2. Neutralisation (COMPLETE)

Question 1

what is neutralisation? is it precise?

Answer 1

Neutralisation is the determination of the power of an unknown lens by combining it with known lenses so that there is no
apparent movement of a target viewed through the combination

It is an approximate method to determine back vertex powers

Question 2

what sort of movement does a positive lens give? and negative lens?
what can be located with this method?

(diagram of positive/negative movement p.4)

Answer 2

positive lenses give “against” movement
negative lenses give “with” movement
locating optical centre

Question 3

describe the process of neutralization/movement with cylindrical lenses. describe the process and what is expected when neutralising +4.00 x 90 lens.

Answer 3

1. along vertical axis meridian
2. along horizontal power meridian
3. rotation of cylindrical lens crosslines appear to rotate
4. distinguishes cylindrical/spherocylindrical lenses from spherical lens

+4.00 x 90
Rotation test: Scissors movement
“against” in vertical movement
“with” in horizontal meridian

-4.00 x 90
Reverse to above

Question 4

describe movement with sphero-cylindrical lenses

Answer 4

transverse lens movement
movement according to powers
rotation identifies principal meridians

Question 5

draw what it would look like with neutralization of lens +6.25/-4.00 x 90

Answer 5

diagram p.6

Question 6

describe the process of neutralisation. (don’t know what type of lens)

Answer 6

1. Sphere or sphero-cylinder?
   Use rotation test

2A Spheres
(a)Transverse movement +ve/-ve lens
(b) If +ve lens, neutralise with -ve lenses
If -ve lens, neutralise with +ve lenses
(c)Write down power of lens

2B Sphero-cylindrical lenses
(a)Rotation test
locates -ve cylinder axis by “with” direction - mark this
axis

Question 7

describe the sphere-sphere method of neutralisation

Answer 7

(b1) Sphere-sphere method
(i) Neutralise along marked cylinder axis with spheres. Record power of correcting lens as sphere
(ii) Remove correcting lens. Neutralise along cylinder power meridian. This is sphere + cylinder. Subtract power in (i) from this power to give cylinder
(iii) Reverse signs of correcting lenses

Question 8

describe the sphero-cylinder method of neutralisation

Answer 8

(i) Neutralise along marked cylinder axis with spheres. Record power of correcting lens as sphere
(ii) Leave correcting lens in place. Add +ve cylinders with axes || to axis of unknown lens, and neutralise along power meridian of cylinder
Record power of correcting cylinder
(iii) Reverse signs of correcting lenses

Examples:
+2.00, +6.25/−4.00 x 90
+2.00/ −4.00 x 90, −2.00/−4.00 x 90

Question 9

Problems

1. Assuming that each of the lenses +0.50 DS/−0.25 DC x 180, −1.75 DS/ −1.50 DC x
   90, +4.25 DS/+1.75 DC x 180 and −2.00 DS/+4.00 DC x 90 is infinitely thin, find the
   power of the fifth thin lens which must be added to their combination in order to
   neutralise them. Express the power of this lens in each of its sph cyl forms (Jalie, Chapter 3, question 8).
2. A crossline chart is viewed through the lens −1.00 DS/+2.00 DC x 180 which is held
   so that the crosslines appear unbroken. Draw diagrams to show the appearance of
   the crosslines when the lens is moved a) upwards through a small distance; b) to the
   right through a small distance; c) rotated anticlockwise through 10° (Jalie, Chapter 3,
   question 17).
3. Briefly describe the steps involved in hand neutralising the following lenses, and give
   the relevant results obtained at each step:
   a) +3.00 DS, b) +3.00 DS/−3.25 DC x 180, c) +6.25 DS/+2.00 DC x 90
4. The sum of the principal meridional powers of a thin sphero-cylindrical lens is zero. The lens is placed in contact with a second sphero-cylindrical lens and the
   combination is neutralised by a −4.00D sphere. When the first sphero-cylinder is
   rotated through 90°, the combination is neutralised by a −2.00 D sphere along one
   meridian and a −6.00 D sphere along the other. Find the powers of the two
   sphero-cylindrical lenses (Jalie, Chapter 3, question 20).

Answer 9

n/a