiring a stronger infrared light source.
Choosing an infrared bandpass filter depends on your application's specific requirements for concealment, light throughput, and purity.
Embedded vision applications that require IR bandpass filters
IR bandpass filters are core components of many high-precision embedded vision applications. Their selling point is their ability to reveal information that visible light cannot.
- Biometrics and security: In facial and iris recognition, infrared light of specific wavelengths can penetrate the skin's surface to capture unique vein patterns or iris features, unaffected by makeup, lighting, and other factors. A 940nm filter enables completely invisible night vision monitoring without visible light, ensuring stealth.
- Industrial quality inspection: In the food and manufacturing industries, infrared light can penetrate surfaces to detect internal defects. For example, industrial cameras using infrared filters can detect internal rot in fruit, liquid levels in plastic bottles, or the integrity of tablet packaging.
- Transportation and autonomous driving: In adverse weather conditions, such as fog, haze, rain, and snow, infrared light has greater penetrating power. Therefore, in-vehicle vision systems utilize infrared (IR) lenses and infrared pass filters to enhance night vision and weather adaptability.
- Scientific Research and Medical Imaging: In the fields of machine vision and medicine, infrared spectroscopy is used to analyze material composition, cell activity, and blood oxygen saturation.
Summary
Infrared pass filters and infrared bandpass filters are essential optical components in the embedded vision engineer's toolbox. By precisely controlling light, they enable cameras to capture non-visible light, greatly expanding the application boundaries of machine vision. From understanding the operating principles of IR longpass filters to weighing the pros and cons of 720nm and 950nm filters, every decision directly impacts the system's ultimate performance.
A successful infrared vision system is a perfect combination of infrared pass filters, IR lenses, sensors, and algorithms. Only by comprehensively considering these technical key points can the vision of "seeing the invisible" truly be realized.
Muchvision can help you with your infrared solution
Does your project require infrared light to address challenges encountered with visible light? Contact our expert team today and we'll provide you with professional infrared pass filter selection and integration solutions to help you build a high-performance embedded vision system!
