The colour perceived by the human eye depends both on the effective colour of the observed object and on the composition of the light illuminating it. For instance, sunlight at dusk or artificial lights modify the perception of colours significantly. For this reason, the measurement of colour is carried out with a reference white light which reproduces sunlight as much as possible.

The phenomenon called "metamerism" occurs when colours appearing identical to the eye under a certain light show different tones if illuminated by a different light.
So for example two filters can appear identical if observed at midday and different if observed at sunset, when the sunlight composition varies. The phenomenon occurs when the spectral curve indicating the level of absorption of light at various wavelengths presents a different shape in apparently identical colours.
In practice, the phenomenon takes place when the spectral curves do not overlap, but "cross" at one or more points. (see figure 1)

Fig.1

These colours, appearing identical under clearly defined light conditions with differently shaped spectra, are defined "metameric colours".
Since the performance of a filter depends not only on the quality of the materials, but also on the shape of the spectral curves determining the colour, filters characterised by the same colour, but with different spectral curves can be characterised by completely different performances. In the realization of a sun filter, the cosmetic colour is second to the spectral curve shape, since it must permit the absorbtion of the light in the most appropriate in order to favour the requested performance (maximum protection, increased contrast, colour vision etc.).
Metamerism is therefore a clear demonstration that the same visual or protective performances are not achieved by simply reproducing a colour.

For approximately six centuries, glass has been the only material used to manufacture ophthalmic lenses and this has continued until after of the Second World War with the advent of the first transparent plastic materials. At present, various types of lenses are manufactured using not only glass, but also different organic polymers which are called "optical polymers" for their physical characteristics and their refractive power.
Considering the various optical materials, the main characteristics are the following ones:

• Refraction index
• Abbe Number or Chromatic dispersion coefficient
• Specific weight
• Impact and scratch resistance

The refraction index expresses the capacity of an optical means to deviate the luminous ray passing through it.
The Abbe number is a number expressing the level of chromatic dispersion of an optical material. In practice, it expresses the attitude of the optical material to separate white light in the primary colour making it up, thereby affecting the neatness of the image. The optically best materials are characterised by high values.
The specific weight of the material determines the weight of the glasses: the lenses of optical polymer weight approximately half of glass. Impact resistance: it is a problem for glass to be tempered to be safe. Optical polymers are instead impact resistant.
Scratch resistance: the situation is reversed. Glass is highly scratch resistant while optical polymers, except for CR39 and its compounds, require an anti-scratch treatment to achieve the appropriate scratch resistance. A more detailed analysis of the various materials follows.

Glass

In the manufacture of afocal and corrective sun lenses, the most common glass used is typically "Crown glass" together with other more specialised types.
The main glass defects are high weight and fragility; the need of a thermal or chemical tempering treatment to pass the Drop Ball test, (the fall of a steel ball from 16mm height), is indispensable for the sale on the US market.

Optical polymers

As an alternative to tempered glass, optical polymers are increasingly popular due to their high optical characteristics as well as lightweightness and resistance.

Polycarbonate (Lexan)
APX
Cellulose esters
Polymethilmetacrilate

The table contains the chemical-physical characteristics of the two most used optical polymers with respect to crown glass.

Poly Dietilenglycole bis Allilcarbonate CR39®

CR 39® is undoubtedly the most common optical polymer in the ophthalmic field.
It is a heat hardening resin and as such it is not attacked by any chemical agent.
The lens comes from pouring the liquid polymer into a mould of optically treated glass. This enables to obtain a lens with the same characteristics of the originating glass.

It has a higher scratch resistance than the other plastic materials and does not require any anti-scratch treatment.
The specific weight is 1.3 versus 2.53 of glass and this means that, with the same thickness, the weight of a CR 39 lens is approximately 48% lighter than a similar one made of glass.
As for impact resistance, afocal lenses with a thickness of 1.8 mm, easily pass the test of the steel ball falling from 16mm height (Drop Ball Test) imposed by the FDA to sell lenses in the USA. They are four times as resistant as Crown Glass and always more resistant than tempered glass.
CR39 lenses are easy to process and surface tintable with uniform and gradient colours, enabling to obtain a wide range of colours. However mass colours are also possible.

POLYCARBONATE

It is a thermoplastic resin and the lens is manufactured by pouring the material into metal moulds at high pressures and temperatures.
It has excellent anti-impact properties which make it ideal for sports and industrial uses.
The specific weight is slightly lower compared to CR 39's. When used in rigid frames or in contact with non compatible materials, phenomena of "stress cracking" might occur (i.e. cracks on the anti-scratch coating).

APX

It is a new material created to prevent the recurrent problems of polycarbonate lenses (stress cracking with metal frames, migration-related crazing with acetate frames and electrostatic labels) causing microcracks visible on the filter surface.
Lenses are manufactured by injection moulding and, like polycarbonate, are covered by an anti-scratch coating to improve their resistance to abrasion. While they maintanin a high mechanical resistance, APX lenses are totally compatible with all the materials used for frames. The impact resistance is the same of Polycarbonate lenses. In particular, the lens with 2 mm thickness passes all the high and low energy tests established by the ANSI Z87.1-1989 standard for sunglasses and shields for industrial protection. These tests include:
a) High speed impact: the lens resists the impact of a steel ball with diameter of 6.35 mm shot at a speed of 45.7 meter/second.
b) Impact with high mass dart: Intercast SINTERŽAPX lens passes the test with a steel dart with sharp conic point weighing 500g dropped by gravity from a height of 1.3 meters.

Cellulose esters

They are essentially 3: acetate, propionate and acetobutyrate.
They are the materials normally used for frames and lenses.
Similarly to polycarbonate, the lenses obtained by injection moulding show good optical properties and keep a good mechanical resistance also at low temperatures (below 0°C), making them suitable for skiing goggles.
Scratch resistance is limited.
Their use in the optical field is limited to low quality lenses, ski goggles and motorcycle goggles and also to the manufacturing of certain types of polarizing lenses.