Glass lenses are available in White, Pink and Photo-chromatic (Grey and Brown). Glasses are heavier compared to CR39 lenses. The risk of breakage is also very high in the case of glass lenses.
Trifocals are those lenses which have three distinct fields of vision in the same lens; one for distance, one for intermediate and one for near. Each field is divided by a dividing line. The chief advantage of these is that they provide a more extended visual field. They have two major disadvantages:
These are multifocal lenses that provide the full range of focus from near to intermediate to far - gradually, without separations or interruptions. These lenses have no seams or lines and look exactly like normal single vision lens. These require an initial period of training and adjustment for the first-time wearer. These are available in plastic and glass.
VARILUX is an example of a progressive lens available in Plastic, High index(Polycarbonate) & Ultra-High index with Photochromatic options (Transition Lenses).
Progressive lenses, also called progressive addition lenses (PAL), progressive power lenses, graduated lenses,and varifocal lenses, are corrective lenses used in eyeglasses to correct presbyopia and other disorders of accommodation.
The wearer can adjust the additional lens power required for clear vision at different viewing distances by tilting his or her head to sight through the appropriate part of the vertical progression.
The lens location of the correct addition power for the viewing distance usually only requires small adjustments to head position, since near vision tasks such as reading are usually low in the visual field and distant objects higher in the visual field. Progressive addition lenses avoid the discontinuities (image-jumps) in the visual field created by bifocal and trifocal lenses and are more cosmetically attractive.
Since bifocal and related designs are associated with 'old age', proponents have suggested the lack of segments on the lens surface of a progressive lens appears more 'youthful' since lenses associated with younger wearers [single vision] lenses tend to be free of segments or lines on the surface.