Phys And Optical Characteristics Of Lenses 2 Flashcards
Light incidence on a surface suffers a combination of three fates
Transmission
Absorption
Reflection
Tranmission + absorption + reflection=
100%
When a light passes through a lanes, it is _____ on both lens surfaces and _____ by the lens material
Reflected
Absorbed
The anoint of light energy that gets through an optical system
Transmittance (T)
When light falls upon a lens, it is lost in two ways
Reflected
Absorbed
Specifies the amount of light reflected from a surface
Fresnel law
Spectacle lens in air fresnels law
R= (n2-1/N2-1)^2
Converting transmittance
Ts-1-R
To find transmittance (not absorbed)
T=TsTsTm
Ts is transmittance at each surface
TM amount transmitted, not absorbed by the medium
How the amount of light transmitted changes based on a change of thickness of the absorbing material
Beer-lambert law
The fraction of light transmitted per unit lenejgth of solid tinted material
Transmittance factor (q)
If given:
Q=0.8/mm
Material thickness=4mm
How much light is transmitted
(0.8)^4=0.41=41%
The ratio of the density of a substance to the density of a reference substance
Specific gravity
Higher the specific gravity of a material,
The more material is packed into a smaller area
Substances with specific gravity >1
More dense than water (will sink)
Substances with specific gravity <1
Less dense than water (will float)
A diopter equals
D=1/f
Light passing through a plus lens
Converges
Light passing through a minus lens
Diverges
Rays of light entering a prism always do what at base of prism
Bends towards
The image through a prism
Shifted towards apex
A curved interface between two media
Single spherical refracting inferfaces
An optical system with two spherical refracting interfaces- the front and back surfaces
A spherical lens
Total lens power =
Front lens surface + back lens surface
Interface wraps around the lower n medium
Concave
Interface wraps around the higher n medium
Convex
Concave interface is always
Diverging
Convey interface is always
Converging
Power of SSRI
F=(n2-n1)/r
n2=RI of final media
N1=RI of original media
R=radius of curvature of SSRI
F=power of the ssri
Total lens power
F=F1+F2
Refractive power of a surface
P=(N2-n1)/r
N1=refractive index of medium
N2=refractive index of emerging rays
R= distance (m)
Simplified to F= (n-1)/r
Total power of the lens ignores what
Thickness
This is approximate power
Front surface power is
+
Back surface power is
-
Back vertex power
Fv= P1/(1-d/n*P1)+P2
Front vertexpower
Fn=F2/(1-t/nF2)+F1
Vertex distance and tilt
When lens is tilted about the optical center the vertex distance doesn’t change along line of sight
Optical axis and tilt
The optical axis of the lens no longer passes through the center rotation of the eye
-paraxial theory no longer applies
Tow forms of tilt
Pantoscopic tilt
Face form
Horizaontal tilt
Panto
Vertical tilt
Faceform tilt
Tilt on lens and cyl and sphere
Sphere power reduced in meridian that is tiled
Panto tilt of a minus lens
Axis at 180
Panto tilt of a plus
Axis at 90
Faceform tile of plus
Axis at 90
Faceform tilt of a plus
Axis at 180
Compensating power for tilt
A Rx modified for the as worn position when fitted to the frame or mount
-Rx measured on a lensometer does not equal Rx/ordered RX
Single vision compensating for tilt
Ensure optical axis of the lens passes through the center of rotation by lowering the optical center
Compensating for panto tilt
Lower it 2mm per 1 degree of tilt
Equation for compensating for lens tilt
X=dcorsiny
Lens form characterizes
The relationship between the front and back surface geometries of a lens
Form of a lens
Specified by the power of the front surface BC
BC can be manipulated for 4 reaseons
Thickness and wt
Peripheral lens aberrations
Cosmetic considerations
Image size and magnification
Plano-convex and Plano-concave
One surface is flat and the others s curved
Meniscus
Convex front surface and concave back surface
Most common
Biconvex and biconcave
Both surfaces are either convex or concave
Equiconvex and equiconcave
Half total power is due to the front surface and half is due to the back surface
Plano cylinder
One flat surface and one cyl surface
Toric
One topic surface and spherical surface
Standard lens curves used by manufactures
Base curve
Spherical lens BC
From spehre curve
Toxic base curves and cross curve
Back surface
Base curve of minus cyl
From sohere curve
Spherical equivalent
Sum of the spherical component and one half of the cyl component of a lens or surface
Curve from which all other curves are measured
Base curve theory
To find the best curve for plus RX, add the sphere power or the spherical equivalent to +6,
Vogels rule
To find the best curve for a minus Rx, add half the sphere power or half the spherical equivalent to +6
Vogels theory
You want the BC of the front surface to be as close to _____ as possible
+6
The height or depth of a lens surface
Sag
The sagitta of a curve is the
Perpendicular distance from the vertex of the curve to a chord intersecting the curve
Sag formula
S=H2/2r
The distance between the posterior pole of ophthalmic lens and perpendicular plane constraint the posterior edge of lens
Vertex depth
The distance between the anterior pole of the lens and the plane containgin anterior edge of the lens
Lens bulge
The height of the lens when laid down on a flat surface
Plate height
Sag of back surface
Vertex depth
Sag of front surface
Lens bulge
Sag of front surface + edge thickness
Plate height
How to account for different RI in lens clock
Pclock= (n2-n1)clock/r
Rearranged to
R=(n2-n1)clock/Pclock
Ultimately
Ptrue=Pclock((n2-n1)true)/(n2-n1)clock
When a frame is put on a patients face, it should touch at three points
The fitting trial
-ear and skull
Bridge of nose
4 pt alignment
Tighten the screw X-int Co-planar Skewed bridge Faceform Panto tilt Parallel temples Nose pads Open temple angle Clean
Check for misalignment when viewing the frame from above
Co planar
-occurs when the bridge us pushed forward/backward while the lenses remain in correct tilt
Check for misalignment when viewing from above
X-int
Lenses twisted where eyewires meet the bridge
Check for misalignment when looking straight on
Skewed bridge/eyewires
Lenses are rotated inward or outward around the bridge
Check curvature of fram from above
Faceform
Too much=changes in perceived lens power
Check for alignment where viewed from
The side
Some panto is necessary for cosmetics and optics
Check for misalignment by placing the eyewear temples up on flat surface and checking from the side
Parallel temples
Both temples should make contact with flat surface
Check from above for 90 angle
Open temple angle
Should never be less
Check from back when temples are closed
Temple fold angle
Temple tips should not touch the lenses, be parallel and cross at middle
Check all three of these for alignment, frontal, splay, vertical)
Nose pads
