2. Neutralisation (COMPLETE) Flashcards
what is neutralisation? is it precise?
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
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)
positive lenses give “against” movement
negative lenses give “with” movement
locating optical centre
describe the process of neutralization/movement with cylindrical lenses. describe the process and what is expected when neutralising +4.00 x 90 lens.
- along vertical axis meridian
- along horizontal power meridian
- rotation of cylindrical lens crosslines appear to rotate
- 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
describe movement with sphero-cylindrical lenses
transverse lens movement
movement according to powers
rotation identifies principal meridians
draw what it would look like with neutralization of lens +6.25/-4.00 x 90
diagram p.6
describe the process of neutralisation. (don’t know what type of lens)
- 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
describe the sphere-sphere method of neutralisation
(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
describe the sphero-cylinder method of neutralisation
(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
Problems
- 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). - 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). - 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 - 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).
n/a
