Both sides are concave, which can effectively refract light in a narrow space. Used to focus collimated light in the shortest possible distance, or to focus as much light as possible from a point source.

Model
Outer diameter
φD(mm)
focal length
f(mm)
Edge thickness
Te(mm)
central thickness
Tc(mm)
rear focal length
fb(mm)
radius of curvature
r(mm)
WNSL4K9-6-6
6
-6
3
1.5
-6.7
6.75
WNSL4K9-10-10
10
-10
4.6
2
-10.3
10.38
WNSL4K9-10-15
10
-15
3.6
2.0..
-15.3
15.57
WNSL4K9-10-20
10
-20
3.2
2
-20.3
20.76
WNSL4K9-12.7-15
12.7
-15
4.7
2
-15.4
15.57
WNSL4K9-12.7-25
12.7
-25
3.6
2
-25.4
59.95
WNSL4K9-15-15
15
-15
5.9
2
-15.3
15.57
WNSL4K9-15-20
15
-20
4.8
2
-20.3
20.76
WNSL4K9-15-50
15
-50
3.1
2
-50.3
51.9