Since the first human adventures with glass during the Roman Empire, this transparent solid has come a long way: not only because of the techniques involved in its production (e.g. tempering, which means cooling slowly) but moreover the panoply of additives that can drastically change its properties.
Glass has a refractive index: the light incident on a block of it will be ‘bent’ according to this ‘n’ number, in truth the ratio of the sines of the angle that one shines the beam into the block to the angle that the refracted beam makes, all with respect to the normal line (see diagram). Normal lenses usually have a biconvex (‘oval’) shape, and focus light from infinity to a specific focal point.
In scanners and other applications, lenses with a very low height are required so that the lens can be moved quite close to the object to be magnified. The problem encountered by optical engineers when utilising standard lenses for these applications is twofold. Firstly, the lenses are too large, meaning that to achieve good levels of magnification these microlenses would have to be moved closer than is physically possible to the paper. Secondly, traditional lenses suffer from all sorts of aberrations, for example the type than can be observed when looking through a pair of thick reading glasses at a slight angle.
There is, however, still reason to smile as you scan in your holiday snaps. GRIN technology, standing for ‘GRadient INdex’, involves the production of lenses that are not only disk shaped, but that outperform normal thin lenses. The idea is that, with a normal lens, the refractive index is constant throughout the material, and so you have to create curved glass to attain the desired refractory effects. However, changing the actual refractive index of the glass as a product of the distance travelled through it can do away with curvy lenses.
It has been established that this reduction in height and increase in optical performance is important, but how are these marvelous little microlenses manufactured?
It turns out that various techniques exist. One that caught my eye while researching this topic was a method that involves bombarding a piece of boron-rich glass with neutrons so that the concentration of boron at various points in the glass can be altered and thus a refractive index gradient can be created. However, even more interesting than this, I alighted upon a method entitled ‘Ion Exchange’. By submerging a piece of glass in a bath containing a solution of ions, the ions that already exist in the glass (eg. sodium) can be replaced by other ions and a gradient can be set up, with a high concentration at one end and a low concentration at the other.
This is where the green shoot breaks its way into the discussion. Thallium, known by some as the ‘poisoners poison’ and others as the ‘inheritance powder’ because of its use in the 1950s as a very effective but equally unknown poison, is a soft, silvery metal that forms a thick oxide coating in air. Etymologically, Thallium is so because of the bright green line observed by Crookes when he spectroscopically analysed a residue that he suspected to be Tellurium-containing. He later went on to isolate this toxic element, but the name Thallium – meaning green twig or shoot – stuck fast.
Thallium, however, along with Lithium, can be used as a dopant in GRIN optics. This is achieved by leaving the glass cylinders in a solution of Thallium Halide(s) where it permeates the vitreous structure and replaces the Na ions. Why thallium? Why choose such a toxic, rare and quite frankly weird element for these lenses?
Consider making a microlens for a scanner. It’s best to harvest light from all around as opposed to a tiny point, as this increases the amount of light entering the lens and therefore shortens the exposure time. The numerical aperture of a lens, turns out to be quantitative extension of this concept, defining for a particular lens or glass the maximum angle at which light can be collected (see below):
When Thallium is impregnated at varying concentrations through a glass cylinder, lenses which have a very good NA (numerical aperture) are produced. It’s as simple as that. Lithium? Works too, but its NA is inferior.
Next time you wait for that glowing white line to travel under your document in the scanner, consider the effort that has gone onto each of its components. Occurrences like these reveal over and over again that every element in the periodic table truly does have its purpose – be it in the big things or the microscopically small.
Also, though, there is a moral dimension to this glass. Exposure to beyond even half a milligram of Thallium, it has been anecdotalised, leads to hair loss, stomach cramps and the sensation of “hot coals everywhere on the skin”. Should we as rational human beings restrict the use of toxic elements such as Thallium? I think not. Not only are the amounts used tiny, but their encapsulation within the glass makes them pose little to no threat to the end user. In sum, it is foolish to label certain elements as ‘good’ or ‘bad’: they are collections of certain atoms which we as humans utilize.