Spectacle can be classified into the following parts like Front, End piece, Shield, Temple and Curl. While fitting each of these parts together into a spectacle some steps like Temple spread Triangle of Force, Temple force, Equality of Lens Vertex Distance, Pantoscopic Angles, Straightness of the Frame on the face, Reference Points, Height of the Frame, Pantoscopic Angles etc.. have to be concentrated. These steps vary according to the frame and lens varieties.

TEMPLE
Temple Spread: The temple spread angle of the frame should be such that the shafts of the temples exert no pressure, even if touching, on any area of the face or temple prior to the point of the head at which they should exert pressure-just above the root of the ears-usually the widest portion of the head.

If the temples are not spread far enough, there will be too much pressure on both sides of the head, causing the temples to bow out. This forces the frame forward until the temples are opposite a narrower part of the head and the pressure that tends to bend them is somewhat relieved. If the glasses are fitted in this manner and the temple-spread is never corrected, the glasses will not only tend to slide down, but as they do so, the bent down portions of the temples will pull against the backs of the ears. Now not only do the glasses slip down as if they were loose, but they hurt behind the ears as well. The wearer experiences the disadvantages of both loose and tight glasses.

If the temple angle is too wide for the patients head, the glasses will tend to slide down the nose. More often this occurs if the frame has been worn for some time.

Triangles of force: The triangle is comprised of the three points where the spectacles contact or put pressure against the head. The apex of the triangle is the pressure point on the crest of the nose, and the base of the triangle is the two pressure points just above the roots of the ears, one on each side of the head. Since pads are often used for frames, there may actually be two resting points at the apex of the triangle.

Equality of Lens Vertex Distance: At this point it is advisable to check the glasses for equality of vertex distance. This is done by having the wearer tilt his head forward, viewing the glasses from above. If the glasses have been properly aligned and the wearers head is symmetrical, the vertex distance will be equal. If, however the temple spread is unequal with one temple being in or out farther than the other, or if one side of the wearers head is somewhat wider than the other, inequality of vertex distance will result.

Temple Pressure: Usually, if the head is erect and still, the glasses will stay in place even if the temples are exerting no pressure against the head but merely residing in place in the ear. Such a situation totally relieves any effect of the temples on the fit of the font and bridge to be adjusted very precisely to the nose. Some fitters feel that a better fit can be obtained, especially when working with the adjustable nose pads, if the temples are not actually exerting pressure anywhere against the head. When nose pad adjustment has been accomplished, temple pressure can be introduced to help hold the frame in place when the head changes from this primary straight-ahead position.

FRONT
Pantoscopic Angles: The usual inclination of the front may be anywhere from 4 to 18 degree from the vertical, varying to the upper extreme or more in the case of exceptionally protruding eyebrows. The lenses should touch neither the brows nor the cheeks. The pantoscopic angle permits the line of sight to pass through the optical center when the gaze is directed ahead toward the ground at approximately walking observation distance. This presents a reasonably angled plane for both distance viewing and reading. In addition, the angle has a more appropriate cosmetic effect when viewed from the side.

Straightness of the Frame on the face: Adjustment of the pantoscopic angle at each end piece also permits leveling the frame as viewed from the front. Most heads are not symmetrical, and one ear is often slightly higher than the other. In such instances, perfectly trued glasses with parallel temples will result in a vertical discrepancy of the lenses with respect to each other when the temples rest on the top of the roots of the ears.

Reference Points: Although it is helpful to refer to the eyebrows when determining the level of the frame, facial asymmetry can cause one lens to appear higher than the other when this is not really the case. The frame front should not be aligned solely on the basis of eyebrow height or the position of the eyes in the head, since either of these features may be asymmetrical, but rather by overall appearance. If, due to facial asymmetry, the frame conforms better to facial features when fitted slightly higher on one side, such an alignment is acceptable.

Height of the Frame: Once the pantoscopic angle has been established satisfactorily, the next step is to place the optical center of the lenses, or the height of the bifocal segment, at its desired position. In plastic frames and other non-adjustable fixed bridge frames, the height can be affected only by altering the distance between the pads-either by narrowing or widening the bridge. The adjustment forces the frames to ride on either a wider or narrower part of the nose. Since the nose widens from the top to bottom, the frames will be correspondingly either higher or lower than they were before the change in bridge width.

Flat surface touch test: a test for temple parallelism in which the spectacles are positioned upside down on a flat surface with temples open

Secondary Functional Capability

• Cosmetic Value in terms of look
• Comfort in terms of weight and size
• Durability
• Resistance to water, temperature, scratch etc.

1. Decentration: The displacement of the lens optical center from the geometrical center of the frames lens aperture and the displacement of lens optical center away from the wearers line of sight for the purpose of creating a prismatic effect
2. Effective Decentration: The distance from the axis of a decentered cylinder to the point from which decentration began.
3. Effective Power: The vergence power of a lens at a designated position other than that occupied by the principal point of the lens itself. That power lens required for a new position which will replace the original reference lens and yet maintain the same focal point
4. Geometrical Center distance: The distance between the geometrical centers of the lenses or lens openings in a frame
5. Focal point: a point to or from which light rays converge or diverge
6. Focal power: a measure of the ability of a lens or lens surface to change the vergence of entering light rays