topic 4 optical characteristics Flashcards
what is lens aberration?
- Aberration causes degradation of image quality.
- Aberration happens mostly when the eyes are viewing through the lens periphery, away from the optical centre.
- Lens Aberration can be reduced but not totally eliminated.
what is Lateral chromatic aberration?
Due to the material of the lens, when polychromatic lights is used.
The inability of a lens material to focus different colours of light to the same spot.
what effects does LCA have on the wearer?
Lateral CA causes wearers to complain about colour fringes around the images of high contrast objects
Make the image blurry especially when the contrast is low.
what is abbe number?
Abbe number is a measure of a transparent lens material to disperse lights.
Traditional lens materials has the highest Abbe number.
higher abbe number= lower dispersion= lower chromatic aberration
what is Chromatic Aberration Formula?
chromatic aberration(Lateral) = cF/V
c= distance away from OC, c increase, CA increase
F= power of a lens, F increase, CA increase
V= Abbe number, Abbe number increase, CA decrease
what is the solution to LCA?
choose lens material with high Abbe number
how does Oblique astigmatism occur?
Rays of light from an object strike the lens obliquely.
These light rays will not form a sharp, point image.
Oblique astigmatism is minimum when the lens is in a certain lens forms.
Best Form Lens are made to have zero oblique astigmatism within 30 degree from the lens optical axis
What is Distortion
Lens magnification/
minification effect increases with the increase of distance away from Optical center
minus lens distortion=barrel
plus lens distortion= pin cushion
Describe the 3 positions of wear (POW) can affect vision and visual comfort.
- Vertex Distance changes effective power
- Pantoscopic tilt is the vertical tilt of lens axis
- Wrap angle (face form) is the horizontal tilt of the lens axis
how does pantoscopic tilt angle differ depending on ear and frame?
- higher ear=more tilt
- the forward tilt angle differs depending on the tilt angle of the frames
how does OC need to be changed depending on pantoscopic tilt?
Position of the OC needs adjustment as the tilt changes,
for every 2 degree tilt, lower the OC by 1 mm
what are the 3 types of face forms?
- zero (patient’s PD= frame PD)
- positive (ideal, wraps around face, frame PD is larger than patient’s PD and is correctly adjusted on face)
- negative (patient PD > frame PD)
what are the 5 properties of lens materials?
- Refractive index
- Density
- abbe number
- UV cut off
- Tensile Strength
what is refractive index and how does it affect lens thickness?
Ratio of the velocity of light of a given wavelength in air, to the velocity of light of the same wavelength in the refracting medium.
The higher the refractive index, the thinner the lens.
effect of high index material in reducing lens thickness is only evident when the power is greater than ± 4.00D
what is the formula for predicting effect of higher RI?
n1-1 / n2-1
result will show the thickness of new lens as percentage of current lens thickness
n1=current lens material RI
n2=new lens material RI
what is the RI of CR39, Trivex, ormex, polycarbonate and polyurethane ?
CR39=1.49 trivex= 1.53 omex=1.56 polycarbonate=1.59 polyurethane= 1.60
- Polyurethane is more commonly called plastic HI 1.6
- The rest of the materials all have RI in their names
Low-index: <1.53 (1.49)
Mid-index: 1.53 to 1.58
High-index: 1.59, 1.60, 1.61, 1.66
Super-high index: >1.66 (1.67, 1.71, 1.74)
what is the rs between density and RI?
In general, the higher the RI the higher the density
what is the rs between density and glass materials? what material has the least density?
All glass materials is much denser and therefore heavier. Glass lens is around 2-4 times the weight of plastic.
Trivex has the least density
what is abbe value? what is the formula?
- Degree of chromatic aberration which the wearer will experience
- Amount of dispersion caused by lens material
- purely a lens material property, not affected by power or thickness
formula;
v = 1/dispersion
*Higher the Abbe value, lower the chromatic aberrations
what is considered low, medium and high dispersion?
> 45 : low dispersion
39 < 45 : medium dispersion
< 39: high dispersion
what is the uv cut off for glass and plastic lens?
plastic lenses have UV cut off ranging from 355 to 400
glass lenses have UV cut off ranging from 320-340nm
All glass lens materials provide insufficient protection against UV
what is tensile strength? what instrument is used to measure it? what is considered good tensile strength?
Tensile strength is the resistance of a lens against breakage when it is mounted into rimless frames.
It is measured using an instrument called Instron (in unit of kilogram of force)
Tensile strength of 35 kg and above is considered good.
List the plastic materials from lowest to highest tensile strength
*lower RI=lower tensile strength generally
CR39, omex=HI plastic 1.74, polycarbonate, HI plastic 1.71, trivex, HI plastic 1.66, polyurethane
(Since glass materials are not indicated for rimless., no values given)
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