Output lenses for LEDs and a method of forming an output lens

1. An optical system, comprising:

a set of inner portions, each for mounting over an LED or LED arrangement, and each of which performs a different first optical function, wherein each inner portion has an outer contact surface; and

an outer portion, for mounting over the inner portion, and which performs a second optical function, and wherein the outer portion has a cavity with an inner contact surface, and an outer lens surface,

wherein the outer portion is adapted to be mountable over each one of the set of inner portions, thereby to form an output lens when the outer portion is mounted over a selected one of the set of inner portions, which output lens has an overall optical function which depends on the selected inner portion,

wherein the inner and outer portions have the same refractive index, and

wherein the outer contact surface of the inner portions and the inner contact surface of the outer portion have a matching shape such that an interface between the inner portion and the outer portion does not have a lens function.

2. An optical system as claimed in claim 1 , wherein the inner and outer portions are made from the same material.

3. An optical system as claimed in claim 1 , further comprising a glue for insertion between the inner portion and the outer portion.

4. An optical system as claimed in claim 1 , wherein the inner portions are formed of silicone.

5. An optical system as claimed in claim 1 , wherein the inner portions comprise a cavity for mounting over the LED or LED arrangement.

6. An optical system as claimed in claim 1 , wherein the inner portions comprise a flange and the outer portion comprises a recess for receiving the flange.

7. An optical system as claimed in claim 6 , wherein the outer portion comprises a channel around the recess.

8. An LED output lens formed from the optical system as claimed in claim 1 .

9. A lighting unit comprising:

an LED or LED arrangement; and

an LED output lens as claimed in claim 8 mounted over the LED or LED arrangement.

10. A lighting unit as claimed in claim 9 , wherein the LED or LED arrangement comprises a chip on board LED arrangement or a set of chip scale packages.

11. A lighting unit as claimed in claim 9 , comprising a plurality of combinations of an outer lens portion with an inner lens portion, wherein at least two of said combinations have a respective, but mutually different, optical lens activity.

12. A method of forming an output lens for an LED or LED arrangement, comprising:

selecting an inner portion from a set of different inner portions, for mounting over the LED or LED arrangement, wherein each inner portion of the set performs a different first optical function, wherein each inner portion has an outer contact surface; and

mounting an outer portion over the selected inner portion, which outer portion performs a second optical function, wherein the outer portion has a cavity with an inner contact surface, and an outer lens surface, and the outer portion is mounted over the inner portion with the inner contact surface contacting the outer contact surface to form an interface and to define a unitary optical component with an overall optical function which depends on the first and second optical functions and wherein the outer contact surface of the inner portion and the inner contact surface of the outer portion have matching shape and have a same index of refraction such that the interface does not have a lens function.

13. A method as claimed in claim 12 , further comprising providing a glue between the inner portion and the outer portion.

14. A method as claimed in claim 12 , wherein the inner portion comprises a cavity, wherein the method comprises mounting the lens over the LED or LED arrangement with the LED or LED arrangement in the cavity.

15. A method as claimed in claim 12 , comprising mounting a flange of the inner portion in a recess of the outer portion.