How are spectacle lenses manufactured?

Lenses are the most important part of a pair of glasses and do the most to ensure you enjoy optimum vision. But first the lenses go on quite a journey before ending up in your frames. BETTER VISION explains how both glass and plastic spectacle lenses are made. What are lenses made of? And what exactly sets individualised lenses apart from those you get "off the shelf"?

Cornea The cornea, the eye’s external layer, is moist on account of the tear fluid that covers it. It is embedded in what’s known as the sclera (white part of the eye); together, they form what experts call the tunica externa bulbi. The cornea acts like a window: it is disc-shaped and transparent, and lets light into the eye. It also protects the eye against external influences such as dirt, dust or superficial injury. It is naturally very resilient. What’s more, its curvature lends it optical qualities and plays a key role in helping us see clearly.

This light is then reflected back by the object and processed by our visual apparatus. If we look at a tree, our eyes absorb the light it reflects: the rays first penetrate the conjunctiva and the cornea. Next, they pass through the anterior chamber and the pupil. Then, the light arrives at the eye’s lens, where it is collected and transferred to the photosensitive (=light-sensitive) retina.

There, the visual information is gathered and sorted: the rods are responsible for light-to-dark vision, and the cones are in charge of clarity and colours. This information is transferred to the optic nerve, which takes it directly to the brain, where it is once again assessed, interpreted and consolidated to form the image that we ultimately see. Even though we have detailed findings relating to the anatomy of the human eye and its structure, many questions about how our consciousness works remain very much unanswered. So while you know what parts of your brain are the most active when you see something, no one quite knows how we perceive the world as a result of this. Seeing things close up and far away

This dynamic ability to see objects clearly at different distances is known as accommodation. It is based on the elasticity of our eye’s lens. As long as there is no impairment, it can change its shape and thus adapt to objects close up or far away, depending on what we want to see. A normal eye’s lens is flat and long – which is ideal for looking at objects in the distance. But if we look at an object close up, the lens becomes more curved: it switches to close range and allows us to see nearby objects clearly. Accommodation is always triggered when objects appear blurred on the fovea. Seeing objects during the day – how our eyes workSeeing objects when there’s plenty of light (photopic vision or daytime vision) is a task assumed by the sensory cells responsible for colour vision: the cones. The pupil is also involved in daytime vision: the brighter it is, the smaller the pupil becomes. It adapts to different light intensities and regulates the amount of light that enters the eye. This quality is known as adaptation.

Night and dusk vision At night, our eyes switch from daytime vision (photopic vision) to nighttime vision (scotopic vision). Healthy eyes need around 25 minutes to adapt to the dark. The less light that is available, the more active the eye’s sensory cells will become; they’re responsible for our light-to-dark vision and are known as rods. At the same time, the pupils widen to “let in” as much light as possible. Healthy eyes have no trouble adapting to changing light conditions. Hereditary diseases, certain medication, injuries and a vitamin A deficiency can all result in limited vision at night or dusk. This is a problem that affects many people who wear glasses. Pupils need to dilate more in reduced lighting conditions.

The i.Scription^® Technology from ZEISS takes into consideration the wearer’s dilated pupils at night in the lens design, helping to considerably improve visual performance in low light conditions. And did you know that our light-and-dark vision also plays a role when it comes to safety on a plane? During takeoff and landing, the cabin lights are dimmed so that passengers’ and crew members’ eyes can adapt immediately to the new light conditions in the event of a crash. This can save precious seconds in an emergency. Vision problems and eye diseases – what to do if your vision is restricted Short-sightedness, long-sightedness, presbyopia – there are many vision problems that can limit our visual perception. In most cases, an optimally fitted pair of glasses with the right lenses can help you see clearly once more. BETTER VISION explains: What type of lens is suitable for different visual impairments? Many eye diseases can have an impact on our vision – and dire consequences for the way we perceive the world around us. This includes everything from the more minor eye diseases such as chronically dry eyes, vitreous opacities and squinting to cataract, glaucoma and macular degeneration.

But what are the most common eye diseases and how can you recognise them? So are all of these terms and processes mixed up in your head? Don’t sweat it!

Hardly any other one of our senses gives us so much information about our environment, daily life or the people around us – and, ultimately, about ourselves. This might also interest you The complexities of the human eye – from the blind spot and macula to focused/central (or foveal) and peripheral vision How does colour vision work? What is colour blindness and what can people affected by it do?