DISPENSING TIPS OF PHOTOCHROMIC LENS

Story of Photochromic Lens

    Photochromic lenses are lenses that visibly darken and fade indefinitely under bright and dull light respectively. They were developed by Dr WH Armistead and SD Stookey at Corning Incorporated in 1964.

    A photochromic glass lens contains billions of microscopic crystals of Silver Halide.It is this metallic silver that absorbs the light. This reduces the amount of light passing through the lens and the lens turns darker. With the disappearance of UV rays or sunlight, the lens returns to its faded state by the reconvertion of deposited metallic silver into silver halide. 

    Since these crystals are within the lens material, the process of darkening and fading can be repeated.Factors affecting Photochromatism

    Although, exposure to ultraviolet light is one of the conditions that influence photo chromic lens transmission most, several other factors also contribute to lighting and darkening phenomenon. They are somewhat temperature dependent.

    Assuming the same degree of illumination, they will be darker when it is colder and lighter more than when it is warmer. This is slightly ironic as of course they are looked upon as a sunglass lens to be used in warm conditions.

    Because they react more in colder climates, there are some photochromic lenses where the manufacturer uses a warning stating that they should not be worn in freezing or cold conditions as the lens would darken to virtually zero transmission. Lens thickness has an effect on the density of the tint.

    The silver halides run evenly through the lens, and therefore the thicker the lens, the more of them is there to absorb the light, hence the tint would be darker. Care must be exercised when looking at transmission graphs or curves put out by the various photochromic lens manufacturers. They are all usually based on 2 mm thick sample at a specific stated temperature.

    Any variance in these values will affect the transmission characteristics. Photochromic lenses achieve their full changing range and speed only after a “break - in” period, i.e., they tend to improve with age. The more light/dark cycles that they go through, the better they perform. It is, therefore, very common for patients supplied with a new pair of photochromic lens to complain that they do not change as good as their old pair.

    Because of the above it is recommended that these lenses are always replaced in pairs to ensure best possible tint match. TINT OPTIONS IN PHOTOCHROMIC LENSES Glass photochromic lens usually come in two basic colours – grey and brown.

 

    Technically it is possible to obtain other tint also, but demand has to be sufficiently to justify the production of the particular tint. It has been attempted in the past to make pink and green photochromic lens, but it seems to have settled down to a choice between grey and brown.

    
Resin Photochromic Lens

    Photochromic resin lenses utilize a significantly different photochromic technology than that is used for glass. Photochromic compounds can be incorporated into a resin lens by applying either a photochromatic coating or dye to the lens or by penetration of the surface, known as imbibitions.

    Using the imbibing technique it is possible to provide a uniform distribution of millions of photosensitive molecules within the front surface to depths of several microns. They become an integral part of the lens and not a coating that can wear or rub off. The process of imbibing can be illustrated by imaging a piece of standard laboratory filter paper saturated with the photochromic compounds and then allowed to air dry.

Dispensing Tips for Photochromatic Lens

    Photochromic lenses are not as effective in hot climate. When temperature increases, the reaction slows. So they are not recommended to replace sunglasses. However, they are useful lens to have, as the variation allows comfortable vision in almost all conditions with one pair of glasses.

    Photochromic lenses do not lighten instantaneously when going from a bright to a dim area. For this reason, the elderly often experience problems wearing photochromic as the need for good illumination increases with age. The convenience gained from a darkened lens in a bright environment is offset by the inconvenience experienced when coming into a darker indoor from outside.

    Also because the photochromic do not always return to their maximum transmission, the additional reduction in illumination when driving at night may prove hazardous.

    No coating that absorbs UV light should ever be used on the front surface of a photochromic lens. Such a coating interferes with the darkening of the lens. Such a coating can, however, be placed on the rear surface without interference.

    Anti-reflection coating will not reduce the range of the photochromic cycles but as with any lens, will increase the transmission in both the lightened and darkened status. Theoretically, a multicoated photochromic lens will transmit as much light as an uncoated clear glass lens, in its lightening state.

    Photochromic lenses may be treated either through chemtempering for optimum mechanical strength, or air tempering. In the latter case, the treated lenses become lighter a little more slowly and darken rather more in the sun.

    When replacing a single lens, the problem of a colour mismatch between the lenses often arises. There are three useful points to remember in order to avoid this problem:

    1. Always make sure that the new lens is of the same type as the old one, i.e., same brand, same thickness etc.

    2. If the old lens has been chemtempered, have it re-strengthened along with as the new lens.

    3. If the old lens has not been treated in anyway, place it with the new lens in boiling water for half an hour (The infrared in the hot water restores original clearness to the old lens).