What is optical glass?

Optical glass, as with all glasses, is basically made of sand plus a few other bits and bobs, melted and then left to cool. Of course the devil is in the detail.

There are well over 1000 optical glasses from 5 or 6 main manufacturers [Schott, Ohara, Hoya etc]. Many of these are essentially just different versions of the same glass, so there are probably about 1-200 actual glasses.

Why so many - surely glass is glass? Well no. Those 'bits and bobs' make all the difference. Most people, for example know of Waterford crystal - this is a glass with a very high percentage of lead. That lead gives the glass its weight, but also a high index and dispersion which makes it so attractive when made into its final shape. Other additives impact on the materials in various ways.

Each glass has a name, or more precisely, an identifier. One of the most common is known as BK7. Others include SF6 or LaKN6 - hardly inspiring - but crucial to the lens designer.

How do optical glasses differ?

Optical glasses are designed, in the first instance, for their optical properties - namely refractive index. However each glass has a whole host of other properties:

  • Transmission - a 'black' glass would not be much good for your camera!
  • Mechanical properties - strength, density etc
  • Thermal properties - deformation point, melting point, chane of index with temperature
  • Chemical properties
  • Cost - prices/kilo can vary by over 100x between different glass types.

Chemical properties are interesting - they measure how the glass responds to various environments - water, alkali, acid etc. These can be crucial for what is known as handling - what problems the glassshop might have working with that particular material. Some glasses stain just by the perspiration on your hand, while others can hold boiling sulphuric acid without problem!

Optical glass and lens design

The correct choice of glasses is fundamental to good optical design. If a lens was composed of one material only, then it would suffer from what is known as 'chromatic aberration' - that is red light would be bought to a sharp image at a different plane than blue light. Either you would have a sharp red image with blue halo's or vice-versa.

The glasses in different elements have to be chosen to compensate for one another, to correct chromatic aberration, and this is non trivial! A task made more difficult by the fact that we do not have a continuous range of properties.

We can define the separation between elements to be any value. However when it comes to refractive index, we only have a certain number of real values to choose from, because we only have those 1-200 glasses. This is a real problem for optical design software, which really requires continuous variables when doing optimisation. So, a lot of the skill in lens design lies in choosing the glasses almost manually!

Furthermore the optical design algorithms dont take account of the non-optical properties. Frequently, a theoretically 'good design' will tuen out to be impracticable for reasons of cost or handling

In short, optical glasses make possible most of the lenses that we have, but also make difficult the optical designers life; blessing and bane!

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