Hand Neutralisation Flashcards
What is hand neutralisation used for
There are several ways available to determine the power of a lens, one of these ways is by hand neutralisation. If you look at an object through a lens and move the lens, the object will appear to move.
Info
A lens does not deviate light that passes through it’s optical centre (nodal point). The amount of deviation increases as we move away from the optical centre.
With movement is when…
If we move a minus lens away from the optical centre the image will move in the same direction as the lens is moved.
Against movement is when…
If we move a plus lens away from the optical centre the image will move in the opposite direction as the lens is moved.
Spherical lens
When we have a spherical lens, it doesn’t matter which direction we move the lens. The lens has the same power in all directions and shows rotational symmetry.
Spherical lens is same in all directions
With movement
With movement is when image moves in direction glasses are moved in
Against movement
Against movement is when image moves in opposite direction
Easiest way to assess this movement
Up down
but can do side to side
If there is a with movement add
PLUS LENS until there is an agaist movement therefore fore add minus
If there is a against movement add
MINUS LENS until there is a with movement therefore add plus
Method of hand neutralisation
Have glasses legs away from you
Find a DISTANCE target
And go up in 1D steps
Only look at Optical centre not periphery
Neutralised = No movement
The power is the opposite of the neutralised lens
To note
- Keep the lenses in contact with one another.
- Use a distance target
- Hold the lenses in the spectacle plane (not at arms length)
- Ignore the periphery of the lens as this will not be in contact and therefore have a different total power
