Week 8 & week 9 - prisms

What is a prism?

• deviates light without chan going Vergence

• Deviation if light displaces image

• Prism has no effect on magnification or size ophthalmic lenses behave Kure or uses due to Change in thickness

Where is light always deviated towards to in a prism?

• the base

How do you calculate deviation?

• d = (n’-1)a

How do you calculate prism diopters?

P = 100 x tan d

In what increments are prism dioptres prescribed in?

0.25

How do you calculate tan (0)

Displacement/distance

How do you calculate resultant power of a prism?

• Pythagoras

• A² = b² + c²

How do you calculate tan - in order to find the axis

• opposite/adjacent

• Vertical/horizontal

How do you find the axis for the right eye?

• subtract feta from 360

How do you find axis for left eye?

• subtract feta from 180

How do you calculate prism power, using decentration and power?

P = cF

Decentration - when up or in

Decentration + when out or down

When combining prisms, in the vertical meridian what should prisms add up to in both eyes?

2

When combining prisms, in the horizontal meridian what should prisms add up to in both eyes?

5

How to calculate difference in thickness between base and apex?

P = 100 (n’-1)g/d

G = dP/ 100 (n’-1)

• g = thickness difference between base and apex

How do you calculate prismatic power of a toric lens, describe the steps?

1. De-transpose to powers

2. Know powers are perpendicular to axis - so basically swap around

3. Decide which d is for the horizontal/vertical meridian

4. If it mentions optical centre, flip it

5. Use P=cF, calculate for both 90 and 180

How do you avoid inducing unwanted prism?

• correctly centring lenses

• Lenses should be placed correctly so OC coincides with patients pupil centre

• Choose appropriate frame

What is the optical centre?

• mid point of lens

• Light through this point is undeviated

What is pupillary distance?

• distance between pupil centres

Boxed centre distance?

• distance between mid-points of frame apertures

What is DBL?

Distance between lenses

How do you calculate decentration of OC from pupil?

Boxed centre distance - pupil distance

• divide by 2 for both eye

What is blank size?

• the finished prescription lenses will be ordered in the smallest appropriate diameter

• Standard blank size approx 50-75mm in 5mm steps

Why is appropriate blank size an issue for + lenses?

• minimise edge thickness

How do you calculate the minimum size uncut?

MSU = longest diameter + (2x decentration) + waste (2mm)

What is MSU?

• unwanted prism from trying to use a too small finished lens

• Or frame too large

• We need blank size that allows OC to align with pupil centre