Chromatic Aberration In The Human Eye Flashcards
What are monochromatic aberrations
Lower order aberrations
Higher order aberrations
What are the lower order aberrations that we normally deal with when correcting with glasses
Defocus and astigmatism
What kind of aberration can you correct with glasses
Lower order aberrations
What accounts for 90% of aberrations in the human eye
Lower order aberrations
Defocus and astigmatism
What are the higher order aberrations we normally deal with in the human eye
Spherical aberration
Coma
Trefoil
Can we correct higher order aberrations with glasses
No
What accounts for 10% of aberrations in human eyes
Higher order aberrations
What is monochromatic aberrations produced with
A single wavelength of light
When a mixture of different wavelengths of light (polychromatic light) are considered, then ______________ occur
Chromatic aberrations
The transmission speed of light within a refractive medium depends upon the wavelength
Dispersion
What is the refractive index for each wavelength in a refractive medium
Each is different
N=vc/Vmed
_______ wavelengths of light are more refracted than ________ wavelengths
Shorter
Longer
The separation of white light (polychromatic light) into its component elements by an optical element is referred to as ____________
Chromatic dispersion
Quantifies the amount of dispersions produced by an optical element
Dispersive power
What wavelengths are considered for dispersive power
486 (B)
589 (G)
656 (R)
What is the formula for dispersive power
w=(nf-nc)/nd-a)
What happens to the dispersion of the optical element (prism or lens) as the dispersive power increases?
It also increases
More or less dispersion causes more chromatic aberration?
More
What is the inverse of the dispersive power
Constrigence or Abbe number (v) of the refracting element
What happens to dispersion as the Abbe number increases
Decreases
Watch optical element is selected based on two important factors
- refractive index
- Abbe number
Which gets focused first, blue or red wavelength
Blue
How do we get the red and blue wavelength to focus on the same point
Glue two lenses together to create an achromat doublet
Is the eye a singlet or an achromat doublet
Singlet
What is an example of an achromat doublet
Video camera lenses and some optometry instruments
Dispersion of water
Not constant, but varies approximately 1% across visible spectrum (0.98%)
For dispersion, when the wavelength increases, what happens to the refractive index?
Decreases
What is the RI for blue wavelength (400nm)
1.343
What is the RI for the red wavelength (700nm)
1.330
Chromatic dispersion for the ocular media of the human eyes
0.02
Cornea, aqueous, vitreous all have similar RIs
Lens a little different
When the chromatic dispersion occurs, the image of source formed by blue rays will be formed in front of the image formed by the red rays. This chromatic difference of focus is called ____________
Longitudinal (or axial) chromatic aberration (LCA)
What is LCA specified by
The ‘distance’ between image planes
What is LCA measured in
Diopters
Longitudinal chromatic aberration (LCA) can be quantified in two ways:
- The variation of power with wavelength-chromatic different of power
- The vergences of source for which the source is focused at the retina for a range of wavelengths-chromatic difference of refraction
The variation of power with wavelength
Chromatic different of power
- a way to quantify LCA
- happens IN IMAGE SPACE
- not ideal to test patients this way
Of the two ways that LCA can be quantified, which one happens in image space
Chromatic difference of power
Chromatic difference of refraction
The vergences of source for which the source is focused at the retina for a range of wavelengths
- a way to quantify LCA
- happens in OBJECT SPACE
- ideal to test patients this way
Of the two ways you can quantify LCA, which one happens in object space
Chromatic differnce of refraction
Of the two ways to quantify LCA, which one is easier to test on patients
Chromatic difference of refraction
What do you need to test chromatic difference of refraction to find LCA?
- filters or laser to isolate red and blue light
- blue target
- get the blue target to focus and then put the red filter on and move it till its in focus, measure the distance between the two and thats the LCA
The variation of position of the image on the retina with wavelength is called _________
Transverse (or lateral) chromatic aberration (TCA)
What is TCA specified by
The angel between the refracted chief rays for different wavelengths
If you increase the angle, what happens to TCA
Increase
What are the two ways in which transverse chromatic aberration (TCA) be quantified
- the variation of postion with wavelength - chromatic difference of position
- the variation of magnification with wavelength- chromatic difference of magnification
Chromatic difference of position
The variation of position with wavelength
-a way to quantify TCA
For TCA, how is blue positioned compared to red
Below red
If the object is off axis, which can be measured, LCA or TCA?
BOTH
The off axis object is needed to measure TCA but you can still measure LCA with this
Chromatic difference of magnification
The variation of magnification with wavelength
- one of the ways to quantify TCA
- image on the retina for 400 and 700nm will be different magnifications
What axis does TCA=0?
Visual axis
What line do we use as a reference line for TCA
Achromatic axis
The angle between the visual axis and the achromatic axis
Is usually just a few degrees and is essentially zero for some individuals
What are the two types of abberations
Monochromatic
Chromatic
What are the types of monochromatic aberrations we deal with
Defocus Astigmatism Coma Trefoil Spherical aberration
What are the types of chromatic aberration we deal with
Longitudinal (LCA)
Transverse (TCA
What are the ways to quantify LCA
- Chromatic different of POWER
- chromatic difference of REFRACTION
What are the two ways we quantify transverse (TCA) aberration
- chromatic difference of POSITION
- chromatic difference of MAGNIFICATION
LCA between 400-700nm range is
About 2.1D
The human eye has too much power for
Shorter wavelengths
-too myopic
Studies of LCA
It is constant across all studies over 70 years
- 2.1D
- the small variation in CA is because the main constituent of the ocular media is water, whose dispersion cannot vary between subjects
Why does the eye have too much power for shorter wavelengths
Human eye is more myopic for shorter wavelengths and so a negative spec Rx is required to correct this focusing error
LCA and age
LCA does not change significantly over the life span
-small changes in the RI of the eyes media which occurs with age leads to age-dependent changes in refractive error. However, the eyes chromatic aberration is determined by dispersions of the media rather than by the refractive index itself
What if chromatic aberration does change with age?
- important consequences for ocular measurement (optometer) and vision itself
- you would need different equipment for each age group
____ wavelength is usually in focus for low accommodation stimuli
Long
_____wavelgnths are in focus for higher accommodation stimuli
Short
What is blue light on the retina a cue for
Accommodation
What wavelength is associated with lead accommodation
Red
What wavelength is associated with lag accommodation
Blue
How does TCA vary
Unlike LCA, TCA varies in magnitude among studies and subjects
What does the magnitude of the TCA depend on
Field angle of the object (theta)
What is the averaged TCA across population
Zero
TCA for any given eye
NOT zero
-averaged TCA is zero, but not the TCA for any given eye
Mean magnitude of foveal TCA for red and blue light is
0.82arcmin of visual angle
