How to compare cooled MWIR and uncooled MWIR lens when assembly?

From an engineering design perspective, this document provides a clear comparison of the key design points and core differences between cooled mid-wave infrared (MWIR, 3–5 μm) lenses and uncooled mid-wave infrared lenses. The content focuses on optics, mechanics, and assembly, and can be directly used for scheme reviews and technical disclosure.

I. Fundamental Differences

Cooled MWIR:Paired with InSb or HgCdTe cooled detectors, operating at cryogenic temperatures of 77K~100K. Equipped with a dewar, cold shield and cold filter, it features ultra-high sensitivity and is used in military applications, temperature measurement and remote sensing.

Uncooled MWIR:Paired with VOx or a-Si uncooled detectors, operating at ambient temperature without a dewar or cold shield. It is applied in security surveillance, industrial temperature measurement and lightweight payloads.

The design differences between the two types of lenses stem entirely from the operating temperatures and structural constraints of the detectors.

II. Common Design Considerations (General for MWIR)

Band and Materials

• Operating band: 3.0–5.0 μm
• Common lens materials: Ge, Si, ZnSe, ZnS, Sapphire
• Must be coated with MWIR anti-reflection coating to control absorption and reflection.

Chromatic Aberration and Secondary Spectrum

• Ge and Si have large dispersion differences, resulting in significant chromatic aberration in the mid-wave band. The design requires at least one Si lens and one Ge lens for achromatization.
• High-performance systems shall consider correction of secondary spectrum.

Thermal Difference (Thermal Drift) Control

• MWIR materials have a large dn/dT value; temperature variations will cause focal plane shift and MTF degradation.
• Thermal optical analysis (athermalization) must be performed in the design.

Stray Light and Ghost Images

• Mid-wave infrared has strong background radiation; reasonable design of stops, baffles and internal surface extinction is required.
• Avoid highly reflective surfaces and paraxial parallel surfaces.

III. Cooled MWIR Lenses: Core Design Considerations

1. Must Match the Dewar Cold Shield

Cooled detectors are integrated with a cold shield. The lens must meet the following requirements:

• Exit pupil strictly coincident with the cold shield (100% cold shielding)
• No vignetting at the field edge and no stray light leakage outside the cold shield
• Relative aperture F-number must match the cold shield, usually with a fixed F#

Consequences of mismatch: severe stray light, non-uniform responsivity, inaccurate temperature measurement, dark image edges.

2. Strictly Fixed Back Focal Length

• The cooled dewar features a rigid, fixed structure with non-adjustable back focal length.
• Extremely tight tolerance for lens image distance: ±0.01~±0.03 mm
• Fine-adjustment structures or trimmable spacers must be reserved in the design.

3. Mandatory Athermalization / Passive Athermalization

• Large temperature difference exists as the detector operates at cryogenic temperature while the lens works at ambient temperature.
• Passive athermalization must be achieved through material combination and structural thermal compensation.
• Common solutions: Al barrel + Si/Ge combination, or Invar, titanium alloy.

4. Ultra-High Requirement for Cold Optical Axis Alignment

• Optical axis of lenses → Dewar window → Cold filter → Detector photosensitive surface
• Coaxiality requirement: ≤0.01~0.02 mm
• Tilt control: ≤1′

Failure to meet these will result in vignetting, non-uniform responsivity and dark edge patches.

5. Key Structural Design Points

• High rigidity of the lens barrel for anti-vibration and anti-deformation
• High flatness of the dewar mating surface to avoid clamping stress
• Internal filling with dry nitrogen to prevent window condensation
• No focusing mechanism; generally fixed-focus

6. Ultra-High Image Quality Requirements

• Cooled detectors have small pixels (typically 10~15 μm)
• Performance close to the diffraction limit and high MTF required
• Strict control of distortion, coma and astigmatism

IV. Uncooled MWIR Lenses: Core Design Considerations

1. No Cold Shield Constraint, Flexible F-Number

• No cold shield, so no exit pupil matching required
• Flexible F-number design: F1.0~F4.0 commonly used
• Larger field of view and simpler structure

2. Back Focal Length Compatible with Focusing

• Uncooled Focal Plane Arrays (UFPA) tolerate slight defocusing
• Lenses are generally equipped with focusing mechanisms to compensate for thermal drift and assembly errors
• Loose back focal length tolerance: ±0.05~±0.1 mm

3. Relatively Low Thermal Difference Control Requirement

• Detector and lens operate at the same temperature
• Thermal drift can be compensated by focusing + software correction
• Low-cost systems may even omit strict athermalization

4. Lightweight, Low-Cost Structure

• Aluminum alloy commonly used for lens barrels
• Small number of lenses, typically 2 to 4 elements
• Supports various configurations: zoom, fixed-focus, macro, etc.

5. Moderate Image Quality Requirements

• Uncooled detectors have larger pixels: 12~17 μm
• Diffraction-limited performance not required; clear imaging and stable temperature measurement are sufficient
• Certain distortion and field curvature are acceptable

V. Summary Table of Core Differences: Cooled vs. Uncooled MWIR Lenses

VI. One-Sentence Summary of Design Differences

• Cooled MWIR lenses: Designed around “cold shield, cryogenic temperature, high image quality, rigid fixation”, with all efforts focused on coaxiality and athermalization.
• Uncooled MWIR lenses: Designed around “ambient temperature, low cost, focusable, lightweight”, with higher design freedom and fewer constraints.

If needed, I can further provide:

• Typical configurations of cooled MWIR lenses (Double-Gauss, Cooke, telecentric)
• Typical topologies of uncooled MWIR lenses with 2/3/4 elements
• Design specification comparison sheets ready for bidding