Saturday, July 3, 2010

The lenses THEORY

The lenses THEORY 
Glass eye is a visual tool, which consists of frames and lenses, that is in use as: 
1.     Rehabilitation of the sharp disparity in eyesight,
2.     Eye protection from harmful environmental eye
3.     Eye protection from harmful rays or light eyes, such as UV light, A, B,and Infra Red that comes from the sun and the computer.

The lenses are good to have three basic elements, that is: 
1.     The result of visual acuity in need of materials, design, and a good lens coatings,
2.     Cosmetic terms, the lens looks thin and clear,
3.     User comfort, lightweight and no lens distortion.

Optical parameters, including four cases, which consists of: 
a.      Refractive index.

Is the ratio between the speed of light in a vacuum with the speed of light in certain media. If the light that came through the two media of different refractive index, refraction will occur and will reflect a small part. The bigger the refractive index difference between the two media, the greater the angle and the percentage of light reflection in reflect. Refractive index is inversely proportional to the thick center of the lens, so the higher the refractive index of a lens, the thinner the lens can be made.

b.     Dispersive power (the nature of the lens material which refract colors).

Optic materials that refract the color purple to red with a lot of angles different materials that have called dispersive forces large (small Abbe value). Due to that produced from the decomposition of the light color of the color aberrations that influence the sharpness of the object. 

c.      Clarity.

A good lens material must be clear and colorless, like a crystal or pure water. Standards in use to determine the clarity of the internationally is Haze Value. 
Haze is the particles - small particles, refractive dirt, dust, air bubbles, or pigment to absorb ultraviolet light which accidentally in the mix in the lens material.Particles - the particles in the considered block the light, if the spread is greater than 2.5%. While both materials have more eandah haze value of 1%. 
d.     Lens Color.

As a rule the lens color is good or not good for his assessment on the use of international standards, namely, (YI) Yellowness Index. Yellowish in base degrees deviation from the white water to the yellow color, with the calculation of wavelength 570-580 nm (nanometers). 


A.   Glass Lenses

Refractive index glass lens is standard in the life of Crown Glass (1.523)

B.    Plastic Lenses

Refractive index plastic lens wear is standard on CR 39 (1.4) 
C.    Advantages and disadvantages of plastic material of glass lenses.

    1. 40% lighter,
    2. Not easy to break, so safe in use,
    3. Can in the given color,
    4. Not easy dewy,
    5. Available a larger diameter,
    6. Aspheric lens available for a high plus power.
1.     More easily scratched,
2.     By comparison of refractive index, then the plastic lens is thicker in comparison with glass lenses for the same power.

Quantitatively FOCUS LENSES 

Consists of three parts, namely: 
1.     Monofokus or Single Vision Lens
2.     Bifokus lens has two focusing lenses are:
Dp = Distant portion
Rp = Reading portion 
Example: Shape Flattop, Kryptok, Curve tops, etc. 
3.     Proggresive lens.
Lenses that have a lot of focus and variations such power without limit, which allows the presbyopia able to see clearly at all distances good distance, medium, or close. 
Based on the above criteria, the glass eye itself according to its function serves to correct refractive disorders such as: 
1.     Myopia, the correction with lens spheres minus (-)
2.     Hypermetropia, the correction with lens spheres plus (+)
3.     Astigmatism, the cylinder correction lens with spheres
4.     Perbyopia, the correction with lens plus Addition
5.     Heterophoria, the correction with prism lenses, which are customized with the direction of the base.

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