How prescription glasses are made
You select your frames, choose your lens type, place your order and a few days later your new prescription glasses arrive. But what happens in the time between ordering and receiving? People often think that a pair of prescription specs can be taken off the shelf, ready made - but the process is far more lengthy and involved. Here is the story of how your glasses are made:-
When you visit your optician they will determine whether you need glasses and if you do they will issue a prescription. This prescription consists of a few main elements - the sphere, the cylinder, the axis and sometimes a reading addition. (For more information click here). Another measurement that does not usually appear on your prescription is the PD (pupillary distance). This is not always necessary as we can use an average in most cases. However if we think it is advisable we may ask you to measure your own PD. (For more information click here).
When we process your order it is firstly checked, by a qualified member of staff, for any irregularities. It is easy for errors to be made when prescription details are entered and if we are concerned we will contact you. Your order is then placed in a tray with your frame and passed on to our lab technician.
There are thousands of different possible prescription permutations and this is why each pair of spectacles is custom made. Prescription lenses come in a round uncut form called a blank. There are a few different blank sizes with 65mm and 70mm being the most common. The lab technician will calculate which blank size is required and this depends on certain criteria including the size of your frame, your PD and the strength of your prescription. By doing this we can ensure that the cut lenses fit into the frame properly - in most cases the optical centre of the lenses should be in line with your pupils. Also the thickness of each lens can differ with different blank sizes.
The lab technician will order your lenses from a lens laboratory and the most straightforward prescriptions are available within a couple of working days. If the prescription is more complicated, or if you require a special lens coating, this will take longer.
When we receive your lens blanks they are checked for any faults and the technician will reorder new lenses if he/she is not happy.
CUTTING THE LENS
In our laboratory the lens cutting process involves two machines – the ‘tracer and blocker’ and the ‘edger’.
The frame and blank are placed in the first machine and the frame shape is traced and the lens power is measured. If the frame is semi-rimless the demonstration lens is used to trace the shape. The PD and cylinder axis are entered and the lens is ‘blocked’ – the machine matches the optical centre of the lens to your PD and aligns the cylinder, and a blocker is attached to the blank holding the lens at the correct orientation.
The blocked lens is transferred to the edger and this machine grinds the lens to the correct shape and size. A bevel is formed around the edge for full-rimmed and a groove for semi-rimless frames.
THE FINAL STAGES
The technician will use a hand edger to make small adjustments to the lens size and shape and will then fit the lens. Most plastic frames need to be heated for the lenses to be popped in, with metal frames screws are used to open and close the rims and with semi-rimless models a plastic wire is used to keep the lenses in place.
The glasses now undergo a final inspection. They are cleaned and the prescription is maesured on a focimeter – this checks that the lens powers are correct.
Your completed spectacles are handed to our dispatch department and are placed in a matching case, packaged up and posted out to you. You receive your bespoke prescription glasses.