DISPENSING TIPS OF ANTI-REFLECTION COATED LENS

 Dispensing Tips of Anti-reflection Coated Lens

    The normal eye is easily adapted to different lighting conditions. Thus, the loss of light due to reflection is usually not serious enough to reduce visibility except where poor lighting condition already exist. It is the reflected light which enters the pupil of the eye along the same path as the light used in “seeing” an object that is of primary importance. Whenever light is incident on the boundary between two medium, some light is reflected and some is transmitted, and the transmitted light undergoes refraction into the second medium.

    Anti-reflection coating are applied on the lens surface to efficiently manipulate the light transmission and reflection through it. It has been seen that the surface reflectance can be reduced by coating the lens with a film of some materials having a lower refractive index than that of the lens. 

    Although, there are now two reflections – one at the exposed surface of the film and another at the lens surface, their combined effect produces less reflection than the uncoated surface. The result is, patient experiences a marked sensation of improved vision. There is no doubt that anti-reflection coating leads to greater patient comfort.

THE REFLECTION OF LIGHT

    Dr Rayton presented some of the problems encountered due to reflections of light in spectacle lenses. There are four definable sources of reflection that can cause annoyance to the spectacle lens wearer:

1. Frontal Reflection: 

    Frontal reflection occurs when some of the light incident on the front surface is reflected back towards an observer as shown in Figure 8.1B. This can be disconcerting for the observer, as the wearer’s eyes are difficult to observe. Communications between human relies just not on spoken or written words but also by facial expressions, much of which are centered on the eye’s reactions. Inability to see the eyes of a person you are talking to can be rather “off - putting” as one cannot deduce whether what is being said is actually being understood. These reflections are also annoying from purely a cosmetic point of view as they detract from the overall appearance.

2. Backward Reflection: 

    Backward reflections occur when some of the light behind the patient is reflected from the back surface of the lens into his eyes as shown in Figure 8.1A. This can be annoying at times when reduced lighting conditions are encountered such as at dusk or while driving at night.

3. Internal Reflection: 

    Internal reflection is caused by light being reflected between the two lens surfaces as shown in Figure 8.1C. The amount of reflection caused in this manner depends upon the power and position of the lens in front of the eyes.

    A.Light reflected from the rear lens surface into the eye

    B.Light reflected from the front lens surface into the eye

    C.Light entering the eye after double reflection within lens

    D.Light reflected by cornea to rear lens surface and back into the eye

    E.Light reflected by cornea to front lens surface and back into the eye

4. Corneal Reflection: 

    Corneal reflection is caused by light being reflected from the corneal surface and then interacting with the lens surfaces as shown in Figures 8.1D and E.

Principle of Anti-reflection Coating

    Anti-reflection coating is applied on the lens surface to reduce the loss of light due to reflection and increase the light transmittance through the lens to the eyes. To understand how it is achieved, we need to remember “Quantum theory” that tells that the light travels in waves similar to those in the ocean.

    The “Principle of Optical Interference” explains that there are two types of optical interference: 

    • Destructive Interference, and

    • Constructive Interference.

Destructive Interference

    Destructive interference occurs when the light waves are traveling out of phase with each other. When this occurs the peak of the wave is lined up with the trough of another wave and they virtually cancel each other out.

Constructive Interference

    Constructive interference we have the opposite effect happening. In this case the light waves are traveling in phase with each other so that the peaks and troughs of one wave line up with the peaks and troughs of another wave. They are now working together creating one larger wave.

    The simple principle of single layer anti-reflection coating is that the substrate (lens) is coated with a thin layer of material so that reflection from the outer surface of the film and the outer surface of the substrate cancel each other by destructive interference. To achieve this we take the advantage of the undulatory nature of light by creating opposition of phases between reflected waves.

    If we place a thin coating on the lens, the light beam, which is a series of waves, hits the coating and breaks up into reflected waves and refracted waves. The refracted waves then hit the lens and split second time into reflected and refracted waves. By a careful calculation of the thickness and index, in such a way that the first series of reflected waves and the second are super imposed while being out of the phase by half a wavelength, we obtain mutual cancellation. Light which is not reflected is refracted.

    

    However, in practice it is impossible to attain total extinction of reflection just by depositing a single layer of coating on the lens. In fact, the reflection could be almost eliminated by meeting two conditions:

    1. The refractive index of the coating is the square root of that of the lens material.

    n film =n Square root Substrate

    2. The thickness of the coating, multiplied by its refractive index, is exactly

    one – quarter of a wavelength of light or any odd number of quarter

wavelengths

Advantages of Anti-reflection Coating

    People see better with anti-reflection coated lenses as anti-reflection coated lenses transmit more lights. Conventional glass or plastic gains 8% in light transmission. High index lenses gain even more 11% to 16% depending on the materials. Increased light means wearer sees things brighter and clearer, with crisper details.

    While looking at a person wearing uncoated lenses, we see reflection of light that is actually coming from the front surface of the lenses. This can be disconcerting for the observer, as the wearer’s eyes are difficult to observe, creating a barrier between the observer and the wearer. By getting rid of the reflections, the lenses seem to “disappear” into the frame.

    Ghost images are a common experience with uncoated lenses, particularly while driving at night. These visual annoyances appear as dull images created by reflections coming from the internal lens surfaces. With minus correction, ghost images are crisp and reflected on the side of the light image towards the optical centre. With plus lenses, ghost images are larger, less distinct and appear away from the optical centre.Most patients drive vehicles and understand the advantage of anti-reflection coating eliminating ghost images.

    Reflections from back lens surfaces can also be annoying. This is most noticeable with sun lenses because the dark lens acts like a mirror. Backside reflections are also more prominent with aspheric lenses as their flatter inside surface reflects more light.

    Strong minus lenses can produce a “Coke bottle” look caused by light reflecting from thick lens edges. The reflection appears as a series of concentric rings. Modern anti-reflection coatings virtually eliminate these unsightly rings.

    Anti-reflection coating has become popular in recent years. As a result, there is a growing perception among consumers that, quality eyewear include anti-reflection coating. Offices that do not routinely recommend anti-reflection coated lenses, risk having patient learn about it from a friend, neighbour or a competitors advertising.

Dispensing Tips

    In dispensing anti-reflection coated lenses, always remember not to sell half pair lens. Whenever only one lens is required, most laboratories will strip anti-reflection coating from the old lens and recoat both lenses together for an exact colour match. Never use a stock anti-reflection coated lens for one eye and order custom anti-reflection coating for the other. Inevitably, there will be a slight colour difference in the anti-reflection coating. The difference in colour has no effect on vision but is always noticed by the patient.

    Be sure to explain that anti-reflection lenses must never be cleaned dry, as this can scratch the coating. The best cleaner is one that is made especially for anti-reflection coated lenses. Patient should not use acetone, windex, caustic solution or soaps to clean the anti-reflection coated lenses. Make sure that every eyewear patient going through the office gets an explanation and demonstration of anti-reflection coatings benefits. This ensures that every patient understands you are trained and skilled in dispensing what has become accepted as the most advanced modern eye wear available.