Lensometry, Prism, and hand neutralization Flashcards
Primary focal point plus lens
- object-space focus
- plus lens, point from which light must originate to emerge parallel from the lens, image is at infinity
Primary focal point minus lens
- object space focus
- minus lens, point towards which the incident light must be directed in order for the image rays to emerge parallel
Secondary focal point plus lens
- image space focus
- the point where parallel rays from a distant point object are rejoined to form an image at that point.
- when parallel rays enter lens, they focus here
Secondary focal point minus lens
- image space focus
- point from which diverging rays seem to come from, after a parallel bundle of rays are refracted by a negative lens
primary focal length
-distance from the optical surface to the primary focal point
Magnify vs minified
Magnify-plus lens
minify- minus lens
Radius of curvature
- points to the right, positive
- points to the left, negative
- determines power
Approximate power
- matches closely with thin lens
- cannot be manufactured because error in approximation can be significant
- ignores distance
Front Vertex Power
- neutralizing power
- used in hand neutralization
- can vary based on lens thickness and shape
Back Vertex Power
- standard for spectacle lens correction
- any lens with the same back vertex power will provide the same correction is it is placed with the same vertex distance.
Equivalent Power
- different lenses with the same equivalent power can provide differing corrective powers
- takes thickness into account
- principal planes change position with surface powers
Effective power (and equation)
- farther the lens, the more power needed
- important for accommodative demand, contact lens calculations, refractive surgery calculations
- P/(1-(t/n2)P)
- P/(1-dP)
Axis
Power is where the axis aint
- skinny babies find sphere and axis
- fat lines are cylinder
What is the effect of a prism?
- deviation of light rays is towards the base
- the image will appear to move towards the apex of the prism
Abbe Number
- constringence/v number
- measure of a transparent material’s chromatic dispersion
- shows the variation of refractive index versus wavelength
- high values of V indicate low dispersion
- achromatic doublets are used to counter balance each other
Wavelength and index of refraction
- higher wavelength= lower index of refraction
- red has lowest n
- violet has highest n
Temperature and index of refraction
-higher temperature=lower index of refraction
How do you measure prism?
- power is defined by its deviation
- 1 prism diopter creates a deviation of 1cm at 1 meter
- 5 prism diopters creates a deviation of 5cm at 1 meter
Prism orientation
Base in- base towards nose
Base Out-base towards temples
Base up-image down
Base down-image up
YOU CAN’T STACK PRISMS
Prentice’s rule
prism=Pd (in cm)
Hand Neutralization
- known trial lens placed against unknown opthalmic lens
- resultant motion determines plus or minus
- time consuming, poor accuracy
- can quick determine plus or minus and if there is astigmatism
Minus lens with hand neutralization
- moves the image in the same direction as the lens
- with motion
- minified
Plus lens with hand neutralization
- moves image in opposite direction as the lens
- against motion
- magnified
Thick lens hand neutralization
- back surface power of first lens is equal and opposite to the neutralizing power of the second lens
- if unknown lens is in back, you are determining the neutralization power of the unknown lens using the back vertex power of the know lens (good to go)
