In simple terms, photochromism is a mechanism by which ophthalmic glass darkens outdoors, when exposed to ultraviolet (UV) rays, and fades back to its initial transparent state indoors or behind a UV-filtered window.
The result is added vision comfort and better protection of the retina.
Since its inception and first commercial release by Corning in 1964, photochromic glass has become more sophisticated and diversified. Yesterday limited to glass, photochromism is today available on most ophthalmic materials.
In more technical terms, photochromic lenses have the capacity to vary their absorption characteristics in response to radiation wavelengths (notably ultraviolet A, or UVA, radiation). This modification is reversible.
Their special property is thus to undergo a change in their range of transmition, according to changing ambient-light levels. Every photochromic lens has its own separate transmission curves in the clear state and the darkened state.
Photochromic Glass Lenses
Photochromic Glass Lenses
A specific mode of action: Silver atoms
The photochromic mechanism that takes place in a glass photochromic lens is very different from what happens with plastic.
The light-reactive properties of silver atoms have long been known. Photography is a key example, but in this field, the process is not reversible.
Photochromic glass lenses, on the other hand, where the reaction is mediated by atoms of silver in the matrix of a specially constituted vitreous environment, are capable of continuous and reversible change between the clear and darkened states. The number of atoms present, and their distribution within the lens, will directly influence transmission levels and lens color. Additionally, the basic composition of the lens determines the rate of change through the darkening-clearing cycle.
The basic principles governing this change of state in the atoms of silver are schematically represented in our animation. Influenced by UV-A or short-wave visible spectrum radiation, an atom is able to modify its external electronic structure by deployment of the electrons present in the vitreous structure. When such radiation is no longer present, the system returns to its original state.
Stable and durable
Laboratory tests involving several tens of thousands of darkening-clearing cycles, corresponding to long years – or even decades – of normal use, have revealed no significant fatigue in this phenomenon. Indeed, it shows remarkable reversibility under natural lighting conditions. This, added to the natural transparency of glass and the in-mass presence of the photochromic agent, explains how stable glass photochromic lenses can be in the long run.
A complex process
Optimum precision in product manufacturing is required to regulate photochromic performance, especially during the critical stage of annealing. After melting and pressing, the carrying of a lens through an annealing lehr (at temperatures up to 700°C depending on the nature and softening point* of the glass) is a commonly used internal stress-reduction process. But when it comes to photochromic glass, annealing is also the way to activate the silver-halide crystals, and its temperature has to be regulated with a precision of 1°C, taking into account the size and number of crystals, the glass composition and volume, and the photochromic characteristics – a very complex process which Corning pioneered in the mid 1960s and never stopped improving since then.
Corning Ophthalmic offers a wide variety of glass photochromic lens materials worldwide. Patented in-mass technology provides photochromics throughout the lens material; guaranteed lens matching, batch after batch; and color consistency during activation – plus, the well-known advantages of natural scratch-resistance and anti-reflective (AR*) glass compatibility.