Frame Theory Flashcards
What is abby value?
Also known as aberration value. Aberration is blurred or distorted image quality that inherent physical properties of optical device. The numerical description of how prone a lens is to breaking light down into component colours. The higher the abbey value of a lens the less likely it is to bend the light too far and create chromatic aberration
what is good and bad abby value
Good Abby valve is high 40 to 50
Bad abbey value is 30 or less
what is abby value dependent on
- Index
- Material
- Curvature
- manufacturer
5 Seidel Aberration
Spherical aberration Coma Oblique astigmatism: Curvature of field Distortion
Spherical aberration
parallel light from an object enters a large area of spherical lens surface.
Coma:
object point is off the axis of the lens, there is a difference in magnification
Oblique astigmatism
when rays from an off-axis point pass through the spectacle lens.
Curvature of field
power error-the spherical component of the lens to have the effect of being off-power in the periphery when worn.
Distortion
occurs because there is a different magnification at different areas of the periphery of the lens in proportion to the distance of those areas from the OC of the lens
Aberration
An optical aberration refers to a defect in a lens design which causes light to spread out instead of focusing to form a sharp image.
longitudinal chromatic aberration
occurs when different wavelengths focus at different points along the horizontal optical axis as a result of dispersion properties of the glass
lateral chromatic aberration:
Produces images of a slightly different sizes at the focal length of the lens, depend on the colour of the light.
(Diff wavelengths, coming in from diff spots. light will bend like to one focal point)
boxing system
The boxing system is based upon the idea of drawing an imaginary box around a lens shape with the box’s sides tangent to the outer most edges of the shape. The system uses the sides of the boxes as reference points for the standard system of measurements.
Monochromatic
Aberrations can occur in the lens even when the light entering the lens is only one colour.
Drop ball test
Withstanding impact of five-eighth steel ball weighing 0.56 oz, dropped from a height of 50 inches.
Four Variable of lens design
Vertex distance
Lens thickness
Refractive index
Front and back lens surface power
The apical angle
The angle formed by the junction of the two non-parallel surfaces is known as the apical angle, and the junction point is known as the apex. The thicker opposite edge is known as the base.
Dispersion
The separation of visible light into its different colors is known as dispersion. different frequencies of light waves will bend varying amounts upon passage through a prism.
Deviation
A turning of the eye from the point of fixation.
Displacement
when looking through a prism, the image of an object appears to be displaced from its actual location
What is image displacement?
A single ray of light is deviated in the direction of the direction of the prism
What is image jump
The sudden displacement of image as the bifocal line is crossed by the eye.
Progressive hard design
Wider area of stable optics in both distance and near
Narrower intermediate longer adaptation
Highest dioptric value of peripheral distortion larger than soft design
Shorter distance down to near viewing
step by step how to take a monocular pd
1) Measure the binocular PD. (Side note: Binocular PD is a single number representing the total distance between your pupils)
2) Before moving the rule, take note of the measurement on the ruler at the center of the nose.
3) Subtract this measurement from the binocular reading to get the left eye’s PD.
3 types of nose pad materials
Acetate pads
Vinyl pads
Silicone pads
four different prism power units
- Apical or refracting angle
- Centrad
- Deviating angle
- Prism Diopter
How do you identify horizontal prism
Always either base in for both eyes or base out for both eyes.
What 2 major groups can aberrations be grouped into
Chromatic aberration (color related) and monochromatic aberration (one color)
What happens when we have unwanted prism in our spectacles
- Blurry vision
- Headaches
- Nausea
- Double vision
6 steps to standard alignment
- check for horizontal alignment.
- 4 point-touch ( vertical alignment)- crossing, face form and planar
- open temple alignment- arms straight (94-95)
- check for temple parallelism (flat surface touch test)
- alignment of the bent down portion of the temple (40 -45)
- Temple fold angle
Rotated lens
will make the top of the eyewire hump up at the nasal bridge or one end piece to appear un swept in shape.
Skewed bridge
when viewed from the front a skewed bridge will cause a lens to appear higher than the other
Face form
the temporal sides of the eyewire should touch and the nasal sides should be equidistant from the ruler.
X-ing-
the frame front may be twisted so that the planes of the two lenses are out of coincidence with each other.
Variant planes
In this situation the lens planes are parallel, but one lens is farther forward than the other.
Open temple spread
the open temple spread or let back, is that angle that each open temple forms in relationship to the front of the frame.
Temple parallelism
For frames to be in standard alignment the temples need to be parallel to one another. Looking at eyeglasses from the side the angle temples with the frame front determine temple parallelism.
Temple fold angle
the temples should fold so that they are parallel to one another or form slight angles from parallel
Four-point touch
check for xing/ check for variant planes or coplanar alignment
Frontal angle nose pads
front view top pads should be closer together than the bottom
