Fan & Block

Question 1

Fan & Block

Answer 1

Used to determine the axis and magnitude of Astigmatism.

The fan is used to determine the presence of any Astigmatism and its principal axes.

The blocks are used in conjunction with cylindrical lenses to neutralise Astigmatism.

Question 2

Fan & Block technique illustrates __________________

Answer 2

the nature of Refractive Asymmetry.

Question 3

What is Fan & Block?

Answer 3

A series of radiating lines spaced at 10’ intervals and arranged like a rising sun.

Question 4

Use of Fan & Block -

Answer 4

Radial lines of thickness about the limb width of a 18m letter are spaced at 10’ intervals around a central panel carrying an arrowhead and two sets of mutually perpendicular lines.

Each line subtends an angle of 1.375 minutes of arc (equivalent to 6/15) at the eye.

The arrowhead is used to refine the determination of the axis of the Astigmatism.

Question 5

What is the objective of Fan & Block?

Answer 5

To bring the 2 focal lines together i.e. move the anterior focal line back to meet the posterior focal line using cyl lenses and essentially collapsing the Interval of Sturm.

Question 6

Advantages of Fan & Block -

Answer 6

• Accommodation is well controlled as the patient is fogged prior.
• Does not require the patient to memorise two images presented sequentially and compare them as is used in JCC.
• Can be used to quickly ascertain if there is actually any astigmatism after Best Vision Sphere.
• Good for detecting irregular astigmatism.
• Alternative method if the responses have been poor for JCC.

Question 7

Disadvantages of Fan & Block -

Answer 7

• Is less sensitive at determining small cylinders.
• It is not reliable for determining astigmatism more than 2.50D.
• If the person has a head tilt, the results cannot be used.
• Computerised charts do not tend to have the best targets