Berkshire-based Polymer Optics has combined refractive and reflective LED optics to produce a narrow beam from a small package, and introduced a novel manufacturing technique to make the device.
Most LEDs are focussed by reflectors or refractive collimators, also known as TIRs - total internal reflectors.
"With a standard TIR, they are a big glob of plastic which gives manufacturing issues," chief designer Mike Hanney old EW. "And there is always some loss with path length, and there are some ray angles that don't hit the TIR. Efficiency is commonly 80-85%."
For the same sized light source, narrower beams need larger optics because a longer focal length is required. "Its the laws of physics," said Hanney.
His design brief was to get a +/-5 degrees beam from Cree's MC-E led which has four 1x1mm die arranged in a square - an active area more than 2mm across.
"We had to make the whole thing to a size which would have been a big moulding challenge and would loose light in the longer light paths," said Hanney.
Instead, he broke with tradition and the result is a reflector with a refractive Fresnel lens suspended at its centre (see the photo). The lens focuses light which would have otherwise missed the reflector and been lost at wide angles.
Reflectors are usually made by metalising a plastic moulding, or by metal 'spinning' where a disc is plastically deformed over a rotating mandrill.
Polymer Optics has developed a completely different technique, using the force available in plastic moulding machines.
It involves bending metal for the reflector into shape at the same time as moulding the plastic.
"The technique is much easier to do than trying to put a vacuum coating onto plastic. We introduce a 98% reflective flat metal shim into the moulding tool which then also acts as a press tool," said Hanney. "While the metal is held in the correct optical form, we can mould plastic around it. There is no spring-back and we can add location features and flanges."
The shim material is hard anodised which not only takes a high polish, but is also environmentally stable.
"Its a quite expensive toolset, but it manufactures considerably cheaper than metal spinning - even from China where the tolerance is typically worse than we want, and it gives a better surface finish, said Hanney.
Once the formed reflector, which is 30mm in diameter, drops out of the press, the Fresnel lens simply clips in.
"We get collection efficiencies in the order of 90% and above," claimed Hanney, who wants to expand the concept. "The technique will broaden out very nicely, we are looking at larger LED arrays."
The MC-E version has a diffuser to remove the image of the gap between its die. "The additional loss is only a few per cent of the total light output as the diffusers are extremely fine and little back-scatter goes back to the reflector," said Hanney.
There is also a version for single 1x1mm emitters that produces a +/-2.5 degree beam - intended for architectural and theatrical lighting with a large number of emitters for high intensity lights.
And there is a colour-mixing version.
"We can take advantage of the higher efficiency of the optic and have developed a little colour mixing chamber - we have made one for clusters of four Rebel LEDs," said Hanney, who claims you can not see and any projected colour variation, or any spots of colour when looking into the optic. "Depending on the LED, you get 60 to 70% efficiency," he said. "This sounds a little low, but we defy anyone else to get anywhere near that."
For a full colour output range, completely variable between the primary colours, red, green and blue LEDs can be mixed, although "you don't get a particularly good colour rendering index [CRI]," said Hanney. "RGB amber gives better CRI."
In many applications, customers want white, but white with a variable colour temperature.
"People want it for accent lighting or in shops, either preset or controllable," said Hanney. "For Luxeon Rebels, typically white LEDs are supplemented with a red or an amber to give a controllable bias to warmer white, or you can start with a warm white and add a little blue."