Diffractive Optics And Iridescence Flashcards
An optical device that blocks alternate zones is called a ___________
Zone plate
A diffraction grating that creates inwardly bending spherical diffracted wave forms a _________
Point image
The focal length of the plate is give by…
F2=r^2/mlambda
Fresnel zone plate blocks wave that contribute to destructive interference at the diffraction maximum, making the image formation much ________
Dimmer
Instead of blocking these waves, they are ________________.
Phase shifted by 180
After phase shift of 180, the phase shifted waves would be ___________ with the axial wave, and would contribute to the constructive interference at the diffraction maximum, making the image much _________.
What is this called?
- in phase
- brighter
Phase plate or phase reversal zone plate
A zone plate with kinoform shaped grooves is referred to as a ______________
Diffractive lens
What is the benefit of the kenoform shaped grooves on an ophthalmic diffractive lens?
It makes it much thinner
A single refractive lens with a phase plate embedded on one side of its surfaces
Hybrid lens
Multifocal
The original bulk of the dashed lens is reduced, leaving a thin lens whose surface slopes match the original lens. This example has __________
Equal step heights
A Fresnel lens with ______________. A fresnel lens is always a mono-focal lens.
Equal zone widths
What does the power in the M=0 come from?
The base curve power
1st order in contact lens
Near vision
0th order in contact lens
Distance vision
Local grating _________ determines ray direction (add power). Spacing gets progressively smaller from center to its edge
Spacing
Local step _____ determines “diffraction efficiency” of light. (Determines how much light put into the add portion)
Height
Allocation of energy into each diffraction order
Diffraction efficiency
When m=0, focal length of _________ is infinite (zero power)
0th order
What is the diffraction efficiency for multifocal
About 80%
40% from 0th order
40% from +1 order
Efficiency for trifocals
Divide 80% by 3
Expected VA for multifocal
Near: 20/40
Distance: 20/20
Cant get 20/20 all around
Unwanted patterns onto the retina which affect the visual performance by obscuring relevant retinal image
Dysphotopsia
What are the two types of dysphotopsia
Positive and negative
Positive dysphotopsia
Bright artifacts onto the retina
Ex. Arcs, rings, streaks, halos, and glares
Negative dysphotopsia
Blockage of light onto the retina
Ex. Shadows, dark spots
Fresenel’s lens increases the luminosity of the lighthouse lamp by a factor of _____
4
The smaller fresnel lens orders, the 4th though the 6th orders, were often situated where?
Lakes and harbors
Biological color effects: physics
- structural pattern
- delivers color due to interference, diffraction and scattering
Natural diffraction grating on a component found on the insect exoskeleton
Chitin
Mollusk outer surface of shell
- fine scale grooves
- 3.4 micrometer
- 296 grooves/mm
- parallels grooves, smooth and even
- diffraction color is strong
Inner surface of mollusk shell
- widely separated
- 11.5micometers
- 87 grooves/mm
- very rough
- diffraction color is weak
Iridescence in shell
Higher orders (3-8) get overlapped
- this causes iridescence
- depend son groove density and surface quality
Pearl
Layer structure diffuses the light and causes less to the iridescence but contribute to the body color of the shell
