Ординатура / Офтальмология / Английские материалы / Quick Reference Dictionary of Eyecare Terminology 4th edition_Ledford, Hoffman_2005

A P P E N D I X 17

English and Metric

Conversion

LINEAR MEASURE

1 centimeter = 0.3937 inch (centimeters x 0.3937 = inches) 1 inch = 2.54 centimeters (inches x 2.54 = centimeters)

1 foot = 0.3048 meter (feet x 0.3048 = meters)

1 meter = 39.37 inches/1.0936 yards (meters x 39.37 = inches)

1 yard = 0.9144 meter (yards x 0.9144 = meters) 1 kilometer = 0.621 mile

1 mile = 1.609 kilometers

SQUARE MEASURE

1 square centimeter = 0.1550 square inch

1 square inch = 6.452 square centimeters

1 square foot = 0.0929 square meter

1 square meter = 1.196 square yards

1 square yard = 0.8361 square meter

1 hectare = 2.47 acres

1 acre = 0.4047 hectare

1 square kilometer = 0.386 square mile

1 square mile = 2.59 square kilometers

WEIGHT MEASURE

1 gram = 0.035 ounce (grams x 0.035 = ounces) 1 ounce = 28.35 grams (ounces x 28.35 = grams)

1 kilogram = 2.2 pounds (kilograms x 2.2 = pounds)

1 pound = 0.4536 kilogram (pounds x 0.4536 = kilograms) 1 metric ton = 0.98421 English ton

1 English ton = 1.016 metric tons

356 Appendix 17

VOLUME MEASURE

1 cubic centimeter (cc) = 0.061 cubic inch (cc x 0.061 = inches3)

1 cubic inch = 16.39 cubic centimeters (inches3 x 16.39 = cc)

1 cubic foot = 0.0283 cubic meter

1 cubic meter = 1.308 cubic yards

1 cubic yard = 0.7646 cubic meter

1 liter = 1.0567 quarts

1 quart dry = 1.101 liters

1 quart liquid = 0.9463 liter

1 gallon = 3.78541 liters

1 peck = 8.810 liters

1 hectoliter = 2.8375 bushels

Adapted with permission from Jacobs K, Jacobs L. Quick Reference Dictionary for Occupational Therapy. 3rd ed. Thorofare, NJ: SLACK Incorporated; 2001: 445.

358 Appendix 18

160 square rods = 1 acre

640 acres = 1 square mile

TROY WEIGHT

24 grains = 1 pennyweight

20 pennyweights = 1 ounce

12 ounces = 1 pound, Troy

CUBIC MEASURE

1728 cubic inches = 1 cubic foot

27 cubic feet = 1 cubic yard

LIQUID MEASURE

4 gills = 1 pint

2 pints = 1 quart

4 quarts = 1 gallon/231.0 cubic inches

AVOIRDUPOIS WEIGHT

27.34375 grains = 1 dram

16 drams = 1 ounce

16 ounces = 1 pound/0.45359237 kilogram

100 pounds = 1 short hundredweight

20 short hundredweights = 1 short ton

Reprinted with permission from Jacobs K, Jacobs L. Quick Reference Dictionary for Occupational Therapy. 3rd ed. Thorofare, NJ: SLACK Incorporated; 2001: 441-442.

A P P E N D I X 19

OPTICAL FORMULAS

It is always vital to use the correct units of measurement when using any formula. See Appendix 17 for English and metric conversions.

FOCAL DISTANCE/LENGTH

Distance from a lens to the point at which rays of light converge to a focal point.

F = 1/D where

F = focal length in meters D = lens power in diopters

DIOPTRIC POWER OF A LENS

D = 1/F where

D = lens power in diopters F = focal length in meters

TOTAL NOMINAL POWER OF A LENS

P1 + P2 = PT where

P1 = front surface power of the lens in diopters P2 = back surface power of the lens in diopters PT = total dioptric power of the lens

360 Appendix 19

OBJECT/IMAGE RELATIONSHIP

U + D = V where

U = vergence of the object rays at the lens in diopters D = power of the lens in diopters

V = vergence of the image rays at the lens in diopters

SNELL’S LAW (THE BASIC LAW OF REFRACTION)

Optical formula defining the refraction of light as it passes from one medium to another.

n sin i = n’ sin i’ where

n = IR of the first medium

n’ = IR of the second medium i = angle of incidence

i’ = angle of refraction

INDEX OF REFRACTION (REFRACTIVE INDEX)

IR = S ÷ M where

S = the speed of light (186,282 miles per second) M = speed of light in the medium

DIOPTRIC POWER OF A PRISM (∆)

P = C ÷ D where

P = prism power in diopters

C = displacement of image in centimeters D = distance from the prism in meters

PRENTICE’S LAW/RULE OF INDUCED PRISM

Optical formula defining the amount that a ray of light deviates (measured in prism diopters, ∆) from its original straight path when passing through a point at a given distance (measured in centimeters, cm)

∆= D * OC where

∆= induced prism in prism diopters D = dioptric power of lens

OC = decentration of optical center in centimeters

Note: If the induced prism is oriented up or down, cylinder must be added to the sphere power if it is horizontal (or nearly so). If induced prism is oriented in or out, cylinder must be added to sphere power if it is vertical (or nearly so). If cylinder is oblique, add half of the cylinder to the sphere power.

SPHERICAL EQUIVALENT

SE = A + (B/2) where

A = spherical component of prescription B = cylindrical component of prescription

TRANSPOSITION

Optical formula for changing plus cylinder refractions to minus cylinder and vice versa.

1.Add sphere and cylinder algebraically, being careful of + and –

2.Change sign of cylinder

3.Rotate axis by 90°

362 Appendix 19

POWER OF A MIRROR

P = 2 ÷ r where

P = power in diopters

r = radius of curvature (in meters) of the mirror

POWER OF MAGNIFIERS

Important Note: A working distance of 40 cm (16 in) is the standard basis for the manufacturing of low vision magnifiers, hence the 4.0 found in the formula below. Some manufacturers base their lens power on a standard of 25 cm (10 inches) rather than 40, and the “X” value should instead be multiplied by 2.5. This is also referred to as “X”. Yes, it is confusing. And yes, it can lead to incorrect powers when ordering low vision aids. You should read the dioptric power of any magnifier you order to be sure it is what the patient needs.

D = X * 4.0 where

D = lens power in diopters

X = the “X” power of the magnifier

MINIMUM (NEEDED) LENS BLANK SIZE

ED + [(A + DBL) – PD] + 2mm where

ED = effective diameter of the frame in mm A = A measurement of the frame in mm DBL = distance between lenses in mm

PD = patient’s pupillary distance in mm

VOGEL’S FORMULA FOR BASE CURVE SELECTION

Plus lens (glass): BC = SE + 6 where

BC = base curve

SE = spherical equivalent of lens

Minus lens (glass): BC = (SE/2) + 6 where

BC = base curve

SE = spherical equivalent of lens