Q1: What is the spectral parameter/working band of a lens? Why is it so important?

A: The spectral parameter of a lens (also referred to as its working band or spectral response) defines the range of light wavelengths over which the lens can form a clear image. This parameter is critical because:

Core Concept:
Light of different wavelengths possesses different physical properties. For example, short-wavelength blue light (~400 nm) and long-wavelength red light (~700 nm) refract (bend) at slightly different angles when passing through the same piece of glass. If a lens is not specifically designed to account for this, light of different colors will not focus on the same plane of the camera sensor, resulting in a blurred image with colored fringes (chromatic aberration).

The Significance of Specialized Design:

Material Selection: Different optical glasses have vastly different transmittance properties for ultraviolet, visible, and infrared light. For instance, common BK7 glass has reasonable transmittance in the near-infrared (NIR) but is nearly opaque in the short-wave infrared (SWIR) band, requiring special materials like Germanium (Ge) or Zinc Selenide (ZnSe).

Coating Technology: Anti-reflective coatings on lenses are not universal. A coating optimized for visible light might have a reflectivity of over 10% in the NIR band, causing significant light loss and ghosting. VIS-NIR or SWIR lenses utilize broadband or multi-layer coatings to achieve average transmittance >99% across their target wavelength range.

Aberration Correction: Lens design must balance various optical aberrations (like chromatic and spherical aberration) across the entire working band. An excellent VIS-NIR lens must deliver sharp images not only with 400nm blue light but also with 1000nm infrared light.

In short: The spectral parameter defines what kind of light a lens can "see" and how clearly it can see it. Choosing the wrong band is akin to asking a nearsighted person to look at a distant object without glasses – you cannot obtain a high-quality image.

Q2: What are the common types of spectral lenses and their core applications?

A: Based on application scenarios and optical principles, they can be broadly categorized into two main classes, comprising five common types:

Class 1: Reflected/Luminescence Imaging Lenses (rely on external light sources or light reflected from objects)

Visible Light (VIS) Lens

Working Band: 400-700 nm

Core Application: Any application requiring true and accurate color reproduction. Examples include media photography, color inspection, food sorting, and microscopic observation. The design goal is to match human visual perception.

Visible-Near Infrared (VIS-NIR) Lens

Working Band: 400-1000 nm (can be extended to 1200 nm)

Core Application: The "versatile workhorse" of industrial machine vision. It can perform color inspection during the day and enable surveillance at night when paired with infrared (IR) illumination. Crucially, many materials exhibit unique characteristics under NIR light, enabling applications like internal defect detection in agricultural products, plastic sorting, paper moisture measurement, and silicon wafer inspection.

Short-Wave Infrared (SWIR) Lens

Working Band: 900-1700 nm

Core Application: An "insight" tool for material analysis. It utilizes the molecular absorption features of substances in this band for identification, such as pharmaceutical component analysis or grain protein/moisture quantification. Simultaneously, silicon is transparent in this band, enabling transmission-mode non-destructive testing for solar cells and integrated circuits. It can also see through certain types of smoke and plastic packaging.

Class 2: Thermal Radiation Imaging Lenses (rely on heat emitted by the object itself)
4. Mid-Wave Infrared (MWIR) Lens
* Working Band: 3-5 μm
* Core Application: The specialist for high-temperature and gas imaging. Many high-temperature targets like engine exhaust flames, welding points, and boilers emit their peak radiation in this band, making imaging highly sensitive. It is also key equipment for industrial gas leak detection (e.g., methane, carbon dioxide).

Long-Wave Infrared (LWIR) Lens

Working Band: 8-14 μm

Core Application: The mainstay for ambient-temperature thermal imaging. The peak thermal radiation from objects at ambient temperature, such as the human body, buildings, and equipment, falls within this atmospheric "window." It is widely used in security night vision, firefighting and search/rescue, building energy auditing, electrical preventive maintenance, and medical fever screening. The proliferation of uncooled thermal imagers has significantly reduced costs.

Q3: Why is it essential to match the lens spectrum to the camera sensor? What happens if they are mismatched?

A: The camera sensor and lens form a precisely matched imaging system. The sensor determines what light is received, and the lens controls what light is delivered. A mismatch leads to severe degradation of system performance, manifesting in the following specific issues:

Analysis of Specific Problems:

Severe Loss of Resolution and Contrast (Defocus & Chromatic Aberration)

Scenario: Using a standard visible light lens with a SWIR camera.

Consequence: The lens is designed for 400-700 nm light. When 1700 nm infrared light passes through, the focal point shifts significantly away from the sensor plane due to the glass's dispersion properties. This results in a completely blurred image where details are indistinguishable, and the MTF (Modulation Transfer Function) drops sharply.

Degraded Signal-to-Noise Ratio and Uneven Illumination (Low Transmittance & Vignetting)

Scenario: Using a standard lens for NIR imaging.

Consequence: The lens coatings have high reflectivity in the NIR band, potentially losing over 50% of the incoming light. To achieve acceptable image brightness, gain must be drastically increased or exposure time lengthened, introducing significant noise. Furthermore, light loss is more severe at the image edges, worsening the "vignetting" effect (center bright, corners dark).

Unavoidable Image Artifacts (Ghosting & Lens Flare)

Scenario: Using a mismatched lens in environments with strong light sources.

Consequence: Light outside the lens's designed band is repeatedly reflected between lens surfaces and the lens barrel, creating blurry spots that overlap with the scene (ghosting) or a hazy veil that reduces overall contrast (lens flare). These artifacts can interfere with detection algorithms or even mask defects.

Completely Inaccurate Thermal Imaging Data (Self-Emission Contamination)

Scenario: Attempting thermal imaging with a standard glass lens.

Consequence: Standard glass is opaque to mid- and long-wave IR, yielding no image. Even if IR-transmissive materials are used but the design is inappropriate, the radiation emitted by the lens itself due to its temperature adds directly to the target signal. This results in temperature readings that are a meaningless mix of "target + lens" radiation.

Conclusion: Choosing a matched lens is an absolute prerequisite for ensuring the imaging system achieves its specified performance (e.g., resolution, sensitivity, accuracy). It is never an optional consideration.

Q4: How do I systematically choose the right spectral lens for my application?

A: Follow this four-step method for efficient and accurate selection:

Step 1: Identify the Sensor – The Foundation of Choice

Check the Camera Datasheet: Locate the "Spectral Response" or "Working Band" parameter.

Key Correspondences:

Silicon-based CMOS/CCD (e.g., Sony, onsemi) → Typically paired with VIS or VIS-NIR lenses.

"Day/Night" or "IR-enhanced" cameras → Always require VIS-NIR lenses.

InGaAs cameras → Require SWIR lenses.

Cooled MCT (Mercury Cadmium Telluride) or InSb detectors → Require MWIR lenses.

Uncooled/Cooled VOx (Vanadium Oxide) or a-Si microbolometer detectors → Require LWIR lenses.

Step 2: Define Core Requirements – Determining Functional Focus

Ask yourself these key questions:

Do I need to see color? → If yes, the lens must cover the visible (VIS) spectrum.

Will I work in complete darkness (no visible light)? → If yes, you need NIR or SWIR (with illumination) or should opt for thermal imaging (LWIR).

Do I need to see through materials (e.g., plastics, silicon) or differentiate chemical composition? → If yes, strongly consider SWIR.

Is my goal to measure temperature or visualize heat distribution? → If yes, go directly to MWIR/LWIR selection, then refine based on temperature range (MWIR for high temp, LWIR for ambient temp) and budget (uncooled LWIR is generally lower cost).

Step 3: Evaluate Key Performance Parameters – Quantitative Comparison

Spectral Transmittance Curve: Verify that transmittance is high and flat across your required band (e.g., >90%).

Chromatic Aberration Correction: For VIS and VIS-NIR lenses, look for "apochromatic" designs. For SWIR/MWIR/LWIR, check if MTF curves are consistent across different wavelengths.

Relative Illumination (Vignetting): Ensure edge brightness fall-off is acceptable for your used band and sensor size (e.g., >70%).

Step 4: Consider Environment & Cost – Practical Implementation

Environmental Suitability: Does the thermal lens require an anti-condensation window? Does the industrial lens need liquid-protection coatings?

Total Cost of Ownership: Consider not just the lens price but also the system performance improvement, maintenance costs, and service life it enables. A properly matched, professional lens prevents countless hours of debugging and image quality issues down the line.