• DCV-Lenses-CaF2-1

Calcium Fluoride (CaF2)
Bi-Concave Lenses

Bi-concave or Double-concave (DCV) lenses are negative lenses which are thicker at the edge than in the middle, when light passes through them, it diverges and the focus point is virtual. Bi-Concave lenses have an equal radius of curvature on both sides of the optical system, their focal lengths are negative, as well as the radii of curvature of the curved surfaces. Negative focal length causes collimated incident light to diverge, they are often used to diverge a convergent beam. Due to their features, bi-concave lenses are generally used to expand light in the Galilean-type beam expanders or increase effective focal length of a converging lens by using in pairs in existing systems such as light projection systems. They are also useful when it comes to imaging reduction. In optical systems, it is common to choose their optics carefully so that the aberrations introduced by the positive- and negative-focal-length lenses approximately cancel. These negative lenses are commonly used in telescopes, cameras, lasers or glasses to help magnification systems be more compact.

Bi-concave lenses (or double-concave lenses) are the best choice when object and image are at absolute conjugate ratios (object distance divided by image disdance) closer to 1:1 with converging input beams, as is the case with bi-convex lenses. They are used for replay imaging (virtual object and image) applications. When the desired absolute magnification is either less than 0.2 or greater than 5, plano-concave lenses are usually more suitable.

Due to its high transmission from 0.18 µm to 8.0 μm, Calcium fluoride 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, it has a refractive index of 1.428 at 1.064 µm. CaF2 is also fairly chemically inert and offers superior hardness compared to its barium fluoride, and magnesium fluoride cousins. Its extremely high laser damage threshold makes it useful for use with excimer lasers. Paralight Optics offers Calcium Fluoride (CaF2) Bi-concave lenses with antireflection coatings for either the 3 to 5 µm wavelength range. This coating greatly reduces the average reflectance of the substrate less than 2.0%, yielding a high average transmission greater than 96% across the entire AR coating range. Check the following Graphs for your references.




Calcium Fluoride (CaF2)


Uncoated or with Antireflection Coatings

Focal Lengths:

Available from -15 to -50 mm


Suitable for Use in Excimer Laser Applications, in Spectroscopy and Cooled Thermal Imaging


Common Specifications:


Reference Drawing for

Double-Concave (DCV) Lens

f: Focal Length
fb: Back Focal Length
ff: Front 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.


Ranges & Tolerances

  • Substrate Material

    Calcium Fluoride (CaF2)

  • Type

    Double-Concave (DCV) Lens

  • Index of Refraction

    1.428 @ Nd:Yag 1.064 μm

  • Abbe Number (Vd)


  • Thermal Expansion Coefficient (CTE)

    18.85 x 10-6/℃

  • Diameter Tolerance

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

  • Thickness Tolerance

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

  • Focal Length Tolerance


  • Surface Quality (scratch-dig)

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

  • Spherical Surface Power

    3 λ/2

  • Surface Irregularity (Peak to Valley)


  • Centration

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

  • Clear Aperture

    90% of Diameter

  • AR Coating Range

    3 - 5 μm

  • Transmission over Coating Range (@ 0° AOI)

    Tavg > 95%

  • Reflectance over Coating Range (@ 0° AOI)

    Ravg < 2.0%

  • Design Wavelength

    588 nm



♦ Transmission curve of uncoated CaF2 substrate: high transmission from 0.18 to 8.0 μm
♦ Transmission curve of AR-coated CaF2 Lens: Tavg > 95% over the 3 - 5 μm range
♦ Transmission curve of enhanced AR-coated CaF2 Lens: Tavg > 95% over the 2 - 5 μm range


Transmission Curve of AR-coated (3 µm - 5 μm) CaF2 Lens


Transmission Curve of Enhanced AR-coated (2 µm - 5 μm) CaF2 Lens