• Si-PCX
  • PCX-Lenses-Si-1
  • Si-plano-convex

Silicon (Si)
Plano-Convex Lenses

Plano-convex (PCX) lenses have a positive focal length and can be used to focus a collimated beam to the back focal point, to collimate light from a point source, or reduce the divergent angle of a diverging source. To minimize the introduction of spherical aberration, a collimated light source should be incident on the curved surface of the lens when using a PCX to focus a collimated light source; Similarly, the diverging light rays should be incident on the planar surface of the PCX lens when collimating a point source of light. These lenses are used in infinite and finite conjugate applications.

When deciding between a plano-convex lens and a bi-convex lens, both of which cause collimated incident light to converge, it is usually more suitable to choose a plano-convex lens if the desired absolute magnification is either less than 0.2 or greater than 5. Between these two values, bi-convex lenses are generally preferred.

Silicon offers high thermal conductivity and low density. However it has a strong absorption band at 9 microns, it is not suitable for use with CO2 laser transmission applications. Paralight Optics offers Silicon (Si) Plano-Convex Lenses are available with a broadband AR coating optimized for the 3 µm to 5 μm spectral range deposited on both surfaces. This coating greatly reduces the surface reflectance of the substrate, yielding high transmission and minimal absorption over the entire AR coating range. Check the Graphs for your references.




Silicon (Si)


Low Density & High Thermal Conductivity

Coating Options:

Uncoated or with Antireflection & DLC Coatings for the 3 - 5 μm Range

Focal Lengths:

Available from 15 to 1000 mm


Common Specifications:


Reference Drawing for

Plano-convex (PCX) Lens

Dia: Diameter
f: Focal Length
ff: Front Focal Length
fb: Back Focal Length
R: Radius
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

    Silicon (Si)

  • Type

    Plano-Concex (PCX) Lens

  • Index of Refraction

    3.422 @ 4.58 μm

  • Abbe Number (Vd)

    Not defined

  • Thermal Expansion Coefficient (CTE)

    2.6 x 10-6/ at 20℃

  • Diameter Tolerance

    Precison: +0.00/-0.10mm | High Precision: +0.00/-0.02mm

  • Thickness Tolerance

    Precison: +/-0.10 mm | High Precision: -0.02 mm

  • Focal Length Tolerance

    +/- 1%

  • Surface Quality (Scratch-Dig)

    Precison: 60-40 | High Precision: 40-20

  • Surface Flatness (Plano Side)


  • Spherical Surface Power (Convex Side)

    3 λ/4

  • Surface Irregularity (Peak to Valley)


  • Centration

    Precison: <3 arcmin | High Precision: <30 arcsec

  • Clear Aperture

    90% of Diameter

  • AR Coating Range

    3 - 5 μm

  • Transmission over Coating Range (@ 0° AOI)

    Tavg > 98%

  • Reflectance over Coating Range (@ 0° AOI)

    Ravg < 1.25%

  • Design Wavelength


  • Laser Damage Threshold

    0.25 J/cm2 (6 ns, 30 kHz, @3.3μm)



♦ Transmission curve of uncoated Si substrate: high transmission from 1.2 to 8 μm
♦ Transmission curve of AR-Coated Si substrate: Tavg > 98% over the 3 - 5 μm range
♦ Transmission curve of DLC + AR-Coated Si substrate: Tavg > 90% over the 3 - 5 μm range


Transmission Curve of AR-Coated (3 - 5 μm) Silicon Substrate


Transmission Curve of DLC + AR-Coated (3 - 5 μm) Silicon Substrate