Double concave lenses are concave on both sides and have the same radius of curvature. Bifocal lenses with negative focal lengths are commonly used for beam collimation, increasing (decreasing) focal lengths, or enlarging (decreasing) images.
| SPECIFICATION | |
| Material | BK7 or Fused Silica |
| Design Wavelength | 546.1nm |
| Design Index | 1.5183+0.0005 |
| Diameter Tolerance | +0.00/-0.15mm |
| Thickness Tolerance | ±0.2mm |
| Paraxial Focal Length | ±2% |
| Centration | <3 arc minutes |
| Clear Aperture | >85% |
| Surface lrregularity | λ/4@632.8nm |
| Surface Quality | 40-20 scratch and dig |
| Protective Bevel | 0.25mm x 45° |
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