Biomedical Engineering Reference
In-Depth Information
KEY FORMULAE
Power in an oblique meridian (sine-squared formula):
F
=
(
F
cyl
)sin
2
θ
θ
Spherical equivalent:
cylindrical correction
spherical equivalent
=
spherical correction
+
2
REFERENCES
Brooks, CW, Borish IM.
System for Ophthalmic Dispensing
, 3rd ed. St. Louis: Butterworth-
Heinemann, 2007.
Gwiazda J, Scheiman M, Mohindra I, Held R. Astigmatism in infants: changes in axis and
amount from birth to six years.
Invest Ophthalmol Vis Sci.
1984;25:88.
Howland HC, Atkinson J, Braddick O, French J. Infant astigmatism measured by photore-
fraction.
Science
. 1978;202:331-333.
Thorn F, Held R, Fang L. Orthogonal astigmatic axis in Chinese and Caucasian infants.
Invest Ophthalmol Vis Sci.
1987;28:191.
SELF-ASSESSMENT PROBLEMS
1. A lens has the following formula:
090. (a) Draw a lens cross.
(b) Write the prescription in minus-cylinder form.
2. A point source is located 50.00 cm in front of the following lens:
−
1.00
−
1.00
×
180. (a) Which meridian is focused closest to the lens? (b) Is the line that is
focused closest to the lens horizontal or vertical? (c) How far is this image
located from the lens? (d) Answer questions
b
and
c
for the image that is located
furthest from the lens (i.e., the image formed by the other meridian of the lens).
(e) Calculate the linear extent of the interval of Sturm. (f) Locate the circle of
least confusion.
3. A point source is located 25.00 cm in front of the following lens:
+
4.00
+
2.00
×
+
5.00
+
2.00
×
090. Answer all the questions asked for Problem 2.
4. A cross is located 40.00 cm in front of the following lens:
090.
(a) At what distance is the horizontal line imaged? (b) At what distance is the
vertical line imaged? (c) What is the linear extent of the interval of Sturm?
(d) Locate the circle of least confusion.
5. A cross is situated 40.00 cm in front of the following lens:
+
6.00
−
1.00
×
+
6.00
−
2.00
×
180.
Answer all the questions asked for Problem 4.
