OPTICAL PROPERTIES OF OPHTHALMIC LENS

 OPTICAL PROPERTIES OF OPHTHALMIC LENS

    When a beam of light falls on lens surface, some light passes through lens, some of the light is reflected from lens surface and some of light is absorbed by lens material.

    The measure of the proportion of light reflected from surface is called reflectance.

    The measure of the proportion of light absorbed is the absorption.

    The measure of the proportion of light transmitted is the transmittance.

    Each one is expressed as a fraction of the total quantity of light in the beam. 

    If the intensity of the beam of light is represented by number 1, reflectance by R, absorption by A and the transmittance by T,  the intensity of light may be expressed as follows:

R + A + T = 1

    The aim of Ophthalmic lenses is to restore clear vision to ametropic eye by placing a corrective lens between the viewed object and eye and dioptric function of lens depends on the characteristics of lens material. 

    Some of the properties of these materials are Optical Properties and Mechanical Properties. 

OPTICAL PROPERTIES

    It helps to calculate the dioptric function and control of optical performance. These properties describe the following features of lens material:

1.ABBE VALUE

    The number which helps to measure of degree to which light is dispersed when entering a lens material. Dispersion stands for amount that material spreads out the different wavelengths of light passing through it.

    Abbe number represents relative degree of distortion generated while looking through off centre area of the lens.

    The lower the abbe value, the greater the dispersion of light, causing the chromatic aberration in periphery of lens. The higher the abbe number, the better peripheral optics. 

    Abbe number of 60 is considered to have the least chromatic aberrations and abbe number of 30 is most chromatic aberrations.

    When the wearer moves the eyes away from the centre and looks through the periphery of lens, the prism is created. The amount of prism created through with dispersion value of lens material affects the amount of “colour fringes" the wearer sees.

    Abbe value is property of lens material and cannot be affected by any surface technique. 

    Generally, higher the index of refraction, the lower the abbe value, but this is not a linear proportion.

    Lenses of same index can have somewhat different Abbe value.Standard plastic lenses have an abbe value of 58.

    Most high index materials have lower Abbe value.However, the effect of dispersion can minimized by correct centration and by reducing the vertex distance, putting edges of lens farther from normal line of sight.

2.REFLECTANCE

    Reflectance is phenomenon of light reflection occurs at each of lens surfaces. The result is loss of lens transparency and undesirable reflections on lens surfaces.

    The reflectance of lens surface is calculated from refractive index of the material. When the light is normal on lens surface, the percentage of light reflected at each surface is given by:

Reflectance = 100 (n-1)2/ (n+1)2 %

    Material of Refractive index is 1.5 has a reflectance of 

= 100(1.5-1)2/(1.5+1)2

Or, 100(0.5)2/(2.5)2

Or, 100X(0.5/2.5)2

Or, 3.9% per surface.

    Therefore, the higher the refractive index, the greater proportion of light reflected from the surfaces. The unwanted reflection can be almost completely eliminated by applying an efficient anti-reflection coating, the need of which is more in high index material.

Refractive Index % of light reflected

1.5 7.8%

1.6 10.4%

1.7 12.3%

1.8 15.7%

1.9 18.3%

3.ABSORPTION

    The amount of light which goes through a lens can be reduced because of absorption by lens material. 

    This is negligible in case of non-tinted lens, but constitutes an intrinsic function of a tinted or photochromatic lens.

    Absorption of an ophthalmic lens generally refers to its internal absorption, i.e the percentage of light absorbed between the front and the rear lens surfaces. Lens absorption occurs according to Lambert’s law and varies exponentially as a function of lens thickness.

4.REFRACTIVE INDEX

    The refractive index of a transparent medium is raito between the velocities of light in air to the velocity of light in given medium and is denoted by ‘n’

n  =  Velocity of Light in air
                                Velocity of light in medium

    This is a number, which has no unit and is always greater than 1. When light passes from air to, say glass, ie rarer medium to denser medium, the speed of propagation slows down. 

    How slowly they pass through various transparent material is called index of refraction of light.

    The higher the index of material, the more it is able to bend light. A higher index of refraction results in a lower angle of refraction which means the bend light ray comes closer to the normal (perpendicular) line.

    In other words, for a given angle of incidence, the higher the index, the more light is bent. That is why “high index”  lenses can be thinner for a given prescription compared to lower index lenses although the reduction is not quite pro-rata.

    As the velocity of light in a transparent medium varies with wavelength, the value of refractive index is always expressed for a reference wavelength.

    In Europe and Japan, this reference is ne… 546.1 nm (mercury green line), whereas in other countries like USA, it is nl … 587.6 nm (helium yellow line).

    The problem that this can cause is that a lens manufacturer may calculating the surfacing curves for a lens base on one refractive index, while the user may measure the same lens on a focimeter caliberated for another.

    But the difference only changes the 3rd decimal of the value of the index. So in fact, it has no real effect.

1.74 Very High Index

1.54 to 1.64 - Mid-Index

Anything lower is Normal Index.

If we assume that nothing less than 1.498 will be dispensed