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OPTICS / LENSES / Spherical Plano Concave Lenses
Diverging Lenses (Negative Lenses): Thinner in the middle, they cause parallel light rays to spread out, making them appear to emanate from a virtual focal point.
Negative Focal Length: Their focal length is considered negative, indicating their diverging nature.
Plano-Concave Lenses: One surface is flat (plano), and the other is concave.
Applications include: Stand-alone Expander; Galilean Beam Expander; Reducing Power Density; Aberration Correction (in multi-element systems) – common practice in achromatic doublets; Projection Systems
Users are most concerned with Reduced Spherical Aberration via different orientation/configurations.
Decide your required focal length (f), Numerical Aperture (NA), Anti-reflection (AR) coatings, Material (fused silica for UV, N-BK7 for visible, Germanium for IR)
PCV-SERIES
2.1) CaF2 / MgF2 / BaF2 Plano-Concave IR Lenses
Infrared (IR) is broadly categorized into:
Near-Infrared (NIR): 0.75 – 3 µm
Short-Wave Infrared (SWIR): 1 – 3 µm
Mid-Wave Infrared (MWIR): 3 – 5 µm
Long-Wave Infrared (LWIR): 8 – 14 µm
Materials for IR Lenses:
Traditional optical glass used for visible light is often opaque or performs poorly in the IR spectrum. Therefore, specialized materials are crucial e.g. Germanium (Ge); Silicon (Si), Zinc Selenide (ZnSe), Calcium Fluoride (CaF2) and Barium Fluoride (BaF2)
Users may be concerned with Thermal Effects (Athermalization) i.e. refractive index of IR materials can change significantly with temperature, causing focal shift (thermal defocus). Athermalization techniques (combinations of different materials or mechanical compensation) are critical for systems operating over varying temperatures (e.g., drones, outdoor surveillance).
2.2) Uncoated BK7 Plano-Concave Lenses
Two commonly encountered optical glasses are BK7 (or its lead-free variant, N-BK7) and B270.
SCHOTT’s N-BK7
This Borosilicate Crown Glass is a widely recognized lead and arsenic-free variant. Excellent optical quality with high homogeneity in refractive index, uniform transmission, and very low inclusion and bubble content.
Refractive Index (nd) approx. 1.517 (at 587.6 nm). This is considered a relatively low refractive index for optical glasses.
SCHOTT’s B270®
This super-white modified Soda-Lime Glass (Crown glass) offers outstanding optical performance with consistent, high transmittance across a broad range of wavelengths, from UV-A into the near-infrared; Refractive Index (nd) approx. 1.523 (similar to N-BK7);
Benefits from a fire-polished surface during manufacturing, which results in an outstandingly smooth surface quality.
2.3) Uncoated UVFS Plano-Concave Lenses
UVFS is a high-purity glass material derived from silicon dioxide. Unlike natural quartz, it’s synthetically produced, allowing for exceptional control over its chemical composition and internal structure. The “Ultra-Low Expansion” characteristic is achieved through a carefully controlled manufacturing process that results in a highly stable material.
Key Reasons for UVFS material:
Low Coefficient of Thermal Expansion (CTE) – In precision optical systems, temperature changes can cause lenses to expand or contract, leading to changes in focal length, aberrations.
Excellent Transmission from UV to Near-Infrared (NIR) – UVFS boasts high transparency across a very broad spectral range.
High Laser Damage Threshold – Crucial for applications involving high-energy lasers, such as in scientific research, laser material processing
2.4) DIA25.4mm BK7 Plano-Concave Lenses AR@400-750nm
BK7 features High Transmission (BK7 offers high linear optical transmission across the visible spectrum, approx. 380 nm to 700 nm) and Low Dispersion (BK7 has a relatively high Abbe number, around 64.17)
AR coatings are thin-film structures applied to optical surfaces to reduce reflections and increase transmission. They typically consist of one or more layers of dielectric materials with specific refractive indices and thicknesses. The primary principle behind their operation is optical interference.
Without an AR coating, around 4% of light is reflected at each air-to-glass surface of a BK7 lens due to Fresnel reflections. This loss can accumulate in systems with multiple optical elements, reducing overall light throughput and potentially causing ghost images or stray light.
2.5) DIA25.4mm BK7 Plano-Concave Lenses AR@600-1100nm
BK7 features High Transmission (BK7 offers high linear optical transmission across the visible spectrum, approx. 380 nm to 700 nm) and Low Dispersion (BK7 has a relatively high Abbe number, around 64.17)
AR coatings are thin-film structures applied to optical surfaces to reduce reflections and increase transmission. They typically consist of one or more layers of dielectric materials with specific refractive indices and thicknesses. The primary principle behind their operation is optical interference.
Without an AR coating, around 4% of light is reflected at each air-to-glass surface of a BK7 lens due to Fresnel reflections. This loss can accumulate in systems with multiple optical elements, reducing overall light throughput and potentially causing ghost images or stray light.