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abo optical
math review
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08/28/2015

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Term

Transpose a prescription written in plus cylinder form to minus cylinder form as follows:

1. Add the sphere and cylinder powers to determine the new sphere power.
2. Change the sign of the cylinder.
3. Change the axis by 90 degrees.

Definition

Example:


Transpose -3.00 +2.00 x 30

1. Add the sphere and cylinder powers to determine the new sphere power.

(-3.00) + (+2.00) = -1.00

2. Change the sign of the cylinder

-2.00

3. Change the axis by 90 degrees.

120

The transposed prescription is:
-1.00 -2.00 x 120

Term

write the rx for the follwing true power

(REMEMBER BRIAN STROME )

Definition

TP+6.00@90 +4.75@180

(-6.00+4.75=-1.25)

RX +6.00-1.25X90

TP-10.00@180 -6.50@090

(+10.00-6.50=+3.50)

RX -10.00 +3.50X180

TP +1.00@168 -0.50@078

(-1.00-0.50=-1.50)

RX +1.00 -1.50X168

TP plano@045 -3.00@135

(0.00 -3.00=-3.00)

RX PLO -3.00X045

TP -3.25@090 PLANO@180

(+3.25 0.00=+3.25)

RX-3.25 +3.25X090

TP -3.25@135 -4.50@045

(+3.25-4.50=-1.25)

RX -3.25 -1.25X135

TP -8.00@055 -3.50@145

(+8.00-3.50=+4.50)

RX -8.00+4.50X055

Term

determine the true power of these lenses

(REMEMBER BRIGE AND LOVE STORE )MOVE AXIS 90DGRESS

Definition

RX +6.00-1.25X008

+6.00@008(BRIGE)

+6.00-1.25(LOVE STORE)=+4.75@098

RX -10.00+3.50X096

-10.00@096

-6.50@006

RX +1.00-1.50X168

+1.00@168

-0.50@078

RX PLANO-3.00X045

PLO@045

-3.00@135

Term
AXIS formula change 90 degress
Definition

axis NEVER TO PASS 180

so if axis is 90 add 90 (90+90)=180

if axis is 135 (take the first two number add togther (1+3=4)axis now is 45

if axis is 101(take the first two number add togther

(1+0=1)axis is now 11

if axis is 163(take the first two number add togther

(1+6=7)axis is now 73

Term
basic sv decention formulas
Definition

A+DBL=FPD

FPD-PPD/2=HORIZONTAL DEC

B/2-OCHT=VERTICAL DEC

2XHIGHEST DEC+ED=MBS

Term
bifocal decentration formulas
Definition

A+DBL =FPD

FPD-(NEAR)PPD/2=HORIZONTAL

B/2-SEG HT =VERTICAL

2XHIGHEST DEC+ED=MBS

Term
LENSOMETER KEY RULE TO ALWAYS RMEMBER
Definition

if signs are the same in the sph and cly(rx or true power)add them together and keep the sign

if sign are not the same in the sph and cly(rx or true power )subtract the different between the too and keep the sign to the biggest number

Term
sph equivalent
Definition

We are going to take a look at the prescription +1.50 -0.50 x090. What are the major power/meridian combinations?

Well, the +1.50 and the axis 090 go together. +1.50 -0.50 = +1.00, and 090 + 90 = 180, so the other leg of the optical cross is +1.00 at 180.

OK, in one major meridian we have a power of +1.50, and in the other major meridian we have a power of +1.00. In-between those two meridians the power on the lens is not really good, accurate power, but what is there ranges in-between +1.00 and +1.50.

If this lens were the one we are discussing, then the meridian with the +1.00 power would have that nice clear line focus at one meter, [since a +1.00D power has a focal length of 1/1D = 1m -- remember?] and the meridian with the +1.50 power would have that nice clear line focus at 0.67m [1/1.5 = 0.666 . . .]

If I had that +1.50 -0.50 x 180 lens outside in the nice bright sun I would not get a round image of the sun that I could use to start a fire. Instead, when I held it one meter from the paper I would have a line image, and when I held it 2/3 meter from the paper I would get another line image perpendicular to the first line image. What would I have in-between these two distances? I would have a varying amount of fuzzy image, going from the first clear line, to elliptical in one direction, through round, to elliptical in the other direction, to the other clear line.

There is no good power between the two major meridians. That is why, when you look at a spherocylindrical lens in the foci meter, you do not get clear images for powers between the sphere lines and the cylinder lines.

The sun is a round object, but the images that we get from the toric lens are line images and, at one point, a fuzzy round image. The spherocylindrical lens distorts the image of the sun. At one particular length, however, the image of the sun IS round, even though it is fuzzy. This distance corresponds to what we call the circle of least confusion which is really the position of no distortion. This focal length can be converted to a power. The power that we get is called the spherical equivalent of the lens. We can find the spherical equivalent by taking the average power on the lens: the average of the two major meridian powers.

OK. So. The lens in our example has +1.50 in one meridian and +1.00 in the other. The average of +1.50 and +1.00 is [(+1.50) + (+1.00)] [image]2 = +1.25. So the focal length of +1.25 is the spherical equivalent of the lens, the focal length corresponding to the power of +1.25 is 0.8m, so the circle of least confusion is located at 0.8m from the lens.

Another Example:
+1.00 -3.00 x 90

  • okay the +1.00 and the 90 go together.
  • at 180 the power is -2.00
  • so the average is [(+1.00) + (-2.00)] [image]2 = -0.50
  • so our spherical equivalent is -0.50

Term
cylinder power
Definition

[image]
Optical cross - used to diagram prescriptions


Prescriptions with cylinder axes of 180 or 90 will have all or none of the cylinder power in the meridian of concern. Prescriptions with oblique cylinders will only have a percentage of the cylinder power at 180 or 90.

It would be impractical to memorize the percentage of power in effect at every increment of a cylinder from zero to 90 degrees. Instead the following chart may be used as a handy reference.

Example #1 --Using the chart to determine the percentage of oblique cylinder at axis 180.

Rx +2.00 +5.00 x 30
[image]
Depending on the direction moved axis 30 deg is either 30 or 150 degrees from the 180 meridian of concern. Refer to column 1 for 30 to 150 line, now refer across to column 2 which gives a percentage to the corresponding line. In this case the corresponding percentage is 25%. Multiply the cylinder power by the percentage.
+5.00 X .25 = +1.25
In this case +1.25 diopters of cylinder power is in effect at the 180 line. Now, algebraically add the effective cylinder to the sphere power for the total which in this case would be

+2.00(sphere) + 1.25 (effective cylinder) = +3.25 (total power at 180)

Term
cylinder power
Definition
Degree from Axis
Percentage of Cylinder Power
0 or 180
0
5 or 175
1%
10 or 170
3%
15 or 165
7%
20 or 160
12%
25 or 155
18%
30 or 150
25%
35 or 145
33%
40 or 140
41%
45 or 135
50%
50 or 130
59%
55 or 125
67%
60 or 120
75%
65 or 115
82%
70 or 110
88%
75 or 105
93%
80 or 100
97%
85 or 95
99%
90 or 90 100%
Term
prism
Definition
[image]
[image]

Proof of Prentice's rule.

[image]

Distance(cm) X power = Prism , All it is is a way to determine the amount of prism you have induced taking into account the amount of power and the distance it is from where you actually need that power's optical center.
You figure you have someone wearing a +4 sph. OU with a pupilary distance of 60 mm but you find the PD in the frame is 64 then by using the formula you can find out how much that 4 mm of distance is inducing prism. .4 cm X 4 (D^) = 1.6^ (prism)total or .8^ in each eye.

Prentice's rule
A method of determining prism power at any point on a lens. Prism power equals the product of the dioptric power and the distance, in centimeters, from the optical center.


In a Prism the light ALW
AYS bends twords the BASE. No matter how you turn the prism the light will ALWAYS bend twords the base.
[image]
Here are some examples of a minus lens and a plus lens. In a minus lens the base is out in a plus lens the base is in. [image][image]
Here is a example of how a plus lens would sit on a patient. Notice the relation of the base to the nose. [image]
Here is the same thing in a minus lens [image]
This is where the optical center is on a plus and minus lens. On a plus lens it is where the two base portions of the prism are together. In a minus lens it is where the two apex point meet. [image]
[image]
Okay, in this example notice where the actual pd (60mm) matches up with the ground pd (64mm). Notice where the base is pointing on the prism that the patient is looking out of, twords the nose. The base in this case is base in. [image]
Okay, in this example notice where the actual pd (60mm) matches up with the ground pd (64mm). Notice where the base is pointing on the prism that the patient is looking out of, away from the nose the nose. The base in this case is base out. [image]

Now let's try to put this in a real question.

Example 1) The lab ground the following lens at 64mm PD. The patient has a actual PD of 60mm. For the following perscription what is the amount of prism induced?
RX: OD +5.00 -2.00 X 90
OS +3.00 -1.50 X 90
First step to this is to obtain the equivalent sphere power, so let's put this on a optical cross.
Right Lens:
[image]
Left Lens:
[image]

Okay now lets look at our decentration of 4mm. Now since that is a binocular (both eyes) measurment lets divide that in have so we have a monocular decentration of 2mm. With that lets figure our answer for both eyes:

Right Eye Prism: Prentice Rule is
Prism = Decentration (cm) X Diopters of power
In the Right eye we figured our power at +3.00 and our decentration at 2mm or .2cm so
Prism = (.2cm) X (+3.00D Power)
Prism = 0.6
Now lets figure out where this 0.06 prism is, Base in or Base out?

[image]
Looking at where the eyes lineup, where is the base facing? is it facing the nose? No, then it is facing out away from the nose. So our base is base out.

Our answer is: 0.60D Base Out
Left Eye Prism: Prentice Rule is
Prism = Decentration (cm) X Diopters of power
In the Left eye we figured our power at +1.50 and our decentration at 2mm or .2cm so
Prism = (.2cm) X (+1.50D Power)
Prism = 0.3
Now lets figure out where this 0.03 prism is, Base in or Base out?

[image]
Looking at where the eyes lineup, where is the base facing? is it facing the nose? No, then it is facing out away from the nose. So our base is base out.

Our answer is: 0.30D Base Out
Term
the dioptric systerm
Definition
The dioptric system. The diopter represents a unit
of measure for lenses. It defines a one-diopter (1.00 D.)
lens as a lens whose focal length is one meter (40 inches),
with its power being the reciprocal of its focal length. In
other words, a one-diopter lens focuses parallel rays of
light at a distance of one meter; a two-diopter lens focuses
parallel rays of light at a distance of one-half meter; a one-half
diopter lens focuses rays of light at a distance of two meters;
and so on.
[image]
Dioptric system of focal-length measurements.
A +1.00 D. lens focuses parallel rays of light at a
distance of one meter.
Power Focal Length
+4.00D 250mm
+3.00D 333mm
+2.00D 500mm
+1.00D 1 meter
+0.50D 2 meter
+0.25D 4 meter
-0.25D 4 meter
-0.50D 2 meter
-1.00D 1 meter
-2.00D 500 mm
-3.00D 333mm
-4.00D 250mm
  • Lens power = 1/focal length
  • Focal length = 1/lens power
Term

visible light spectrum

[image][image]

  • Wavelengths are measured in Nanometers (nm)
Definition
  • Wavelengths are measured in Nanometers (nm)

Of the radiation that reaches the earth's surface 60% is infrared, 37% visible, and 3% Ultraviolet (UV). The UV spectrum composes the 200 to 400 nanometers (nm) wavelength band, and is divided into 3 groups:

  • UVA is made up of wavelengths 320 to 400 nm
  • UVB 280 to 320 nm
  • UVC 200 to 280 nm

The earth's atmosphere absorbs UVC rays and only UVA and UVB reach the earth's surface.

  • UVB rays are more important in the development of skin cancer than UVA.
  • UVA rays cause aging and wrinkling of the skin, but also increase the damaging effects of UVB.

Table 1: The Regions of the Electromagnetic Spectrum

RegionWavelengthFrequency / Hz
Radio-Frequency > 30 cm < 109
Microwave 3mm - 30cm 109 - 10 11
Infrared 1000nm - 3mm 1011 - 3 x 10 14
Visible 400nm - 800nm 4 x 1014 - 8 x 1014
Ultraviolet 300nm - 3nm 1015 - 1017
X-Rays and Gamma-Rays < 3nm >1017

Table 2: Colour, frequency and wavelength of light

RegionFrequency / 1014 HzWavelength / nmEnergy per photon / 10-19 J photon-1
Microwaves and Radiowaves 3x1011 Hz and below 3x106 and above 2.0x10-22 J and below
Infrared 3.0 1000 1.9
Red 4.3 700 2.8
Orange 4.8 620 3.2
Yellow 5.2 580 3.4
Green 5.7 530 3.7
Blue 6.4 470 4.2
Violet 7.1 420 4.7
Ultraviolet 10 300 6.6
X-rays and Gamma Rays 103 and above 3 and below 660 and above
UV is generally divided into four ranges. We are actually only interested in three of them.

VACUUM UV
Rays in the range of about 100nm to 200nm exist only in a vacuum, and are not important to us. This is why most discussions about UV only include the next three types.

FAR UV or UV-C
The rays in this range, from 200nm to 290nm, are generally absorbed by the ozone layer of the atmosphere, and therefore are not interesting to us. As we damage the ozone layer of the atmosphere with pollution, (and no, I am not going to get into a discussion with anyone on whether this is actually happening, whether it is the result of man-made pollution, or whether it is worth worrying about) UV-C may become more important to us.

UV-B
The rays in this range, from 290nm to 320nm, are almost entirely absorbed by the tear film, the cornea, and the conjunctiva. Thus they do not reach the retina, which is the light-sensitive portion of the eye.
But 'absorption' does not mean 'no worries.' When biological tissues absorb any portion of the electromagnetic spectrum a change occurs. In this case, the changes are slow and reversible; but long intense exposure will result in permanent changes. An example of a temporary change is 'photokeratitis' or snowblindness, which is simply a sunburned cornea. This is uncomfortable and results in red-eye, dryness, scratchiness, and discomfort. Continued exposure results in permanent changes, for example, pterygium and pinguecula, which you will learn about in anatomy, and which are permanent thickening in the clear covering of the eye and are unsightly at best. Welders flash is another condition caused by UV-B exposure of the cornea.

The portion of UV-B that does reach the lens of the eye is thought to be a factor in the development of 'brown' or 'sunshine' cataracts, one of the forms that cataracts can take.

NEAR UV or UV-A
Near UV, 320nm to 380nm, is transmitted by the cornea and is partially absorbed by the lens. In a very young child the lens transmits most of the UV-A. As we age the lens begins to turn mildly yellowish, and it absorbs progressively more UV-A. This yellowing of the lens is NOT the same as cataract formation. However, one form of cataract, the 'brown' or 'sunshine' cataract, may possibly be caused by VU-A as well as UV-B.
As in sunburn and suntan, which are the result of UV exposure of the skin, UV exposure causes changes in the cornea and lens which are partially corrected over time. But the correction is never complete, and eventually the changes to the tissues become irreversible damage.

Many medications increase the body's reaction to UV exposure. Examples are analgesics, antibacterial agents, tranquilizers, diuretics, antifungal agents, and contraceptives.

ANSI standards set the upper limit of the UV range as 380nm. All other limits indicated here are approximate; there is no set wavelength where one attribute stops abruptly and another begins. We frequently use 400nm for a UV cutoff because that is the wavelength used by UV absorption dye manufacturers to classify their product.

Crown glass absorbs UV below 300nm, but transmits 90% of the UV over 300nm.

Untreated CR39 absorbs UV below 360nm, and transmits 90% of UV-A above 360nm. We routinely use dyes to increase CR39 absorption of UV to 400nm.

Clear coated polycarbonate absorbs all UV below 380nm. The absorptive pigments are in the clear scratch resistant coating.

Photochromatic glass lenses such as Corning's PGX absorb all UV below 315nm, and transmit only 4% of the UV-A up to 380nm in the full darkened state.

UV absorption is the mechanism used by both glass and plastic photochromatic materials to cause their color change. Thus, plastic photochromatic lenses also absorb UV. I have not seen the absorptive ratings for plastic photochromatics published in textbook form yet, so I am not sure of the transmission and cutoff ranges

Term
lens guide
Definition

Lens Guide Correction and Thickness

[image]

Lens Correction

Condition Explination Correction
Myopia The condition of simple myopia or nearsightedness arrises as a result of light coming to focus before reaching the retina consequently resulting in blurred vision. Placing a diverging minus lens in front of a myopic eye will move the focal point back on to the retina and, therefore, correct the blurred vision.
Simple Myopic Astigmatism One meridian focuses on the retina while the other meridian focuses in front of the retina. Plano lens with a minus cylinder lens.
Compund Myopic Astigmatism Both meridians focus in front of the retina but at two separate points. Minus sphere with a minus cylinder lens.
Hyperopia The condition of simple hyperopia or farsightedness arises as a result of light coming to focus at a point theoretically behind the retina, consequently resulting in blurred vision. Placing a converging plus lens in front of the hyperopic eye will move the focal point forward on to the retina and, therefore, correct the blurred vision.
Simple Hyperopic Astigmatism One meridian focuses on the retina while the other meridian focuses behind the retina. Plano lens with a plus cylinder lens.
Compund Hyperopic Astigmatism Both meridians focus behind the retina but at two separate points. Plus sphere lens with a plus cylinder lens.
Mixed Astigmatism One meridian focuses in front of the retina while the other one focuses behind the retina. A lens which is plus in one meridian and minus in the other.

Material Guide

Material Index Approximate Thickness
CR-39 1.49 5% thicker than Crown Glass
Crown Glass 1.523
Optical Tools & Gauges 1.53 Calibration Index
Polycarbonate 1.59 10% thinner than Crown Glass
Polyurethane 1.60 10% thinner than Crown Glass
Highlite 1.74 25% thinner than Crown Glass
Thindex 1.80 35% thinner than Crown Glass

Simple decentering edge thickness guide

Direction of Decentering Plus Lens Minus Lens
In - nasally Thicker nasally - Thinner temporally Thicker temporally - Thinner temporally
Out - temporally Thicker temporally - Thinner nasally Thicker nasally - Thinner temporally
Up Thicker at top of lens Thicker at bottom of lens
Down Thicker at bottom of lens Thicker at top of lens

Lens Types

Plus Lenses [image] [image] [image]
Minus Lenses [image] [image] [image]
Term
multi foucal guide
Definition

Multi-Focal Lens Guide

Bifocal Type Description
[image] The first type of multi-focal lens we will talk about is the Flat-top or "D" shaped bifocal. The most common used is the Flat-top 28. The 28 stands for the widest part of the segment. Other variations of this bifocal are the FT 25, 35, FT 40 and the FT 45.
NOTE: When you measure for a FT style bifocal you measure from lower eye-lid to the bottom of the frame.
[image] Next we will discuss the trifocal which has Three Focal Lengths. They consist of the distance portion, and intermediate segment, and the near segment of the lens. The intermediate segment is usually one half the power of the reading segment and increases the reading distance to twice the distance of the bifocal segment. The focal length is about 24-36 inches. The most common trifocal used is the 7x28. The 7 stands for the depth of the middle segment and the 28 stands for the width of the segment.
NOTE: When you measure for a trifocal you should measure from the bottom of the pupil to to bottom of the frame.
[image] Another type of lined bifocal/trifocal is the Executive. This type of lined multifocal has the seg go the entire width of the lens. Because of the way this lens is fabricated it is almost twice as heavy and thick as other multifocals. This lens should be avoided. This lens is also available in a bifocal or trifocal lens.
NOTE: Fit the same as a Bi-focal and Tri-focal
[image]
[image] A Double D Bifocal has a seg at the top as well as the bottom of the lens. This would be helpful for people who have to be able to read or see up-close above their head as well as below (i.e. electricians)

NOTE: this should be fit the same as a bifocal but the upper seg should be at the upper eyelid.

[image] A Progressive lens (Also known as a "No-line bifocal or No-line Tri-focal) is designed to give the patient the feeling of natural vision in all focal lengths because of the precisely ground corridor in which the power gradually increases to the full reading power of the lens. Due to the design of the lens, there are soft focus areas on the periphery that are not usable. With a no-line, the better the lens the less peripheral distortion. Varilux is the best on the marked as of today.
NOTE: This lens should be measured from the center of the pupil to the bottom of the frame. It is better to err on the side of too low then too high.
Term
lens coating
Definition

Lens Coatings

A simple fact is that a clear uncoated lens will only allow 92% of the afailable light to pass through while the other 8% is lost in the form of reflected glare. Not only does this loss of light diminsh visual acuity, it also creates troublesome reflections which tend to mask the wearer's eyes.
Color coatings can add cosmetic versatility to lens. Color coating can also add reflectance to a glass or plastic surface as in the case of gradient or solid mirror coatings.

Coating Advantage
Blu-Blocker is a lens for use in bright sunny conditions. This lens completely blocks blue light below a wavelength of 540nm so providing maximum protection from glare for those with high degrees of light sensitivity in bright outdoor situations.
Color - Yellow Good for hunting: Good for night driving (A/R Coating is better)
Color - Brown Good Driving lens
A/R Coating
Polarized The principle of a polarized lens is best illustrated by observing the use of venetian blinds. The blind blocks light at certain angles, while allowing light to transmit through select angles. True polarization is achieved by shutting out 100% of undesirable light and allowing 100% of desired light through.
One of the most noticeable effects of a polarized lens is the reduction of squinting. Squinting can cause the patient eyestrain and tension.
UV Filter
Term
the metric systerm
Definition

The Metric System

The optical industry is a world industry and as such it is gauged by the world standard of measurement which is metric. Temple lengths, eye sizes, pd's, lens sizes, blank sizes, and segment heights are all measured in millimeters. To be an effective dispensing technician a good working knowledge of the metric system is necessary. To be successful on the exam it is often required to convert a correct answer in millimeter form into corresponding meter form.

The Metric Values:

1 Meter (M) = 39.37 inches
0.0254 (M) = 1 inch
1 Decimeter (dm.) = One Tenth (1/10) (0.1) Meter
1 Meter (M) = 10 Decimeter (dm.)
0.254 dm. = 1 inch
1 Centimeter (cm.) = One One Hundredth (1/100) (0.01) Meter
1 Meter = 100 Centimeters (cm.)
2.54 cm. = 1 Inch
1 Millimeter (mm.) = One One Thousandth (1/1000) (0.001) Meter
1 Meter = 1000 Millimeters (mm)
25.4 mm = 1 Inch
1 Nanometer (nm.) One One Billionth (1/1,000,000,000) (M)
Nanometer is also the designation used to measure wavelengths

CONVERSION:

Meters to Millimeters:

Move the decimal three places to the right by multiplying the Meter Value by 1000.

Example - Convert 0.012 M to Millimeters
(0.012M x 1000 = 12 mm)

Convert 2 M to Millimeters)
(2M x 1000 = 2000 mm)

Millimeters to Meters:

Move the decimal three places to the left by dividing the Millimeter Value by 1000.

Example - Convert 3000 mm to Meters.
(3000 divided by 1000 = 3M)

Convert 1200 mm to Meters.
(1200 divided by 1000 = 1.2M)

Meters to Centimeters:

Move the decimal two places to the right by multiplying the Meter Value by 100.

Example - Convert 0.012 M to Centimeters
(0.012M x 1000 = 1.2 cm)

Convert 4 M to Centimeters
(4M x 100 = 400 cm)

Centimeters to Inches:

There are 2.54 centimeters per inch, so in order to convert you must divide the number of centimeters to be converted by 2.54

Example - Convert 36 cm to Inches
(36 divided by 2.54 = 14.17 Inches)

Convert 300 cm to Inches
(300 divided by 2.54 = 118.11 Inches)

Millimeters to Inches:

There are 25.4 Millimeters per inch, so in order to convert you must divide the number of Millimeters to be converted by 25.4

Example - Convert 500 Millimeters to Inches.
(500 divided by 25.4 = 19.68 Inches)

Convert 72 Millimeters to Inches.
(72 divided by 25.4 = 2.83 Inches)

Inches to Millimeters:

In order to go from Inches to Millimeters it is necessary to multiply the number of inches to be converted by 25.4

Example - Convert 17 Inches to Millimeters.
(17 x 25.4 = 431.8 Millimeters)

Convert 2 Inches to Millimeters.
(2 x 25.4 = 50.8 Millimeters)

** Remember -The Metric System works on multiples of 10, Based on the standard Meter.

Term
anatomy physiology
Definition

[image]

Frontal View of Left Eye
[image]
  1. Upper eyelid
  2. Lower eyelid
  3. Lateral angle
  4. Medial angle
  5. Lacrimal caruncle
  6. Limbus
  7. Iris
  8. Pupil
  9. Lacrimal papilla
  10. Sclera
  11. Plica semilunaris

Cornea

The clear front window to the eye. It is the most important part of the focusing system of the eye and brings objects into focus on the retina.

Anterior Chamber

The space between the cornea and the iris which is filled with fluid aqueous.

Chamber Angle

The structure formed by the junction of the iris with the corneo-sclera. Normal fluid drainage from the eye occurs through the trabecular meshwork which lies in the chamber angle. Malfunction of the chamber angle is the cause of glaucoma (high pressure in the eye).

Iris

The colored tissue behind the cornea - color varies from pale blue to dark brown.

Pupil

The round hole in the iris. Its size is automatically varied to regulate the amount of light entering the eye.

Lens

A clear biconvex structure behind the iris which works in conjunction with the cornea to focus light onto the retina. When the lens becomes opaque or cloudy, it is called a cataract.

Ciliary Body

A ring of tissue lying at the base of the iris. Its muscle fibers serve to change the curvature of the lens and thereby provide fine focusing of light onto the retina.

Zonules

Threadlike filamentous attachments which hold the lens in place in the eye.

Vitreous

The clear jelly which fills the space between the lens and the retina. Eighty percent of the eye is filled with vitreous.

Sclera

The white, tough protective outer wall of the eye.

Choroid

A spongy layer filled with blood vessels. It lies between the sclera and the retina. The choroid nourishes the outer layers of the retina.

Retina

The vital thin layer of tissue composed of millions of visual cells which lines the inside back two-thirds of the eye. The retina is analogous to a film in a camera. It receives light and sends tiny electrical impulses to the brain to give sight.

Optic Nerve

A cable-like structure composed of thousands of nerve fibers which carry impulses from the retina to the brain where visual perception occurs.

Macula

The highly developed central zone of the retina. The macula gives critical vision for reading and discrimination small objects.


[image][image]

Muscles of the Eye

The Eye has the ability to move in a variety of direction. It achieves this great versatility by vitrue of the
6 Musclesthat are attached to the sclerotic coating of the globe. Working together the following muscles achieve rotation:

Six Muscles of the Eye

Superior Rectus Turns the eye upwards and inwards
Superior Oblique Turns the eye downwards and outwards
Internal Rectus Turns the eye inwards and toward the nose
Inferior Oblique Turns the eye upwards and outwards
Inferior Rectus Turns the eye downards and inwards
External Rectus Turns the eye outwards

When these muscles are working correctly in conjunction with one another the eyes can achieve great range for distance vision as well as Convergence necessary to achieve Binocular Vision. Binocular vision refers to the two eyes Fusing their two images to form a single three dimensional image that the brain can readily interpret

Term
light theory
Definition

Light Wave Theory

A fundamental understanding of light is important to the optical technician since it is throught the vehicle of light that the whole system of vision functions. There are two separate theories of light and while neither one totally explains the mysterious properties of light both are accepted since each approach explains some feature of light which the other cannot. The two theories which we will briefly outline are known as the Corpuscular Theory and the Electromagnetic Wave Theory.

CORPUSCULAR THEORY

Light is composed of a stream of invisible bundles of radiant energy known as photons. This radiant energy moves away from its source in vibrating straight lines of energy through space, air and transparent objects.
This theory can be used to explain how the dial of a camera moves when light enters the exposure meter. The impact of the light bundles or photons releases charged particles called electrons which create an electric current and thus activate the dial. Also this theory can help explain the formation of shadow when an object intercedes and blocks a stream of ongoing particles.
While this theory explain certain properties of light it fails to explain others. These gaps in the corpuscular theory open the way for a differant theory of light
[image]

ELECTROMAGNETIC THEORY

Light leaves its source in a series of pulsations knows as waves. Like sparks from a sparkler light waves proceed from the source in all planar directions. These waves travel in concentric circles at a constant speed of 186,000 miles per second in a medium of air.
The distance between pulsations is known as wavelength. Wavelengths are measured in Nanometers which is the designation of one/one billionth of a meter.
This theory explains how white light is broken into it's component parts when it passes through a prism. When we see various colored light what we are viewing is light of one specific wavelength. Each color has it's own specific wavelength with red being the longest and violet being the shortest and all others falling in between the two extremes. The light that we use for vision is referred to as visible light, which is a combination of all the colors of the spectrum. Light immediately below the visible spectrum is ultraviolet, and light immediately above the visible spectrum is infrared or heat energy.
[image]
Light waves in phase compound on another. [image]
Light waves 180 deg. out of phasedestroy one another. [image]
Light waves 90 deg. out of phase reduce each front by half. If the waves are partially aligned, there will be partial interference
Term
When ordering a six inch temple, its corresponding millimeter value is which of the following?
Definition
1 inch is = 25.4 mm so
6 inches X 25.4mm = 152.4mm
Round that to the closest correct answer and your answer would be D) 150mm
Term
2) What is the focal length of a 2 diopter lens?
Definition
Focal length is equal to 1/diopter value and your answer is in meters
1/2 = 0.5M
0.5M = 50cm
0.5M = 500mm
your answer would be C) 500mm
Term
3) Wavelengths are measured in
Definition
Wavelengths are measured in nanometers. your answer would be D) Nanometers
Term
4) Which of the following has the shortest and therefore most harmful wavelength?
Definition
  • UVA is made up of wavelengths 320 to 400 nm
  • UVB 280 to 320 nm
  • UVC 200 to 280 nm
  • Infrared 1000nm - 3000nm
    Your answer would be A) UVC 280 to 320 nm
Term
5) When two light waves are "in phase" they will?
Definition
Light waves out of phase would either Destroy one another or Reduce one another.
Light waves in phase would Compound one another.
Your answer would be C) Compound one another
Term
6) The cylinder axis tolerance for a 1.00D cylinder according to ANSI Z80.1 is
Definition
ANSI standard for cylinder axis tolerance is Plus or Minus 3 degrees
Your answer is C) Plus or Minus 3 degrees
Term
7) A plus lens decentered "out" will result in a finished lens which is
Definition
Since the thickest part of a plus lens is in the center, the closer the center is to the edge of the frame
the thicker that part of the edge will be. So if a plus lens is decentered out twords the temples the finished lens would be thicker temporally.
Your answer is B) Thicker temporally
Term
8) A minus lens decentered "out" will result in a finished lens which is
Definition
Since the thickest part of a minus lens is on the edge, the closer the center is to the edge of the frame
the thinner that part of the edge will be. So if a minus lens is decentered out twords the temples the finished lens would be thicker nasally.
Your answer is C) Thicker nasally
Term
9) An uncoated normal plastic lens will reflect approximatley _____% of light.
Definition
A clear uncoated lens will only allow 92% of the available light to pass through while the other 8% is lost in the form of reflected glare.
Your answer is C) 8%
Term
10) A lens having a minus curve on both front and back is called
Definition
  • A Meniscus lens has one convex side and one concave side
  • A Biconvex and two convex sides
  • A Biconcave has two concave sides
    Your answer in this case would be C) Biconcave
Term
1) What is a geneva lens measure
Definition
A. Lens Clock
Term
The ophthalmoscope is used to:
Definition
An instrument for examining the interior structures of the eye, especially the retina, consisting essentially of a mirror that reflects light into the eye and a central hole through which the eye is examined
Term
The keratometer is used to:
Definition
A keratometer is a medical instrument that eye care professionals use to measure the curvature and reflection of the anterior surface of the cornea. A keratometer, also sometimes called an ophthalmometer, is primarily used to diagnose the presence of astigmatism and to determine the degree and treatment of astigmatism. Astigmatism is a condition of the eye in which the cornea or lens is misshapen and can result in vision problems.
Term
A tonometer is used to measure:
Definition

The tonometer is used in order to ensure a person's optic nerves are healthy. Optometrists check the pressure placed on them by the fluid in the eyes. This pressure is called intraocular pressure and should measure between 10 mmHg and 21 mmHg. Measurements that are higher than normal can be a sign of early glaucoma or retinal detachment.

The tool used to measure intraocular pressure is called a tonometer. Is used to measure the production of aqueous humor, the liquid found inside the eye, and the rate at which it drains into the tissue surrounding the cornea. Usually, the tests performed are simple and require only a short span of time to complete.

Term
In a minus powered lens light rays ____ and come to a focal point ____ the lens.
Definition
  • Converge, in front of
Term
1 Prism Diopter can be described as deviating a ray of light:
Definition
  • 1 meter / 1 millimeter
Term
Using Prentice's Rule a -4.00 lens must be decentered ___ to induce 1.00 Prism Diopter
Definition
  • 2.5 mm
Term
When the direction of light changes as it passes from one medium to another, it is called
Definition
  • Reflection
Term
The P.D. written on the order is mistakenly noted as 70 mm, when the actual measurement should be 66 mm. What unwanted prism would be found in the prescription if the distance prescription is O.U. -15.00D sphere?
Definition
  • 3D base out each eye
Term
A +5.00 diopter lens decentered 2 mm creates ____ diopters of prism
Definition
  • 1(FORMULE TO YOU power X dec /10)
Term
prentice rule
Definition

Once it is found that either wanted or unwanted prism is present in any given lens and the direction of the base is determined, the power of the prism can then be calculated utilizing a simple formula called Prentice's Rule. It is important to become familiar with Prentice's Rule since it is used many times each day as a routine matter in the optical dispensary.

Prentice's Rule states: The power of the prism is equal to the power of the lens in diopters times the amount of decentration in millimeters divided by 10.

Stated Algebraically:

[image] =
F x dec (mm)

10

where:

[image] = prism diopters
F = power of the lens in diopters
dec = decentration or distance in mm away from the optical center of the lens.

Example 1: How much prism is induced 4 mm away from the optical center of a +3.00 D spherical lens?

Applying Prentice's Rule, F = +3.00 and dec. = 4.0 mm
so:

[image] = F x dec/10
= 3.00 x 4/10
= 1.2 [image]

Example 2: A patient's PD was mistakenly noted to be 62 mm when the actual measurement is 66 mm. If the power of the lenses are ­2.00 diopters in each eye, how much prism will be induced in the finished spectacles and in which direction is the base?

[image] = (F x dec) / 10 Right Eye: (-2.00 x 2) / 10 = 0.4 [image] Total = 0.8 [image] Base out
Left Eye: (-2.00 x 2) / 10 = 0.4 [image]

How do we know the direction of the prism is base out? The optical centers were positioned a total of 4 mm narrower than the actual PD. By referring to the figure D on page 13 which illustrates this situation with a minus lens. It can be observed that the direction of the base of the prism through which the patient is looking is

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