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  • CaF2-DCX-(1)

Calcium Fluoride (CaF2)
Bi-Convex Lenses

Both surfaces of Bi-convex or Double-convex (DCX) Spherical Lenses are spherical and have the same positive radii of curvature, they are positive lenses which are thicker in the middle than at the edge. When collimated rays pass through them, light converges to a physical focal point. Bi-convex are popular for many finite imaging applications in situations where the object and image are on opposite sides, they are designed to have a focal length of f= (R1*R2)/((n-1)*(R2-R1)).

Bi-convex lenses (or double-convex lenses) perform better when the object is closer to the lens and the conjugate ratio is low. When the object and image distance are equal (1:1 magnification), not only is spherical aberration minimized, but also distortion, and chromatic aberration are canceled due to the symmetry. So they are best choices when object and image are at absolute conjugate ratios close to 1:1 with diverging input beams. As a rule of thumb, bi-convex lenses perform well within minimum aberration at conjugate ratios between 5:1 and 1:5, they are used fo relay imaging (Real Object and Image) applications. Outside this range, plano-convex lenses are usually more suitable.

Due to its high transmission from 0.18 µm to 8.0 μm, CaF2 exhibits a low refractive index varying from 1.35 to 1.51 and is commonly used for applications requiring high transmission in the infrared and ultraviolet spectral ranges. Calcium fluoride is also fairly chemically inert and offers superior hardness compared to its barium fluoride, and magnesium fluoride cousins. Paralight Optics offers Calcium Fluoride (CaF2) Bi-Convex Lenses available with a broadband AR coating optimized for the 2 µm to 5 μm spectral range deposited on both surfaces. This coating greatly reduces the average reflectance of the substrate less than 1.25%, yielding an average transmission in excess of 95% over the entire AR coating range. Check the following Graphs for your references.

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Features:

Material:

Calcium Fluoride (CaF2)

Available:

Uncoated or with Antireflection Coatings

Focal Lengths:

Available from 15 to 200 mm

Applications:

Ideal for Use with Excimer Lasers

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Common Specifications:

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Reference Drawing for

Double-convex (DCX) Lens

Dia: Diameter
f: Focal Length
ff: Front Focal Length
fb: Back Focal Length
R: Radius of Curvature
tc: Center Thickness
te: Edge Thickness
H”: Back Principal Plane

Note: The focal length is determined from the back principal plane, which does not necessarily line up with the edge thickness.

Parameters

Ranges & Tolerances

  • Substrate Material

    Calcium Fluoride (CaF2)

  • Type

    Double-Convex (DCX) Lens

  • Index of Refraction (nd)

    1.434 @ Nd:Yag 1.064 μm

  • Abbe Number (Vd)

    95.31

  • Thermal Expansion Coefficient (CTE)

    18.85 x 10-6/℃

  • Diameter Tolerance

    Precision: +0.00/-0.10mm | High Precision: +0.00/-0.03 mm

  • Thickness Tolerance

    Precison: +/-0.10 mm | High Precison: +/-0.03 mm

  • Focal Length Tolerance

    +/-0.1%

  • Surface Quality (scratch-dig)

    Precison: 80-50 | High Precison: 60-40

  • Spherical Surface Power

    3 λ/4

  • Surface Irregularity (Peak to Valley)

    λ/4

  • Centration

    Precison: <3 arcmin | High Precison: <1 arcmin

  • Clear Aperture

    90% of Diameter

  • AR Coating Range

    2 - 5 μm

  • Reflectance over Coating Range (@ 0° AOI)

    Ravg < 1.25%

  • Transmission over Coating Range (@ 0° AOI)

    Tavg > 95%

  • Design Wavelength

    588 nm

  • Laser Damage Threshold

    >5 J/cm2 (100 ns, 1 Hz, @10.6μm)

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Graphs

♦ Transmission curve of 10 mm thick, uncoated CaF2 substrate: high transmission from 0.18 µm to 8 μm
♦ Transmission curve of enhanced AR-coated CaF2: Tavg > 95% over the 2 µm - 5 μm range

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Transmission Curve of Enhanced AR-Coated (2 µm - 5 μm) Calcium Fluoride

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