| Tom Lucas 2006-09-19, 8:29 am |
| "Ace" <acemanvx@yahoo.com> wrote in message
news:1158644566.241946.152430@m73g2000cwd.googlegroups.com...
>
> Tom Lucas wrote:
>
>
>
> The chisel is smaller than the statue. The laser is far bigger than
> any
> high aberration. They distort vision not because of size but because
> of
> number, there are so many of them. One aberration doesnt matter, one
> million does. Lasik induces more aberrations(as you found out) thereby
> decreasing vision.
The chisel is bigger than the individual hairs it carves. 500um diameter
spot does not mean that is exactly the area ablated - for one thing the
beam is not perfectly coherent in any laser so the edges of the beam are
slightly less energetic than the centre. There are other factors too -
flying spot means that the laser doesn't just move to one place, zap,
and then move to another creating a mosaic - it means that the beam is
smoothly passed over several points like spraying paint on a car. This
gives a much greater resolution that 500um.
As you are probably aware then the orders of the aberrations are derived
from the polynomial equation used to mathematically describe the shape
of the cornea. As you rightly say, one aberration on its own doesn't
affect much however when a lot of them are grouped together they
collectively affect the rate of change of that area of the cornea - i.e.
they act together. Mathematically, rate of change is differentiation and
when you differentiate a polynomial all the orders go down by one. This
means that the higher orders are promoted to lower order status and will
actually affect vision. Myopia is nothing more than a collection of HOAs
working together which is why it can be modelled with a lower order
equation.
However, the Laser can confidently deal with lower order aberrations and
will fix them using the traditonal ablation method - meaning that the
interraction between the previous HOAs is lost. Perhaps a few may remain
but the density of them is greatly reduced and they can now be taken as
individuals and classed as HOAs. It is like a few splinters sticking up
from an otherwise smoothly sanded floor which had previously been rough
everywhere.
If there are enough of these HOAs left to affect vision then it is
because the Lower orders were not completely fixed. Perhaps a term like
medium order aberration would be useful in these cases? Anyway my point
is this - lasers treat lower (and, with wavefront, medium) order
aberrations. There is no need to treat higher order aberrations unless
they are dense enough to be visual which is exactly when they cease to
be higher order and are thus treatable.
This post is a bit complex and mathematical so please shout if anything
is not clear but I think it somes up what I've been trying to say about
HOAs for a while. Anyway the gist of it is this - any anomaly that
affects vision is of a low order regardless of whether it is formed of a
collection of higher orders (which is the definition of low order).
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