>Spectacle frames are the easy part- the lenses are tough.

While there are fatheads and pinheads who need different size frames,

there are many frames around that are satisfactory for a large

proprtion of head sizes. A limited number of frame size can cover

much of what you would need for an adult population. You might need

smaller frames for kids.

The lenses are the big problem.

Generally these need to be customized for the individual. While most

individuals have similar Right eye and Left eye characteristics (and

thus need lenses of similar powers) there can be still substantial

differences between right and left eyes.

For correcting distance vision,first there are spherical power

components, the bulk of which are within a +/- 4.00D range, but a +6D

to -10D range would be required to cover about 95% of the population.

Normally we work in steps of 0.25 D but 0.50D steps could be

reasonably satisfactory,

But then there is the astigmatism component that has both a power

component and an orientation component. Most astigmatism is less

than 3D but the axis may be oriented at angles going up to 180

degrees usually in 5 degree increments While many people have

astigmatism of low power that might be ignored, for others the

astigmatism is the main problem.

Glasses for reading or close work can be simpler. Many people over

age 50 will need separated correction for their distant and near

vision tasks.

There are organizations (such as VOSH) that have a lot of experience

at getting cheap vision care to developing countries. They might be

able to give you some useful advice.

Good luck

Ian

Ian L Bailey

1) What are the main variables in making eyeglasses (I'm guessing things

>like prescription, width of frames, length of frames... What are others?)?

Variables: type of prescription (single vision, multifocal,

occupational); lens type (single vision, bifocal, trifocal, progressive,

blended, occupational); lens material (glass, CR-39, mid-index, high

index, very high index, polycarbonate, Trivex); lens options (clear

lenses, tinted lenses, coated lenses, such as anti-reflective,

anti-scratch); impact resistance requirement (primarily for glass lenses

since all plastic resin lens material are inherently impact resistance);

lenses meeting prescription tolerances as recommended by ANSI standards;

frames (various sizes, colors available) must be in accordance to

specifications as ordered (eyesize, bridge size, temple style and length,

color)

>2) What is the range at which these variables vary? (could you give me

>specific ranges for each variable?)

all above variables must meet as specified on prescription order with no

deviation (otherwise order is incorrect); with reference to the actual

prescription numbers, ANSI standards are available which specifies the

allowable tolerance range where a prescription lenses may deviate from and

considered acceptable.

For example if a prescription asks for -3.00DS, tolerance range is b/w

-2.87DS and -3.13DS.

there are many components to the prescription lens such as the sphere,

cylinder, axis, prism, add power, segment height, lens thickness, lens

curvature,lens optical center separation, etc for which there is an

allowable deviation

>3) Do most people fit within a smaller window? For example, are 75% of

>prescriptions within .25 (or whatever measurement you use) of each other?

not exactly sure what you are asking here...prescriptions can go from + or

- 0.25 (at the lowest possible prescription) to whatever number. since

all prescriptions typically have a sphere, cylinder and axis component,

many permutations are possible.

if you are asking the bulk of the spectacle wearing population, what their

prescriptions fall within, generally the majority of patients will be

within sphere power from +4.00 to -6.00 and cylinder power no greater than

-2.00.

>4) What is the most expensive part of making glasses? Why is this part the

>most expensive?

the cost of the actual production of the glasses (meaning the process of

cutting a lens blank to fit into a frame) is not necessarily all that

expensive. the cost comes from primarily the materials, the cost of the

lens blank and the cost of the frame. the material cost can be very high

or very low, depending on quality, type of material,etc.--there is a great

variance here.

Rich,

it looks like you are embarking on a great humanitarian effort and i have

tried to answer the questions as best as i think you are looking for. if

i can help clarify please don't hesitate. Here at Berkeley, many of our

students participate in providing vision care to 3rd world countries

through volunteering through a program like VOSH (volunteer optometric

services to humanity).

If there is anything that i can help you with regarding your project

please let me know.

[btw, yes we ARE the VERY BEST school!!]

George Lee