1. Subject 18:Progressive Addition Lenses.
Identification of Progressive Addition Lenses
Regions of a Progressive Addition Lens
Markings in a Progressive Addition Lens
Center of a Progressive Lens
Determining the parameters of a progressive lens
Aim. Give the basic concepts to allow the identification and manipulation of this kind of ophthalmic lens.  

2. Identifying Progressive Addition Lenses
IDENTIFICATION THROUGH OBSERVATION
MICROETCHINGS OR MARKS SENSITIVE TO ULRTRAVIOLET LIGHT
As was already stated in subject 16, the main difference between a bifocal lens and a progressive lens is the absence of the separation line in the and the gradual change of power which permits good far and near vision in the progressive lenses, because they possess a zone where it is possible to see objects at intermediate distances.  
There are two methods that can be used to identify and differentiate these lenses from monofocal lenses.
IDENTIFICATION THROUGH OBSERVATION: the first consists of observing a nearby object through the lens, if the object appears to have a specific size but without any alteration of its form, it means that the lens is monofocal, if, on the contrary there are deformations of the object’s image in certain zones of the lens and when the lens is rotated the deformities follow the rotation, then the lens is progressive.
MICROETCHINGS OR MARKS SENSITIVE TO ULRTRAVIOLET LIGHT: The second method is more rigorous and consists of finding the characteristic markings of progressive lenses. Every progressive lens has, in its front surface, a series of permanent markings that can be found easily. The marks are made with a diamond after the surface is polished.

3. The regions of a progressive addition lens.
The concept of a progressive lens is based on the production of a lens where the curvature of the front of the lens is specially designed to produce a gradual increase in the positive power (addition) in the transition from the far to the near zone.
Probably the main inconvenience that this type of lens presents is the existence of lateral zones where it is impossible to have correct vision, normally these zones are called aberrations.
So we have lenses,
With a broad zone for far vision (distance area),
a progressive zone (progressive corridor), for intermediate vision,
And a relatively broad zone for near vision (reading area), which is located about 10 to 16mm under the far zone and slightly displaced towards the nasal region (generally 2 to 2.5mm).
The narrowness of the progressive zone depends on the design of the lens and the power of addition, which determine the undesired ABERRATIONS that appear on the periphery of the lens, limiting the field of useful vision.

4. Marks on a progressive lens
REFERENCE CIRCLES
ADDITION
A diagram of the permanent marks is shown on the image to the left, they consist of two circles separated by a distance of 34 mm.
On the front surface, under the right circle there are a series of marks that correspond to the identity of the manufacturer (in this case V2 and an anagram made up of an eye and an “e”, which used by the manufacturer Essilor), this also identifies the nasal side of the lens, which tells us that this lens is for the right eye.
Under the circle on the other side of the lens are some numbers which identify the addition value of the lens (in this case, 2,00D).

5. Center of a progressive lens
LENSMETER
PUPIL
CENTER MARKINGS: The diagram on the right has a series of temporary markings (which are easily erased) that facilitate placement in a lens-meter and the centering of the lens in frame.
 MEASUREMENT OF PARAMETERS: The exterior circumference arc indicates where the lens stop of the lens-meter should be placed to control the far vision of the lens, the interior circumference is used to determine the near power of the lens, the central cross corresponds to the center point in far vision and the discontinuous horizontal line should be parallel with the Datum line of the frame.
ADAPTATION
Adaptation of progressive lenses requires an adequate horizontal and vertical centering.
If the horizontal centering is not adequate, the user will not see the progressive corridor correctly and an inappropriate binocular cooperation will impede a clear and maximum binocular vision through the zones of addition, and will effect above all the useful field for near vision.
The height of the mount also has to be exactly specified to avoid possible vertical disequilibrium that cause adaptation problems which make comfortable binocular vision difficult..
Finally, it is necessary to respect the horizontal specification of the lens for far vision, near vision and the appropriate localization of the different vision zones.

6. Determining the parameters of a progressive lens
Locate the marks using illumination over a dark background and a marker,
Place the lens on a template provided by the manufacturer and transcribe the other markings,
Place the lens in a lens-meter and determine its powers.
If you have to take the parameters of lens that is already mounted, where the erasable marks no longer exist, you should proceed in the following way:
Locate the marks using illumination over a dark background and a marker,
then place the lens on a template provided by the manufacturer and transcribe the other markings, ,
Finally, place the lens in a lens-meter and determine its powers…
Placement on the lens stop in the lens-meter must be precise and in the appropriate zones, otherwise the values will be incorrect, the horizontal discontinuous line must be observed in order to guarantee a correct measurement of the orientation of the axis.  
Keep in mind that when checking the power of this type of lens, the target is not centered with the reticle.
In this type of lens there it doesn’t make any sense to determine the surface power or curvature radii, since the first surface is not from revolution, and from any such data you can’t make any meaningful conclusion.

7. Bibliography
 1.- Tecnología óptica. Lentes oftálmicas, diseño y adaptación.
Joan Salvadó, Marta Fransoy y otros.
Edicions UPC 1997 colección POLITEXT
2.- El vidrio y la óptica ocular.
Editor. Corning France 1988.
3.- Lents oftàlmiques pràctiques.
Marta Lupon, Marisa Vera, Joan A. Martinez, Jesús Caum

8. Bibliography 4.- Manual de prácticas de óptica oftálmica.
Eloy A. Villegas, Antonio Benito.
Editor. Universidad de Murcia 1999.
5.- Clinical Optics
Troy E. Fannin, Theodore Grosvennor.
Editor. Butterworth-Henemann ª edición.