1.LIGHT
2.OPHTHALMIC LENSES
3.INTERACTIONS OF LIGHT WITH OPHTHALMIC LENSES
4.SIGN CONVENTIONS
DISPENSING OPTICS
Dispensing Optics is science of dispensing eyeglasses and contact lenses based on the prescription of an eye care practitioner.
GEOMETRICAL OPTICS
Light travels in straight line, Laws of reflection and Laws of refraction.
Wave Optics - Interference, Diffraction, Polarization.
VISUAL PERCEPTIONS - 4 types - Light sense, Form sense, Colour Sense, Sense of Contrast.
OPTICAL DISPENSING is also an art - that teaches how to make the spectacle frame, make contact with the patient’s face comfortably, looks good and also serves desired function.
The art can be learned and mastered by PRACTICE AND EXPERIENCE.
Both Anthropometric and Cosmetic demands are to be met.
Process of dispensing from selecting a frame to taking measurements and fitting eyewear on the patient’s face.
Even the best refraction may not provide desired results if the lenses are not dispensed effectively.
Lenses are used to maximize vision and maintain the vision.
A rightly selected lens material coupled with appropriate lens design is critical to achieve desired goal.
1.LIGHT
Visible Light - From 380nm to 760 nm. Light travels very rapidly. Speed - 186,282 m/s or nearly 3,00,000 km/s.
UV rays lying at shorter wavelength, IR Rays lying Longer wavelength end.
Human eyes are most sensitive to yellow color (570-590nm)
If all wavelengths of visible light enter, we interpret them as WHITE light. If no wavelengths in the visible range are present, we interpret as DARK.
Longer wavelength has lower frequency - Eg Red
Shorter wavelength has higher frequency - Eg. Violet
Shorter wavelength of light carries intense energy than that of longer wavelength of light.
Shorter wavelength enough energy to damage ocular structure.
X-rays and gamma rays are danger.
2.OPHTHALMIC LENSES
Ophthalmic Lenses are transparent.
Curvature determines how steep or flat the lens surface.
Curvature is inversely proportional to the radius.
Large radius of curvature will have flatter lens surface and smaller radius of curvature will have steeper lens surface.
Plus lenses are combination of prisms base to base while minus lenses are the combination of prisms apex to apex.
The image viewed through prism always moves towards the apex, it implies that plus lenses will show up motion, when it is taken down as the apex of prism lies at the top edge.
Minus lens will show down motion if the lens is taken down as apex of the prism lies at the center of the lens.
Light traveling from air to prism always bends towards its base. For plus lens, the lens will CONVERGE to a point. Similarly, for concave or minus lens the light rays will be DIVERGE.
Optical center of the lens is singular point through light rays pass without being deviated.
All ophthalmic lenses have 2 prinicipal meridians. A meridian is a imaginary line that passes through the optical center of the lens. A lens has many meridians, but there are two principal meridians those lie perpendicular to each other and meet at optical center of the lens.
The rays of light passing through this meeting point travel undeviated.
When these two meridians carry same power, the lens is called SPHERICAL lens
-1.00 DSPH -1.00 DSPH
When these two meridians carry different power, the lens is called SPHERO-CYLINDER lens
-1.00 DSPH -2.00 DCYL
In case of plano-cylinder lens, one meridian has no power and other carries the lens power.
The two principal meridians are commonly known as Axis meridian and power meridian.
Axis meridian has the minimum lens power and power meridian has the maximum lens power.
3.INTERACTIONS OF LIGHT WITH OPHTHALMIC LENSES
When light travels through medium, it is influenced by Physical, Chemical and other
properties of matter. Interaction might be emission, absorption, transmission, reflection.
ABSORPTION - Lost certain wavelength due to absorption. A red apple appears red because
it absorbs other colors of the visible spectrum and reflects only red light
REFLECTION - Redirection of Light. The reflected light may have same or longer wavelength
as the incident light. Reflection is also knows as light scattering.
TRANSMISSION - Light interferes with transparent medium, refraction occurs.
4.SIGN CONVENTIONS
Used to denote ophthalmic lenses based upon direction of light. Assuming light always travelling from left to right in straight line.
Conventions used are
All dimensions are measured from vertex of lens.
Any measurements taken in course of light is taken as positive and the measurement taken against the course of light is taken as negative.
Light rays emanating from object point and passing convex lens has
principal focus infront of lens and focal length measured from lens surface to
image point is with direction of light travel. So, convex lens is + sign
Light rays emanating from object point and passing through concave lens has
principal focus behind the lens and the focal length measured from lens to image
point is against the direction of light. So, it is - sign.
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