Competency 4.1.5

Question 1

Features of a PAL

Answer 1

• The power varies across the surface of the lens between two fixed values, the distance and near prescriptions
• The progressive power is usually worked on the front surface, the rear of the lens being spherical or toroidal depending on their prescription

Question 2

Problem with Varifocal Lenses

Answer 2

• if you want to have a front surface that reduces in radius/increases in curvature as you go down the lens, you end up getting a lens that is thicker at the top
• Manufactures can put a prism on the back of the lens so that the finished one is a knife-edge, reducing weight and thickness

Question 3

Factors Which Impact on Near Vision Through Varifocals (not Rx)

Answer 3

• Near Vision Effectivity
• Oblique Aberrations
  
  • Occurs as patient is looking obliquely through the lens
• Vertex Sphere Error
  
  • As lens gets further away the prescription becomes more positive

We can account for these factors by using compensated powers which aim to take into account that what the patient is experiencing is different to what appears on a focimeter reading.

Question 4

What is Near Vision Effectivity?

Answer 4

• The difference between the vergence of light actually leaving the lens when the light originates from a near object (L2’) and the anticipated vergence obtained by adding the incident vergence and the BVP of the lens (L1 + BVP) is known as the near vision effectivity error

Question 5

Main Differences Between Adult and Paediatric Anatomy

Answer 5

• Reduced crest height
• Larger frontal angle
• Widened splay angle
• Smaller bridge projection
• Smaller head and temple width
• Less pantoscopic tilt

Question 6

Characteristics of an Ideal Paediatric Dispense

Answer 6

• Pre-adjustment
  
  • Frame is first set-up
• Removing temple pressure
  
  • All pressure removed from temples
• Verify Equal vertex distances
  
  • angle of let back should not create unequal vertex distances
• Alignment of bridge
• Sides adjusted to contours of childs head
• Snugging
  
  • equal tension is returned to temples from each side of head throughout area behind Childs ears

Question 7

How to Cope in Children with Small PD but Large Head

Answer 7

• increase angle of let back
• put a gentle bow in the sides
• will ensure there is no pressure on the temples.

Question 8

Pantoscopic Tilt of Paediatric Dispenses

Answer 8

• An adult frame normally has a pantoscopic angle of between 8º and 12º
• Children’s frames should have a pantoscopic angle of around zero degrees to prevent them resting on the cheeks
• Sometimes adding a little pantoscopic tilt may help prevent the lashes from rubbing on the back of the lenses.

Question 9

Side Styles of Paediatric Frames

Answer 9

• Drop ends
• Curl sides (need fitted perfectly to avoid damage to ear cartilage)
• Loop ends (10mm past ear and strap fitted)

Question 10

Paediatric Eye Positioning Within Frame

Answer 10

• Choose a frame with a boxed centre distance similar to the patient’s PD to reduce the amount of decentration required
• Ideally, the child’s pupils should sit on, or just above the HCL. It is not acceptable for a child to be using only the top 10 to 20% of the lens.

Question 11

Considerations When Dispensing High Powers

Answer 11

• The prescription including the vertex distance
• Lens type, form and material
• Frame selection and fitting
• Field of view
• Spectacle magnification
• Centration
• Surface treatments
• The patient’s previous correction.

Question 12

Reasons for Correct Centration of Spectacle Lenses

Answer 12

• The positioning of the zone of the lens where the paraxial prescription is most effective
• Reducing unwanted differential prismatic effects
• Reducing the possibility of the formation of ghost images.

Question 13

How to Minimise Lens Decentration

Answer 13

• Select a frame that has a boxed centre distance that is as close as possible to the required distance (or near/intermediate) centration distance.

Question 14

Optimum Setup When Dispensing Full Aperture High Minus Lenses

Answer 14

• higher refractive index materials combined with aspherical surfaces and a reflection-free coating often provides the lens of choice.
• Such lenses give the practitioner a reasonable degree of control over the edge substance while providing good off-axis performance in oblique gaze.

Question 15

What is ABBE Number?

Answer 15

• Measure of dispersion
• How much refractive index changes between different wavelengths
• results in transverse chromatic aberration (TCA)

Question 16

How to Keep TCA Low in a High Minus Lens

Answer 16

• Apply correct horizontal and vertical centration
• low Abbe number lenses
• zero pantoscopic tilt
• Small BVD
• UCSC

Question 17

What is a Lenticular Lens?

Answer 17

• Minus lenticular lenses are lenses in which the edge thickness has been reduced or ‘flattened’, resulting in an aperture and a margin
• A line, similar to that of a solid bifocal usually exists between the flattened margin and the aperture

Question 18

Benefits of a Lenticular Lens

Answer 18

• reduction in volume
• reduction in the weight
• the real field of view offered by minus lenticular lenses is also very good.

Question 19

Example of Lenticular Lens

Answer 19

• Super Lenti Lens
• From -12.00DS to -48.00DS

Question 20

Common Problems with Dispensing High Plus Power

Answer 20

• Weight and thickness of the finished lenses
• Oblique performance when viewing off axis
• Magnification
• Lack of accommodation if aphakic
• Restricted field of view
• Centration and prescribed prism.

Question 21

Benefit of Aspheric Lenses In High Plus Prescriptions

Answer 21

• Removes unwanted induced cylinder off axis
• Still causes difference in spherical power when off axis

Question 22

What is a Polynomial Lens?

Answer 22

• With polynomial designs the surface power reduces from the centre of the lens to the edge, where the periphery is afocal
• the field of clear vision is quite small and wearers often turn their head to obtain a wide field of vision
• have replaced many lenticular designs for high plus

Question 23

Advantages of Polynomial Designs

Answer 23

• No visible dividing line
• Good mean oblique power when viewing off-axis
• Reduced distortion
• Very slightly thinner
• Little sensitivity to fitting distance change
• Increased field of view
• Reduction in the ‘Jack-in-the-box effect’
• Flatter
• Less magnification
• Less TCA.

Question 24

What is Jack-in-the-Box Effect?

Answer 24

• Scotoma produced by high plus lenses
• Due to reduction in FoV through a high plus lens