I was in one of our practices recently and witnessed someone being measured for their first ever spectacles. As the patient was completely new to spectacles the dispensing optician was taking extra time to explain exactly what she was doing and why and it occurred to me that this is something even experienced spectacle wearers know little about, hence this blog!
So you have your prescription from the optometrist, which is a bit like getting one from your GP, useless until it is dispensed. The similarity doesn’t end there. In the same way that, should the pharmacist dispense an incorrectly made up prescription the patient will not only just not get better but may actively be further harmed.
Dispensing spectacles is the same. It isn’t just a matter of cutting a set of lenses into a frame and putting them on your nose! Modern lenses are sophisticated optical instruments. Design of the lens, both in the central area and as importantly the periphery is carefully optimized for your particular prescription to minimize any aberrations and distortions caused by the eye looking through different parts of the lens. In fact the most modern lenses are designed to take into account not only the original prescription but the shape and style of the frame, exactly where it fits on your face, and the type of things you will be using them for, only then will the computer design the exact lens for you.
However, whatever lens is prescribed needs accurate fitting and centering and the more accurate this is done, the better you see. That’s why we have invested in the latest technology in dispensing and measuring. In the hands of our fully qualified and GOC registered dispensing opticians this is a formidable tool and ensures you of the most accurate and well-fitted spectacles possible.
Your lifestyle, how and what you intend to use your spectacles for is an important consideration and the most modern and advanced lenses are designed with this in mind, so expect some questions, from the dispensing optician, on your lifestyle prior to the dispensing.
How do we measure so accurately? We use an amazing piece of computerized camera equipment developed by Hoya Lenses called the VisuReal. Not only does this take all the measurements we require with great accuracy but it also acts as a great way of demonstrating lens options and what different frames styles look like on your face. Particularly useful if you struggle to see without your specs on!
So what exactly do we measure? Firstly it is impossible to measure without the chosen frame on the patient's face. Fulfilling the prescription is very much a symbiosis of frame and lenses. It is worthwhile mentioning that not all frames are suitable for all prescriptions so it is really important to be guided in your frame choice by your dispensing optician.
Interpupillary Distance (PD) – Traditionally this has been the distance between the centre of your pupils when looking straight ahead. Today we understand that this just isn’t enough for the production of modern spectacles. Each lens must be individually position using your chosen spectacle frame. This measurement takes into account that, to paraphrase George Orwell, not all noses (or faces) are equal. In fact most are not! This measurement takes into account that the frame may be sitting slightly more to the left (or right) of centre.
Heights – It is important that the lenses are centered vertically so they are exactly the correct height in the frame, this is dependent on the design of lens. Our faces are very asymmetrical and so having the frame in the correct position on the face is vital for this measurement. This applies to modern Single Vision lenses as well as progressive lenses.
Pantoscopic Tilt – This is the angle that the frame sits against the front plane of your face. This is important because if you look through incorrectly angled spectacles aberrations and distortions are introduced.
Front Bow – Likewise the front bow of the frame will affect the horizontal angle that the eye looks through the lens and can also cause aberrations and distortions. Your dispensing optician will give careful consideration to the lens recommendation depending on your frame choice.
Back Vertex Distance (BVD) – This is the gap between the front of your eye and the back surface of the lens. The distance the lens is away from the eye alters the effective power of the lenses thus it is important to compensate for this in the final lens power. Naturally we need the frame on the face to measure this and we also need to know the original prescribing BVD that the optometrist used. If you are a little older and wear progressive or multifocal lenses we also need to know where the optometrist placed the reading addition when you were being tested, before, or behind, the distance lens.
Head width – This is exactly what it says. The distance the open spectacles sides need to be apart to give a snug, yet comfortable fit.
Side lengths – likewise we need to know the distance from the frame to the top of your ear, the length of the drop behind your ear and the angle of the drop.
Finally you will need to discuss with the dispensing optician the best type of coating for your lenses. Not all coatings are the same and that would be a blog in its own right but modern lenses are an amazing combination of incredible design and fantastic materials but, in the same way that all camera lenses are coated to stop reflections, it is the coating which gives you that pin sharp, reflection free vision. Modern coatings are designed to do different things. “Blue Control” for instance is designed for use with the plethora of mobile, tablet, computer and television screens we all use, cutting down on the particular type of glare that cause eye strain and tiredness, while “LongLife UV” cuts out ultraviolet light not just through the lenses but from the reflections off the rear surface.
You will notice that the name “dispensing optician” has appeared in italics throughout this blog and it is worthwhile a paragraph explaining this. In the hands of an expert dispensing optician the VisuReal digital dispensing system and lens design programme combine to give you the best, most effective and accurately fitting spectacles ever.
The Term “dispensing optician” is a protected title and shows that the holder has undergone 3 years of intensive academic and practical training. They will have passed the qualifying exams, and become a fellow, of a body such as the Association of British Dispensing Opticians and importantly be registered with the General Optical Council (GOC) the body charged with safe practice and patient safety in UK optics. Don’t compromise on your eyes, always check that you are seeing a registered dispensing optician.