Chapter 7: Optical Prescription, Spectacle Lenses Flashcards
What does -2.0 DC axis 90 mean
A cylindrical lens of -2 dioptre power, with its axis (of no power) being vertical
NB: axis is shifted 90 degrees to normal sphere degrees
In a toric lens, what is placed on top, spherical or the cylindrical lens?
Cylindrical lens is placed on top of the spherical.
But when writing the Mathematical abbreviation, the sphere is noted first.
What is simple transposition of a spherical lens?
Way of depicting a spherical lens but in different ways. Calculated by adding the surface powers.
Transpose the following cylinders
Aim is to change the cylinder into the opposite power. Used to compare present refraction with new refraction
What is a toric astigmatic lens made of?
A spherical surface and one toric surface which contributes the cylindrical power.
Te principle meridian of the weaker power (closer to zero) of the toric surface is known as the base curve
Transpose the following:
NB: Base curve and cylinder need to be same sign
Transpose the following
NB: Base curve and cylinder need to be the same sign
If you look at a cross through a convex spherical lens and move the lens up and down, what happens to the image?
Image moves in the opposite direction to the movement of the lens. AKA against the movement.
If you look at a cross through a concave spherical lens and move the lens up and down, what happens to the image?
Image moves in the same direction as the movement of the lense. Aka with the movement
What happens when you rotate an astigmatic lens that is infront of a picture of a cross
When lens axis is inline with cross, nothing. When rotated, the cross begins to scissor.
NB rotating spherical lens does nothing to the image.
You can find the optical centre by by moving the lens and one line not being displaced then repeated at 90 degrees.
How does a prism displace a cross.
Lenses in glasses are assigned a power based on their
Back vertex power. To work out the power, a neutralising lens is usually put at the back vertex of a lens. This is not always possible in some lenses because they are highly curved. In this case, you neutralise using front which is inaccurate.
What is lens neutralisation
A way of working out power of lens.
Once you have worked out axis and type of lens, you can use a known lenses of different power and place them behind a lens of unknown power. These are held in front of a cross.
When the cross no longer moves, the known lenses power is equal and opposite to the unknown lenses power.
IE a + 2.0 D lens neutralises a –2.0 D lens
What is a Geneva lens measure
Calculates the curvature of a lens allowing it to calculate the surface power of a crown glass lens (refractive index 1.523)
Describe how a focimeter works
Used to measure power and axis:
- Formed from 2 parts, a focusing system and a viewing system
-Focussing system is made up of a collimating lens (a lens that makes light parallel when an object is held at f1) and and illuminated target. The target is a paper with a ring of circular holes illuminated from the back with green light to eliminate chromatic aberration.
-The viewing system is a telescope that sees the image at infinity which contains a graticule which is basically a ruler/scale
- a test lens is placed on the lens table on its back vertex at the f2 of the collimating lens. This is stationary.
- The target can be moved forward and backward. If it is infront of f1 of the collimating lens, light is slightly divergent. If it further back then f1, light is slightly convergent. You can then use different test lenses and see which distance the target makes the light parallel at the viewing piece.
How do you power and axis of bifocal glasses near segment
Near segment = area that allows you to read close things.
Because the supplementary power of the near segment works by reducing the divergence of light from the near object before it enters the distance lens, the front vertex power is the relevant measurement for the near segment.
Therefore to work out the near add, you do front vertex power of near portion - front vertex power of distance portion = bifocal add.
How do you examine an astigmatic lens using a focimeter?
Astigmatic lens sees lines instead of dot targets.
To examine an astigmatic lens on the focimeter, e.g. the lens shown in Fig. 7.10a, the instrument is adjusted until one set of line foci is in focus (Fig. 7.10a(i)) and the reading (+1.0 D) recorded.
The instrument is further adjusted until the second set of line foci come into focus (Fig. 7.10a(ii)) and the reading (+3.0 D) and axis (180°) of the lines recorded. The first reading gives the spherical power of the lens.
The cylindrical power is calculated by algebraic subtraction of the first reading from the second (+3) – (+1) = +2 D. The axis of the cylinder corresponds to the axis of the second reading, i.e. 180°.
What does a prism look like in a focimeter
Image of the target can’t be put into the centre of the graticule . The image is seen pushed towards the base of the prism.
Can use the scale to work out the prismatic power using
P= FxD
P = prismatic power
F= power
D= distance in cm
How does anti reflective coating work
Lens coating is 1/4 of the thickness of a wave length of light.
The light that hits the surface of the lens is reflected and travels half a wavelength more than a wave of light that is reflected off the coating.
Because they are half a wavelength out of phase, they undergo destructive interference.
