Lens Neutralisation

Question 1

What is lens neutralisation

Answer 1

It is possible to establish the spectacle distance and near prescription (and also prismatic component) of a pair of bi-focal spectacles using a lens box by the principle of lens neutralisation. This estimate may not be as accurate as a focimeter; however, it is still a useful skill to have and one that is assessed in the Refraction Certificate examination.

Question 2

What is the principle of lens neutralisation?

Answer 2

uses lenses of equal but opposite power to neutralise a test spectacle lens so that there is no overall effect, as the combined net power will be zero. For example, a +2.50 ds spectacle is neutralised with a -2.50 ds lens. A2 pd BO prism will be neutralised by a 2 pd BI prism.

Question 3

How to assess if a lens is plus or minus

Answer 3

complete the ‘transverse test. Draw a target or cross formed by two perpendicular lines. Pass the lens horizontally from left to right over the crossing lines. If the image of the lines moves in the same direction (left to right) as the sweep (with), then the lens is minus. If the image of the line moves in the opposite direction to the sweep (against), then the lens is plus

Question 4

What does it mean if the image of the line moves with the sweep?

Answer 4

Minus lens

Question 5

What does it mean if the image of the line moves opposite to the sweep?

Answer 5

Plus lens

Question 6

If a transverse test implies a lens is minus how do you neutralise it?

Answer 6

place a plus lens (say + 3.00 ds) in direct contact and see if this eliminates the movement of the vertical line image. If the movement is still with the sweep, then try a more plus lens; if it is against, then try a less plus lens.

Question 7

When does neutralisation occur?

Answer 7

when the image of the crossed lines remains still as the lenses are swept across them horizontally

Question 8

Where do you place the trial lens relative to the lens being tested?

Answer 8

The trial lens should be held against the back surface of the test lens being investigated, since it is the back vertex power that we are measuring

Question 9

What do you see with an astigmatic lens on lens neutralisation?

Answer 9

scissoring movement