> Yet thinking in millimeters, the angstrom unit is what?
>> about the tenth place of decimals...
One Angstrom is a ten billionth of a meter . [ 1 e -10 m]
This unit is obsolete and is reserved for historical use only.
The SI unit is nm, nanometer, [1 e -9 m]
>In my experience the human eye starts to lose the ability
> to distinguish individual black dots when they are about 400 dpi,
> and so there is a real gap between visual smoothness
> say from above 500 dpi
> and engineering smoothness say from above
> 1000 to 10,000 'bumps' per inch
In real surfaces, roughness below 500 dpi, about 50 micrometers is readily
observable in reflection. Such small imperfections, while not discreetly
visible, scatter light and cause a dull surface. A good polish takes out
scratches into the sub micron range.
-Jay