Chua Optics Corrections Flashcards
What is eclipse burn caused by
Infrared radiation
When is blue/yellow defect found
Retinal dystrophies
Glaucoma
Autosomal dominant optic neuropathy
What does the farsowrth munsell hue 100 contain?
84 colour discs that differ only in hue to be arranged in order
Relation of scattering and wavelength of light
Inversely proportional - the shorter the wavelenght, the more scattering
How much light does the cornea scattter
10%
What can pinhole improve
Ametropia of up to 4D
Birefringent materials have
2 different refractive indicesW
What is normal stereoacuity
60 seconds of arc or better
What are the range of stereoacuity tested by titmus test?
3000 to 40 seconds of arc
Frisby test
600-15 seconds of arc
TNO test
480-15 seconds of arc
Lang
1200-550 seconds of arc
Where is the image located in reflection at place surface
Erect, virtual, laterally inverted, along a line perpendicular to the reflecting surface
As far behind the surface as the object infront of it
What is the relationship between radius of curvature and focal point of a spherical mirror
Focal point is half the radius of curvature
When does total internal reflection occur?
When light travels from a medium with a higher refractive index to one with lower index
Thus to visualise the anterior chamber angle, a contact lens with higher refractive index than the cornea needs to be used to overcome the total internal reflection
What wavelength is devaited more by refraction?
Shorter wavelenght (blue light)
Where are fresnel prisms applied? How do they reduce VA
To the back of glasses lenses
Reduce VA by chromatic aberration
Which direction does base of prims face to correct a strabismus
Opposite direction of the deviation in the deviated eye in vertical strabismus
OR
Opposite direction of the deviation in either eye in horizontal strabismus
What happens when decentring a myope lens nasally? inferiorly?
NAsal - base out prism induced
Infeiror - base up prims induced
What happens when decentring a hyperope lens nasally? inferiorly?
Nasal - base in prism induced
Inferior - base down prism induced
8PD right esotropia correction split R and L
R 4PD base out, L 4PD base out
When does f1 = f2
When the media on each sid eof lens are the same
What is axial magnification?
Magnification of depth of image along the optical axis
What is angular magnification of a loupe
Angle subtended by the image divided by the angle subtended by the same object placed 25cm away from eye
What magnification/minification does cyl lens cause?
In meridian 90 degrees to axis
What is maddox rod used for
Distance phoria measurement
Double maddox rod for cyclotorsion
Against the rule astigmatism
Steeper meridian (more converging power) is at 180 so more plus required at 180 to correct (or weaker plus)
Note: this will be axis 90 on the prescription to act in meridian of 180
With the rule astigmatism
Steeper meridian at 90 degrees vertical (Axis 180)
So more plus required to correct at 90 (axis 180)
More plus cylinder is at 90 (axis 180) and minus cylinder is at 180 (axis 90)
What does the Geneva lens measure do?
Calibrated for crown glass
Measures base curve
Can measure lens diameter and thickness
What can a focimeter measure
Back vertex power
Prism
Axis of cylinder
Optical centre of lens
What does light absorption in a tinted lens depend on ?
Thickenss of lens - absorption higher in area where lens is thicker
What is thickness of anti-reflective coating?
Quarter of the wavelength of incident light
Made from metal oxides
What do photochromic compounds made of glass vs plastic use
Glass - silver halide
Plastic - organic compounds
What temporature do photochromic lenses darken more rapidly?
Low temperature as heat reduces the effect of light on photochromic lenses
