Term
|
Definition
Concave wraps around lower medium and is diverging
Convex wraps around higher medium and is converging |
|
|
Term
|
Definition
| Primary focal point- light from object diverges and is parallel upon exiting the lens (image is at infiinity). Also the point where you place the object and no accommodation is necessary. |
|
|
Term
|
Definition
| Secondary- light enters the lens parallel and converges to a point (object is at infinity) |
|
|
Term
| Total internal reflection |
|
Definition
Any angle greater than the critical angle will be totally internally reflected. N1sin theta critical= n2 sin theta reflected. The reflected angle is equal to 90, so sin critical= n2/n1.
ex= AR coating and gonio lens |
|
|
Term
|
Definition
|
|
Term
|
Definition
| U eff= U/(1-xU) where U is vergence of light at position r=0 and x is the directed distance from position r=0. In summary we are finding the down stream vergence light becomes less divergent as it moves away from the light source. |
|
|
Term
| Why are spectacles a different power than CLs? |
|
Definition
| Plane waves are incident on a spectacle lens: the light gain vergence as they arrive at the cornea. This is why there is a difference between the power of CL and the spectacle. |
|
|
Term
| How do you vertex correct? |
|
Definition
| When solving problems find the focal point of the lens and and or subtract the distance depending on whether you are getting closer to or father away from the focal point. |
|
|
Term
| Are CL more plus or more minus then specs? |
|
Definition
|
|
Term
| effective power equation for thick lens system |
|
Definition
| Fe=F1+F2- (t/n2)*F1F2 (n2= lens material) |
|
|
Term
| back vertex power/front power for thick lens equation |
|
Definition
Fv= F2 + F1)/ 1-(t/n2)F1)
Fn= F2 + F1)/ 1-(t/n2)F2) |
|
|
Term
| When are nodal points not equal to the principle plan? |
|
Definition
| when n1 is not equal to n3 |
|
|
Term
| what is the aperture stop |
|
Definition
| light limter= pupil. Limits FOV |
|
|
Term
|
Definition
| Field stop: limits the size of the object that can be imaged. |
|
|
Term
| what is the entrance pupil? |
|
Definition
| image of aperature stop formed by the lens in front of it. To find it go to object side. The entrance pupil is the element or the image of an element that subtends the smallest angle. Determines size of light cone that enters the system. |
|
|
Term
|
Definition
Exit pupil- image of aperature stop formed by the lens behind it Go to image side to find it.
Determines the size of the cone that exits the system. |
|
|
Term
| What is the aperature and entrance pupil of a telescope? |
|
Definition
| Telescopes- objective is aperature stop. Entrance pupil= aperature stop. |
|
|
Term
| Where is the exit pupil in keplarian? In galilean? |
|
Definition
| Exit pupil- in keplarian outside the telescope closer to the patients eye. Galilean- inside the telescope. |
|
|
Term
| What is the lens difference between a keplarian and galilean? |
|
Definition
| galilean has a negative ocular |
|
|
Term
|
Definition
| Ports: images of field stop formed by lenses in front of or behind the stop constitute. |
|
|
Term
| What is the difference between depth of focus and depth of field? |
|
Definition
Depth of focus: interval surrounding the retina which an eye sees an object in focus. Pinhole.
Depth of field- interval surrounding fixation plan in which an object can reside and still be in focus. |
|
|
Term
| What are the trends seen with depth of field and depth of focus? |
|
Definition
| • a short focal length leads to a large depth of field. An increase in aperature size leads to a decreased depth of field and decreased depth of focus. |
|
|
Term
| What is the field of fov? What kind of lenses will increase it? |
|
Definition
| Field of view the extent of object plan that is imaged. Minus lenses will increase FOC, plus lenses will decrease. |
|
|
Term
| What is the field of fixation? where is the center of rotation? |
|
Definition
| angle made by the optical axis by the entrance port as measured at the center of rotation of the eye. Center of rotation is 14 mm. |
|
|
Term
| What is the COLC and how do we solve for it? |
|
Definition
| the point of best focus for the lens. It is located dioptrically halfway between the two line images. COLC= spherical equivalent. Find the speherical equivalent (gives you the dioptric midpoint) |
|
|
Term
| What is the interval of sturm? |
|
Definition
| the spatial difference between the two line forms. |
|
|
Term
| How do we solve for power in oblique meridian? |
|
Definition
| we cannot use the equation bc there is a sign function. So if they are asking for the power in the meridian exactly 45 degrees between either meridians you can just find the average between the two and this is your oblique power. This may be useful with vertical imbalance problems using prentice’s rule. |
|
|
Term
| What happens in plus vs minus lenses with lens tilt? |
|
Definition
Minus lens the axis is equal to the same meridian you are rotating about. (minus cyl form_
Plus lens the axis is opposite to the meridian you are rotating about. (minus cyl form)
Plus lens- increasing plus cyl- so need to convert it to minus cyl form. That is why the axis is different. |
|
|
Term
| equation for power of mirror? |
|
Definition
| Power F=2n/r (meters) where n is the index of refraction of air and r is the radius of curvature. |
|
|
Term
| How do you solve for a mirror lens combo? |
|
Definition
| F=2F1 +F2 a lens with a silver back (sum of two refractions and one reflection. |
|
|
Term
| equation for Power of SSRI or CL given the radius |
|
Definition
|
|
Term
| What is the difference between a BIO and DO? |
|
Definition
| difference it remember indirect larger FOV, larger depth of focus, less mag and inverted vs direct smaller FOV, more mag and upright |
|
|
Term
| How does a lensometer work? How do you know what sign the power is? |
|
Definition
| Telescopes have parallel light comin in and leaving. In a lensometer the lens makes it so there is no longer parallel light coming in. To view the lines in focus in a lensometer the standard lens must converge at the primary focal point of the test lens. In lensometer you are simply moving the target until this happens. If you move it away from the viewer you have a more minus test lens. If you move it towards the viewer you have a more positive test lens. If x is minus then lens is too, if x is positive then lens is too. |
|
|
Term
| what is newton's formula and what is it used for? |
|
Definition
| Newton’s formula: x=f^2Fv where f is the 1/ power of standard lens |
|
|
Term
| how do you determine the direction of the prism using lenometry/ |
|
Definition
| Base direction= direction of bullseye |
|
|
Term
| how do you neutralize an add power? |
|
Definition
| To neutralize an add: measure the front vertex power in the distance and near portion and find the difference. |
|
|
Term
| Hand neutralization motion? |
|
Definition
| motion is opposite retinoscope. Minus lens is with motion. Plus lens is against motion. When neutralized there is no motion and the power of the lens is equal and opposite in sign to that which was used to neutralize |
|
|
Term
| how does a radioscope work? |
|
Definition
| measures radius of curvature or BC of CL. Forms an clear image at two loactions One when at the surface of the lens and two when at the center of curvature of the lens. The distance between the first and second point is the radius of curvature. |
|
|
Term
| what is the assumed index of the cornea and tear film? |
|
Definition
|
|
Term
| what is the calibration equation for a lens clock? |
|
Definition
|
|
Term
| What is the trend with condensing lenses in terms of FOV and mag? |
|
Definition
| Condesnsing lenses: higher power= increased FOV, but decreased mag. |
|
|
Term
| What are the ansi standards for sphere and cyl power? |
|
Definition
Trend: tolerance for power goes up for increased lens power
Sphere <6.50 +- 0.13 and >6.50 is 2%
Cyl <=2 +0.13, 2-4.5 =0.15 >4.5 is 4% |
|
|
Term
| what is the ansi tolerance for axis? |
|
Definition
Trend: less tolerance the higher power cyl.
<0.25 +-14
.25-0.50 +-7
0.5-0.75 +-5
0.75-1.5 +-3
>1.50 +-2 |
|
|
Term
| What are the ansi standards for thickness, warpage, BC, and impact? |
|
Definition
Thickness +- 0.3mm
Warpage 1.00 D
BC =-0.75
Impact 5/8 in steel ball from 50 in |
|
|
Term
| What do you need to be considered impact resistant? |
|
Definition
| To be considered impact resistant need + and z87.2 marking as well as manufacturers logo. |
|
|
Term
| what do you need to be considered high mass and high velocity safe? |
|
Definition
High mass impacted- pointed projective 500 g from 50 in
High velocity- steel ball 0.25 inch in diameter fired at 150 ft per second. |
|
|
Term
| where is BC located for specs vs CL? |
|
Definition
|
|
Term
| What three equations do you need to solve a lens thickness prob? |
|
Definition
CT-ET=s1-s2
Sags can be related to radius of curvature via s=h^2/2r
Radius of curvature can be related to power by F=(n-1)/r
Draw a pic! |
|
|
Term
|
Definition
Minimum blank size M= ED +2(d) = 2mm
Where d= dencentration per lens |
|
|
Term
| Where are the OC of various segs? |
|
Definition
Flat top 28= 5mm
Franklin=0
Round= radius of seg
All others are usually 4.5 unless ribbon r seg which is 7. |
|
|
Term
| What are the different insets? |
|
Definition
Frame pd- MRP/2
Seg inset= distance pd- near pd/2
Total inset= distance from GC to center of near reference point . frame pd- near pd/2 |
|
|
Term
| What are the different indices a abbe numbers for crown class, CR 39, poly and trivex |
|
Definition
Ophthalmic crown glass n= 1.523 Abb 58.9
Cr-39 1.498 Abbe 58 Light impact resistant
Poly 1.586 Abbe 30 impact resistant but chromatic aberration
Trivex 1.53 Abbe 43-45 light and impact resistnace. |
|
|
Term
| What is longitudinal abberation? |
|
Definition
| A form of spherical. marginal rays focus at different point than paraxial. |
|
|
Term
| Why do we usually ignore spherical abberation and coma? |
|
Definition
pupil limits marginal rays.
consider in rxs >+10 and use aspheric lenses to decrease. |
|
|
Term
| aspheric lenses reduce... |
|
Definition
| mag, weight and spherical abs |
|
|
Term
|
Definition
| Coma occurs for off axis points- results variable magnification difference as the height of rays incident above the the aixs is varied. |
|
|
Term
| trend with aperature size (pupil size) and longitudinal |
|
Definition
| Longitudinal and coma aberration increase with square of aperature whereas lateral increase with cube of aperature. Which essentially says that larger pupils have more aberration. |
|
|
Term
| What is radial astigmatism? how can you minimize it? |
|
Definition
| rays hitting the system obliquely. Power is altered by the tilt of the lens. Tangenital and sagittal rays are altered asymmetrically. Can be altered by picking the correct BC. |
|
|
Term
| what is the Tschernig ellipse? |
|
Definition
| shows the best value for a BC for given back vertex power, which eliminates oblique astig. Can also correct for curvature of field. Oswalt used to correct oblique astig |
|
|
Term
| What is the curvature of field? |
|
Definition
| image plane is warped. Quality of the image on a flat screen decreases for larger distances. Like a projector screen. The retina accounts for this. |
|
|
Term
| how are radial astig and curvature of field related? |
|
Definition
| Image surface curvature is in part due to curvature of field and radial astigmastism. A petzval surface is an image surface created by a a system with no radial astigmatism. A curvature of field will be present any time the petzval surface does not correspond to the far point sphere of the eye. |
|
|
Term
| what is the dif between point focal lens and percival form lens/ |
|
Definition
point of focal radial astig is corrected and curvature of field is not
percival form lens- curvature of field is corrected and radial astig is not.
not compromise between the two. |
|
|
Term
| how do you find the curvature of Petzval surface? |
|
Definition
| Add together the distances from center of curvature to far point. Divide by one to find the curvature of the far point sphere. The radius is equal to 1/K. K=F/n. You can compare the petzval radius to the far point sphere. |
|
|
Term
| What is distortion? How can you minimize it? What are the different types? |
|
Definition
does not cause blur or poor resolution. affect of axis magnification difference. Only light going through optical axis is uncurved. Stops can minimize.
Barrel- minus lenses puncushion is a plus lens. It can be minimized by an othroscopic doublet. It is aproblem with high powered lenses. |
|
|
Term
| What is chromatic abberation? what is the difference between lateral and longitudinal? |
|
Definition
| result from fact that n is dependent on wavenlength. Red bends more then red and light is therefore focused at different locations. The image may vary in size- lateral and location-longitudinal. Both lateral and longitudinal are related to wavelength. Longitudinal is inversely related to Abbe value. Lateral is related to prismatic and also inversely related to the abbe number. |
|
|
Term
| What are the trends seen with CA and abbe? and power? |
|
Definition
Abbe number increases- CA decreases
Power of the lens increases- increase in CA |
|
|
Term
| What equations do you need to solve for achromatic doublet? |
|
Definition
Pt=P1+P2
0=P1/v1 + P2/v2
the ratio of the v’s= the ratio of the powers except sign |
|
|
Term
|
Definition
| use shorter vertex distance, use monocular pd’s, include sufficient panto |
|
|
Term
| the abberations of most concern are |
|
Definition
| oblique astigmatism, curvature of field, distortion. |
|
|
Term
|
Definition
Prism power=y/x
usually x is 1 meter |
|
|
Term
| how do you convert from prism diopters to degrees? |
|
Definition
| Prism diopters-> radians = prism powr/100. To convert to degrees multiply by (180/pi) |
|
|
Term
|
Definition
| Deviation angle d=A (n-1) or Dan-1 , where d is deviation angle in degrees and A is the apex angle |
|
|
Term
| Predict prism power from thickness: |
|
Definition
| 100(g(n-1)/l), where g is there difference in 3.thickness apex-base and l is length from apex-base. |
|
|
Term
|
Definition
| prism power= dP where d is the distance from the optical center in cm. If a lens is decentered both horizontally and vertically use prentices rule for each direction individually then the total prism is the vector sum of the two. |
|
|
Term
| how do you combine vertical and horizontal prisms? |
|
Definition
| simply draw a vector diagram then use a^2 +b^2=c^2 and solve for c. Only works with combining prisms 90 degrees apart. To find the angle tanx= opposite of adjacent or draw a pic and approximate ☺ |
|
|
Term
|
Definition
| Total prismatic effect- calculated for 1 eye. Decide if relative to distance OC or seg OC. Do not use both! may have to find distance from optical center using seg drop and seg OC in order to find decentration from near OC. |
|
|
Term
| How do you calculate the SM? What are the equations for Ms and Mp? |
|
Definition
SM for lens thickenss: shape and power contribute: SM=shape factor X power factor.
Ms= 1/(1-(t/n)P1) T=meters where P1 is the front vertex or bc in spectacle
Mp= 1/(1-hPv) where h = the distance between back vertex of lens and entrance pupil of eye (vertex +3mm), Fv the back vertex power of lens. (meters) or the power of the lens. |
|
|
Term
| what are trends seen with SM for minus and plus lenses? h, t, BC, n |
|
Definition
Increase h- increases SM for plus lenses and decreases for minus lens (think of plus add effect)
Increase in t- increase SM for both
Increase in BC (F1)- increases SM for both
Increase in n- decreases SM for both. |
|
|
Term
| what do you need to do with anisos to keep the shap factor the same (Ms)? |
|
Definition
1) for small differences rx equal BC and thickness
2) large difference rx a thin flat lens for the lens with high RSM. Rx a thicker steeper lens for eye with lowest RSM |
|
|
Term
| what does knapps law say about RSM? |
|
Definition
| According to knapps law RSM is 1 if the lens is placed at the primary focal point of the eye which is about 17mm |
|
|
Term
| axial ammetropes versus refractive? |
|
Definition
best corrected with glasses according to knapps law
best corrected with CL |
|
|
Term
| transmittance/reflectance equaltions |
|
Definition
R= ((n2-n1)/(n2+n1))^2
Ts=1-R
TM=1-absorbed
TM=T0^(t1/t0) can vary with thickness)
T=TSXTSXTm (transmittance through lens) |
|
|
Term
| ideal index for anti glare coating |
|
Definition
| square root of the index of the lens |
|
|
Term
| How to find the primary and secondary principle plans using Fe and Fv and Fn |
|
Definition
FE-FV= location of secondary principle plane. The same concept can be applied to front surface using the front surface
FE-FN= location of primary principle plane. |
|
|
Term
| what must hold to correct an ammetropic eye with gps? |
|
Definition
|
|
Term
|
Definition
front surface positive
back surface negative |
|
|
Term
| power conversion for radius of curvature of CL |
|
Definition
| Approximately every change of 0.05 mm between the BC and cornea is 0.25 D of power. |
|
|
Term
| what is the residual astig? |
|
Definition
| Residual astig=OR-corneal astigmatism. |
|
|
Term
| whawhen is a patient a good candidate for toric CL? |
|
Definition
| When residual astigmatism is large- we use a front surface toric. A patient can generally tolerate 0.5 ATR astig and 0.75 WTR astigmatism. Larger then that in residual astigmatism is a good candidate for toric CL. |
|
|
Term
|
Definition
| when cornela astigmatism is great, that a back surface toric surface is needed to ensure a good physical fit. BC are choosen to parallel both corneal meridian- a back surface toric is chose, the power of the tear leans is plano. One should choose Fv for the contact lens to be equal to zero. Corneal astigmatism greater than 2.50 this is a good option |
|
|
Term
| What are the parameters for toric GP fitting? |
|
Definition
Corneal astigmatism > 2.50 back toric
Coreneal astigmatism <2.50 spherical
Lenticular- front toric
Conreal <2.50 and lenticular- front toric.
Corneal >2.50 and lenticular- bitoric. |
|
|
Term
|
Definition
flexure- when on the eye
warpage- when not on the eye |
|
|
Term
| difference between hydrogels and silicone hydrogels? |
|
Definition
| High water content for hydrogels- high permeability (high dk). High water content for silicon hydrogels- low permeability low dk Silicon is more permeable to oxygen then water. |
|
|
Term
| What is the trick with Hydrogels 1234? |
|
Definition
1234: LHLH: NNII
corresponding to H20 content and nonionic vs ionic. |
|
|
Term
| What is the difference between accommodation and mag between CL wearing hyperopes and myopes? |
|
Definition
Accomdoation required by hyperopic pres is greater, and less for myope when compared to emmetrope.
Mag- myopes have a larger image size with contacts then with spectacles, hyperopes have a a smaller image size with contacts than with spectacles. |
|
|
Term
| What is relative size mag? |
|
Definition
| change in object size. I2/I1. Ex larger print |
|
|
Term
|
Definition
| ratio of image size to object size: m=U/V |
|
|
Term
|
Definition
| Magnification due to a change in object distance. u1/u2 |
|
|
Term
| how can a plus lens act as a magnifier? |
|
Definition
| A plus lens can act as a magnifier, when inside the focal point. MT= mM where m is the lateral mag and M is the relative distance Mag. |
|
|
Term
| What is the magnification of a collimating lens a t 25 cm? |
|
Definition
M=0.25P (collimating)
can be used to find the power of a reading cap |
|
|
Term
| max mag of stand magnifier? |
|
Definition
|
|
Term
| FOV of hand held magnifier |
|
Definition
FOV: w=d/(Fel)
l is distance between magnifier and spectacle plane |
|
|
Term
| Tube length of telescope? |
|
Definition
Tube length: d=fobj + foc
foc is negative for glalilean |
|
|
Term
|
Definition
• Final image is magnified and inverted
• Objective lens forms inverted, real image inside telescope
• Ocular lens is a simple magnifier to view
• Exit pupil is located outside the telescope
• Larger FOV bc you can align the eyes entrance pupil |
|
|
Term
|
Definition
• Ocular lens is negative (diverging) objective lens is positive
• D= fobj +foc (but need to think about sign convention bc one is negative!
• Objective lens forms a real image past the ocular lens , this image serves as a virtual object for the ocular lens.
• The final image is magnified and erect.
• Exit pupil is located inside the telescope, so smaller FOV.
• Only available in mags up to 4X |
|
|
Term
| How do you find the tube length of an adjustable telescope? |
|
Definition
| Just at the object vergence to the objective power and solve for the tube length. |
|
|
Term
| what are two ways to find the mag of a telescope? |
|
Definition
M-Poc/Pobj
M= dEnt/dEx,entrance pupil is often the objective B/(B/A) |
|
|
Term
| What is the A and B of telescope? How do you solve for the exit pupil? |
|
Definition
| A is mag and B is the objective lens diameter in mm |
|
|
Term
| What is the total mag of a telemicrosope? |
|
Definition
| MT=MCMt where MC is equal to the power of the reading cap P/4. |
|
|
Term
| How can you estimate the reading power for a low vision patient using kestembaums rule? |
|
Definition
| The add power is simply 1/V where V is the distance snellen acuity. Which is basically the MAR… then needs to put it at that focal point in order to view with no accommodation. |
|
|
Term
|
Definition
1 M unit is equal to 5 arc min when viewed at 1 m or 20/20
ratio and solve for variables: 1 meter/1M=20ft/20 |
|
|
Term
| How can you convert an M notation acuity to snellen? |
|
Definition
| Ex 0.5m/2M=.25 _> 20/x=0.25 x= 80 so 20/80 |
|
|
Term
| What are the classifications of low vision? |
|
Definition
Normal vision 20/12-20/25
Near normal vision 20/30-20/60 w/20/60 then can resolve 1M at 1/3 of a meter
Moderate 20/70-20/160 must read 1M at a distance of less than 25 cm. convergence becomes a problem and BI prism is often needed if using glasses or a hand magnifier wide as the patient pd. |
|
|
Term
|
Definition
Patient cannot read an letters on the 20/100 line
A visual field diameter of 20 degrees or less in the better eye. |
|
|
Term
| What are the values for the reduced eye model? |
|
Definition
N=1.33
Cornea to posterior focal point 22.22 (axial length)
Total eye power +60 |
|
|
Term
| What are trends for myopia throughout life? |
|
Definition
Myopia trends: 25-50% at birth.
By age 1 few children are myopic.
By age 6, -0.50 or more is found in only 2% of the population.
Increases between age 20-60 and is 20 percent by age 20.
30% by 30 |
|
|
Term
| Difference between facultative and absolute hyperopia? |
|
Definition
Absolute hyperopia- when hyperopia is too large to be corrected by accommodation. The amount left over is called absolute.
Facultative amount that can be neutralized by accommodation. |
|
|
Term
| Hyperopia trends throughout life? |
|
Definition
Hyperopia trends:
6% of children 6-15
hyperopic child that is greater than 1.5 D will likely be hyperopic at age 14.
Emmetropic if between 1.5 and 0.5
And myopic if less than 0.5
Hyperopes and emmetropes show an increase with age, but it remains constant between 20-40. |
|
|
Term
| What is resolution acuity in cpd and MAR? |
|
Definition
| 40-60 cycles per degree. MAR 0.75 |
|
|
Term
| What is the min detectable acuity? |
|
Definition
|
|
Term
| Aphakia- contacts to glasses magnification |
|
Definition
| CL power/spectacle power. |
|
|
Term
|
Definition
|
|
Term
|
Definition
Assume 12 microns per diopter if 6mm diameter cut
If not 6 mm ablation is equal to diameter^2 Xspherical equivalent/3 |
|
|
Term
| What is the min thickness of a lens? |
|
Definition
| t=wavelength/4n (wavelength can be 555 nm) |
|
|
Term
| What properties of light change with higher index? |
|
Definition
| frequency does not change but wavelength and speed can be retarded |
|
|
Term
| difference between circular and elliptical polarization |
|
Definition
Circular polarization- amplitutude is the same
Elliptical- amplitude is not the same. |
|
|
Term
|
Definition
Brewsters angle: tan angle= n2/n1
The light reflected from the interface and the ray refracted are perpendicular, this leads to reflected light plane polarized in the plane of the surface. |
|
|
Term
| How are polarized lenses usually oriented? |
|
Definition
| usually horizontally polarized. Absorbed reflective horizontal light and transmitted that which is perpendicular (vertical) |
|
|
Term
| What is Airy's disc and rayleigh's criteria? |
|
Definition
| As light passes through circular disk it creates an Airy’s disk. The disk is larger for smaller aperatures. Two objects cannot be distinguished when their two corresponding airy’s disks overlap. This is known as rayleighs criteria and can be reduced to: dt=1.22*lambda (lambda = 555*10^-9 and d= diameter of aperature). This will give you T in radians. To convert to degrees multiply by 360/pi |
|
|
Term
| How do you solve for the size of the letter on a snellen chart? |
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Definition
Example 20/100
1) MAR=100/20= 5 arc min
2) MAR x 5(5x to distinguish letter) =25 arc min
3) 25/60= 0.42 degrees
4) 0.42x pi/360= .0072 radians
5) x/20=0.0072 , x= 0.14 ft |
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Term
| How do you solve for the snellen acuity of a large print book? |
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Definition
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