Product Description
Precision λ/20 Fused Silica Reference Mirror – 50mm Diameter, Ex-US Defense Lab Surplus
This elite single-surface reference mirror is constructed from premium fused silica, a high-purity synthetic quartz glass acclaimed for its ultra-low thermal expansion (5.5 × 10⁻⁷/°C) , broad spectral transmission from UV to IR (185nm–2.5μm), and outstanding resistance to thermal shock, chemical agents, and environmental stressors . Boasting a 50mm diameter and robust 16mm thickness, it delivers superior structural integrity, reducing flexure and maintaining stability in high-precision or variable-temperature setups—critical for defense and aerospace environments.
The front surface is enhanced with a defense-grade protected silver coating, featuring advanced multi-layer dielectric overcoats to prevent oxidation, tarnishing, and degradation while achieving exceptional reflectivity of 97–99% (or higher) across the visible spectrum (0.4–0.7μm) , with averages often exceeding 98% at low angles of incidence . This optimized coating, compliant with MIL-C-48497 standards for durability, ensures near-total light reflection without measurable transmission, producing razor-sharp, vivid images. The back surface, inscribed with “λ/20” (denoting the front’s flatness spec), incorporates a matte or commercial polish to suppress stray reflections and avoid measurement artifacts.
The λ/20 flatness tolerance—verified at 632.8nm (HeNe laser wavelength)—signifies a peak-to-valley surface deviation of just 31.6nm, embodying cutting-edge precision through sophisticated polishing and interferometric certification . This level outstrips standard λ/4 or λ/10 optics, enabling sub-wavelength resolution in rigorous applications .
Key Specifications
| Specification | Details |
|---|---|
| Manufacturer | US Defense Laboratory contractor (comparable to Edmund Optics TECHSPEC® grade) |
| Type | Single-surface reference mirror |
| Substrate Material | High-purity fused silica (Corning 7980 equivalent) |
| Diameter | 50 mm |
| Thickness | 16 mm (substantially thicker than standard 5mm commercial mirrors) |
| Surface Flatness (P-V) | λ/20 @ 632.8 nm (measured pre-coating) |
| Surface Quality | 20-10 scratch-dig (estimated, consistent with λ/20 grade) |
| Clear Aperture | >90% |
| Parallelism | <30 arc seconds (typical for precision mirrors) |
| Front Coating | Enhanced protected silver with dielectric overcoats |
| Coating Type | Metal with protective dielectric overcoat |
| Reflectivity | 97-99% average across visible spectrum (450-700nm) |
| Coating Durability | MIL-C-48497 compliant (abrasion, humidity, adhesion) |
| Back Surface | Commercial polish, uncoated, with handwritten “λ/20” marking |
| Edge Finish | Ground, protective bevel |
| Provenance | Ex-US Defense Research Laboratory surplus |
| Condition | Excellent – minimal use, optically pristine |
Material Advantages – Premium Fused Silica
Fused silica is the substrate of choice for precision reference optics due to its exceptional properties :
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Ultra-Low Thermal Expansion (5.5 × 10⁻⁷/°C): Maintains figure across temperature variations – critical for metrology applications
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Broad Spectral Transmission: 185nm to 2.5μm, covering UV through IR
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High Homogeneity: Minimizes wavefront distortion
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Superior Durability: Resists chemical attack and environmental degradation
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Proven Performance: Industry standard for precision optics (Corning 7980 or equivalent)
The 16mm thickness is a significant differentiator from standard 5mm commercial mirrors . This extra thickness provides:
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Enhanced structural rigidity – minimizes flexure under mounting stress
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Superior thermal mass – dampens rapid temperature fluctuations
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Reduced mounting-induced distortion – maintains λ/20 flatness in fixtures
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Long-term stability – ideal for reference standards
Defense-Grade Protected Silver Coating
Silver coatings offer the highest reflectivity in the visible spectrum but are prone to tarnishing without proper protection . This mirror features an advanced protected silver coating with dielectric overcoats that:
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Prevents oxidation and tarnishing – maintains performance over decades
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Achieves >98% average reflectivity across 450-700nm
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Complies with MIL-C-48497 for durability, including abrasion, adhesion, and humidity resistance
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Enables safe handling and cleaning – the protective overcoat allows for careful cleaning without damaging the reflective layer
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Minimizes scatter for clean interferometric fringes
The coating specification is consistent with Edmund Optics’ TECHSPEC® protected silver mirrors, which achieve Ravg >98% across the visible spectrum .
λ/20 Flatness – What It Means
The λ/20 surface flatness specification (peak-to-valley at 632.8nm) represents a surface deviation of just 31.6 nanometers – approximately 1/2000th the thickness of a human hair. This level of precision:
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Exceeds standard λ/10 or λ/4 optics, enabling diffraction-limited performance
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Minimizes reflected wavefront error for accurate interferometric measurements
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Ensures reliable calibration of metrology instruments
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Certifies the mirror as a true reference standard
Surface flatness is measured before coating, ensuring the underlying substrate meets the specification . The coating process maintains this figure without degradation.
Applications and Uses
Reflection Interferometry:
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Ideal for Fizeau, Twyman-Green, or Michelson interferometers to evaluate wavefront integrity, surface flatness, or aberrations in reflective optics like mirrors, lenses, or prisms
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The superior coating enhances fringe contrast for unmatched measurement accuracy
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λ/20 flatness ensures reference-quality wavefronts
Metrological Precision:
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Serves as a reference standard in optical and dimensional metrology for calibrating autocollimators, profilometers, or CMMs
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Quantifies flatness via interference fringes (e.g., Newton’s rings) to sub-wavelength levels, ensuring ISO 17025/NIST-traceable results in quality assurance labs
Laser and Photonics Systems:
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Crucial for beam alignment, cavity tuning, and mode profiling in high-energy lasers, fiber optics, or telecom gear
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Low-expansion substrate and high-reflectivity coating prevent thermal-induced errors
Aerospace and Defense Metrology:
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Employed in inspecting satellite components, targeting optics, or avionics for parallelism and surface quality
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Leverages MIL-spec ruggedness for reliable performance in vibration-prone or extreme conditions
Semiconductor and Precision Manufacturing:
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Verifies wafer flatness, photomasks, or gauge blocks with sub-micron precision
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Supports advanced fabrication in chips or CNC processes
Scientific Research:
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Facilitates holography, spectroscopy, or adaptive optics experiments
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Capitalizes on fused silica’s broadband capabilities and λ/20 accuracy for precise wavefront analysis and correction
Condition
EXCELLENT – MINIMAL USE
OPTICALLY PRISTINE
NO VISIBLE SCRATCHES, COATING DAMAGE, OR DEFECTS
“λ/20” INSCRIPTION CLEARLY VISIBLE ON BACK SURFACE
STORED AND HANDLED WITH CARE
READY FOR IMMEDIATE INTEGRATION
⚠ HANDLING NOTE: This is a precision reference mirror with a delicate protected silver coating. While the dielectric overcoat improves durability , always handle only by edges with clean gloves. Never touch coated surfaces. Clean only with approved optical methods if absolutely necessary. Store in clean, dry environment when not in use.



