Although the title of this article is “High Index” and is accompanied by a series of numbers,it has nothing to do with the FTSE 100 index, which you will know is not terribly high at the moment.  The good thing about the title though is that “high” doesn’t have to apply to price, as there are means nowadays of getting cheap glasses online even for what used to be very expensive lenses. We are talking about what is often referred to when buying glasses online as spectacle lenses which are also sometimes described as “Thin and Light” - again not to be confused with any other product such as the excellent work undertaken by Weightwatchers. 

 

The index referred to in the title is the refractive index of the lens materials used in the production of spectacle lenses.  It is assumed that all readers of this column are entirely familiar with the term “refractive index” (also known by the symbol n).  The alternative is a further twenty pages before reaching the point of the story.  Basically, the higher the refractive index, the thinner the lens needs to be in order to produce a lens of a given power and, of course, the thinner the lens, the less of it there will be and hence it will be lighter ergo “Thin and Light”. 

 

In the days of glass lenses, the basic glass was known as “Spectacle Crown” or simply crown glass (usually n 1.523).  Of a higher refractive index, was dense barium crown and higher still, flint glass.

 

The basic plastic lens is made of a material n 1.5 approximately and the materials of n 1.6,1.67 and 1.74 are proportionally thinner for a lens of the same power.  The optimist may think that as he (or more likely she) is getting less lens for the same prescription, the cost will be correspondingly lower.  Alas the reverse is true and the increase in cost as the n goes higher is not exactly corresponding either!

 

About 60% of all prescriptions dispensed will not benefit much from having lenses of a higher n,  but in the higher powers the difference can be dramatic, particularly in minus lenses.  The lenses of a higher n also, almost invariably come automatically with an anti reflection coating which is a great avdvantage cosmetically,  again particularly with minus prescriptions. 

 

So for most prescriptions, it is probably unecessary to go to the expense of thinner lenses and for the higher powers, it may not be necessary to go for the most expensive

high index material.  Your spectacle supplier wil be glad to advise you as to the most suitable type of lens for your prescription.

 

In deciding whether or not to opt for the more expensive high index lenses, other factors need to be taken into consideration.  For example, the size of the frame to be glazed.  If you previously had a very small eye size (eye size being the horizontal measurement across the middle of the lens in the frame),  then the thickness of the lens would be correspondingly small.  But if you now choose a frame with a larger eye size and decide on thinner lenses, it could well be that the lenses are actually thicker than those in your previous glasses.  they would however be thinner than they would have been if you had not opted for high index lenses.  Ah!  I can hear you saying,  that is now crystal clear, I can now order my lenses with confidence in the appropriate index. 

 

Again one is bound to rely on the expertise of the spectacle supplier.  The cynics among you (and regrettably , I believe that there may be one or two),  may think that such expertise will be influenced  by the one eye focussed firmly in the direction of the till. 

 

May I attempt to reassure you by saying that in my experience, most such vendors are thoroughly decent and honest people (some even as d. and h. as oneself) and can be relied upon to give the best advice and perhaps, more convincingly, there is little financial incentive to recommend the higher priced products.

 

I commend to you, where appropriate, High Index ,  or if you happen to be someone who needs to take account of the FTSE,  you may require to be Thin and Light.  Please refer to the organisation named in paragraph one for

Tags: cheap glasses, cheap spectacles, Glasses Online, high index lens, refractive index

This entry was posted on Sunday, March 22nd, 2009 at 4:52 pm and is filed under Lenses. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.