CaF2 Bi-Concave Lenses
|Substrate Material||Vacuum-Grade Calcium Fluoride|
|Wavelength Range||0.18 - 8.0 μm|
|AR Coating Range||3 - 5 μm |
|Reflectance over Coating Range (Avg.)||<2.0%|
|Diameters Available||1/2" or 1"|
|Diameter Tolerance||+0.00/-0.10 mm|
|Thickness Tolerance||±0.1 mm|
|Focal Length Tolerance||±1%|
|Surface Quality||40-20 (Scratch-Dig)|
|Spherical Surface Power*||3λ/2|
|Spherical Surface Irregularity|
(Peak to Valley)
|Clear Aperture||>90% of Diameter|
|Design Wavelength||588 nm|
*Much like surface flatness for flat optics, spherical surface power is a measure of the deviation between the surface of the curved optic and a calibrated reference gauge, typically for a 633 nm source, unless otherwise stated. This specification is also commonly referred to as surface fit.
- Vacuum-Grade Calcium Fluoride Substrate
- Ø1/2" and Ø1" Versions Available
- Available Uncoated or with a Broadband AR Coating for the 3 - 5 μm Range
- Focal Lengths from -15.0 to -50.0 mm
Thorlabs' Ø1/2" and Ø1" Calcium Fluoride (CaF2) Bi-Concave Lenses, which offer high transmission from 0.18 - 8.0 μm, are available uncoated or with a broadband AR coating optimized for the 3 - 5 μm spectral range deposited on both surfaces. This coating greatly reduces the surface reflectivity of the substrate, yielding an average transmission in excess of 96.8% over the entire AR coating range. See the Graphs tab for detailed information.
CaF2 is commonly used for applications requiring high transmission in the infrared and ultraviolet spectral ranges. Its extremely high laser damage threshold makes it useful for use with excimer lasers. The material exhibits a low refractive index, varying from 1.35 to 1.51 within its usage range of 180 nm to 8.0 μm. Calcium fluoride is also fairly chemically inert and offers superior hardness compared to its barium fluoride, magnesium fluoride, and lithium fluoride cousins.
Bi-concave lenses have negative focal lengths, making them useful for a wide range of applications.They are often used to increase the divergance of a converging beam. In optical systems, it is common for researchers to choose their optics carefully so that the aberrations introduced by the positive- and negative-focal-length lenses approximately cancel. Others use these lenses in pairs to increase the effective focal length of a converging lens.
When deciding between a plano-concave lens and a bi-concave lens, both of which cause the incident light to diverge, it is usually preferrable to choose a bi-concave lens if the absolute conjugate ratio (object distance divided by image distance) is close to 1. When the desired absolute magnification is either less than 0.2 or greater than 5, the tendency is to choose a plano-concave lens instead.
Shown above is a theoretical graph of the percent reflectivity of the AR coating as a function of wavelength. The average reflectivity in the 3 - 5 μm range is <2.0%. The blue shading indicates the region for which the AR coating is optimized.
Shown above is a graph of the theoretical transmission of an AR-coated calcium fluoride plano-convex lens. The blue shaded region denotes the 3 - 5 μm spectral range where the AR coating is optimized. For this wavelength range, the measured transmission is in excess of 95%.
Total Transmission of Optic (CaF2 Substrate, Uncoated)
The table below gives the approximate theoretical transmission of these uncoated optics for a few select wavelengths in the 0.18 - 8.0 μm range. To see an excel file that lists all measured transmission values for this wavelength range, please click here.
|Wavelength (μm)||Total Transmission|
Total Transmission of Optic (CaF2 Substrate + AR Coating)
The table below gives the approximate theoretical transmission of these AR-Coated optics for a few select wavelengths in the 0.18 - 8.0 μm range. To see an excel file that lists all measured transmission values for this wavelength range, please click here. Please note that the transmission values stated for wavelengths outside of the AR coating range are approximate and can vary significantly by coating lot.
|Wavelength (μm)||Total Transmission|
Wavelength-Dependent Focal Length Shift
The paraxial focal length of a lens is wavelength dependent. The focal length listed under the Specs tab for a given lens corresponds to the value at the design wavelength (i.e., the focal length at 588 nm). Since CaF2 offers high transmission from 0.18 - 8.0 μm, users may wish to use these lenses at other popular wavelengths. Below is a table that gives the focal length (in millimeters) for each lens at fifteen popular wavelengths within the 0.18 - 8.0 μm range.
The first column lists the fifteen wavelengths for which the focal length was calculated. The first row lists the item numbers. Trace across the row of choice to the column of choice to find the focal length corresponding to that lens and that wavelength. For example, the focal length of an LD5788 bi-concave lens when used with 8000 nm light is -31.08 mm. Note that the row denoting the focal lengths at the design wavelength is highlighted in orange.
Ø1/2" Bi-Concave Lenses
| 248 nm|| -13.88 mm|| -23.14 mm|
| 405 nm|| -14.73 mm|| -24.55 mm|
| 488 nm|| -14.89 mm|| -24.81 mm|
| 532 nm|| -14.94 mm|| -24.90 mm|
| 588 nm|| -15.00 mm|| -24.99 mm|
| 633 nm|| -15.03 mm|| -25.05 mm|
| 780 nm|| -15.11 mm|| -25.17 mm|
| 850 nm|| -15.13 mm|| -25.22 mm|
| 1064 nm|| -15.19 mm|| -25.31 mm|
| 1310 nm|| -15.24 mm|| -25.39 mm|
| 1550 nm|| -15.28 mm|| -25.46 mm|
| 3000 nm|| -15.58 mm|| -25.96 mm|
| 4000 nm|| -15.90 mm|| -26.49 mm|
| 5000 nm|| -16.33 mm|| -27.21 mm|
| 8000 nm|| -18.67 mm|| -31.08 mm|
Ø1" Bi-Concave Lenses
| 248 nm|| -27.78 mm|| -46.32 mm|
| 405 nm|| -29.47 mm|| -49.12 mm|
| 488 nm|| -29.78 mm|| -49.64 mm|
| 532 nm|| -29.89 mm|| -49.82 mm|
| 588 nm|| -30.00 mm|| -50.00 mm|
| 633 nm|| -30.06 mm|| -50.11 mm|
| 780 nm|| -30.21 mm|| -50.36 mm|
| 850 nm|| -30.26 mm|| -50.44 mm|
| 1064 nm|| -30.38 mm|| -50.63 mm|
| 1310 nm|| -30.47 mm|| -50.78 mm|
| 1550 nm|| -30.55 mm|| -50.92 mm|
| 3000 nm|| -31.16 mm|| -51.92 mm|
| 4000 nm|| -31.79 mm|| -52.97 mm|
| 5000 nm|| -32.65 mm|| -54.40 mm|
| 8000 nm|| -37.27 mm|| -62.09 mm|