Even before the days of PC it was very important to have the correct PD.  It is especially important when considering buying glasses, even when buying glasses online. In most cases, when buying glasses, it is ok to refer to the “default” value – but it is important to understand the need for the measurement as well so you can take it into consideration for your spectacles purchase.

The PD is the distance between the patient’s (or anybody elses’s for that matter) eyes.  This measurement is determined by placing a millimetre scale across the patient’s forehead, lining up the 10 on said scale with the vertical tangent to the patient’s  right iris (i.e.the left one as you look at it), and reading off the distance from that to the vertical tangent of the patient’s left iris (i.e. the one on the r………etc) and then deducting 10mm from the reading.  Now the clever reader will want to know why not line up the 0 on the scale in the first place and save oneself the mathematical exercise of deducting the 10mm.  The answer is that after some years of using the same ruler it tends to become a little rounded at the corners. Indeed the writer’s well rounded ruler has been  in use for over ……. ,however we haven’t got time to go into that now.

 

So, we now have our PD.   All we need to do is to inform whoever is making the spectacles to “centre” them at the same distance as the given PD.  That is to say that the lenses must be cut in such a manner that their optical centres must end up in the frame at the same distance apart as the PD.

 

So what are the consequences of incorrect centering.  In the lower powers,  not very much, but as powers increase, it becomes more important and in the very high powers absolutely vital;  the reason being that if the centres of the lenses are not in the right places, a prismatic effect is introduced, which can produce stressful symptoms and in severe cases diplopia (double vision to the uninitiated).

 

There is one kind of optical dispensing operation where correct centering is absolutely essential, regardless of the powers of the lenses.  This is in the dispensing of Varifocal  spectacles. Here, because of the complex design of the lenses, it is necessary not only to measure accurately the PD, but also to take account of any facial asymmetry, i.e.provide the glazer with “half” PD’s. 

 

For those who might experience difficulty aligning scales with vertical tangents etc. there are now on the market one or two “PD guages”.  These devices are more or less triangular in shape with an eyepiece at the apex.  You place the wide end on the patient’s forehead and look through the eyepiece and lo and behold there you see the patient’s eyes with a millimetre scale superimposed.  Easy!

 

For some practitioners a PD guage is indispensible.  People with only one functioning eye would find it impossible to use the method described above,  these would include people with a condition known as ambliopia ex-anopsia, sometimes referred to as a “lazy eye” . 

 

If you happen to be a practitioner who habitually turns up for the session ten minutes late with a touch of the shakes (for whatever reason) then a PD guage is recommended for you also.

 

Tags: Buying Glasses Online, Glasses Online, pupilliary distance

This entry was posted on Wednesday, May 20th, 2009 at 2:07 pm and is filed under Glasses. 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.