Week 4

Question 1

Resolution

Answer 1

• the closeness of two adjacent points or pixels in an object that can still be seen as seperate in an image
• resolution or sharpness in an image is compromised due to aberrations
• resolution is measured using visual acuity charts

Question 2

contrast

Answer 2

• the closeness of brightness or luminance of two adjacent points or pixels that can still be seen as seperate in the image
• contrast is measured using contrast sensitivity charts
• the contrast in an image decreases as spatial frequencies increase

Question 3

aberrations

Answer 3

aberrations can be defined as:
- failure of light rays from an object to form a clear point image (ray criteria)
OR
- failure of light waves to converge to form a point image ( wave criteria)

Question 4

ray criteria

Answer 4

• deviation of each ray from the chief ray can be represented as a single number, the root mean square which is the average of all ray deviations

Question 5

wave criteria

Answer 5

• light can be treated as a wave
• to from a perfect focused image, the wavefront would need to be spherical and converging to its centre of curvature at the retina
• how the shape of the wavefront is deviated from the reference spherical wavefront at the exit pupil defines wave aberrations

Question 6

seidel aberrations

Answer 6

snells law of refraction. Guess simplified this to n theta = n’ theta ‘ to just consider the first sin expansion
Seidel aberrations suggested that the second term of Sin expansion should also be considered. The second term can be expanded into 5 components:
1. spherical aberrations
2. coma
3. oblique astigmatism
4. distortion
5. field curvature

Question 7

chromatic aberrations

Answer 7

• occurs when different wavelengths of light after passing through a lens or optical system, refract differently and focus at different points along the optic axis
• short wavelengths of blue light are refracted more than longer wavelengths of red light

Question 8

Duochrome Test

Answer 8

-Commonly used in clinical settings during subjective refraction test
-Words on the principle of chromatic aberrations
-By adding +/- 0.25D when the eyes are focused that the peak of the relative luminosity curve of 555nm the focus can be moved approx +/- 50nm on the visual spectrum to check for relative preference for red or green colour

Question 9

wavefront error

Answer 9

-How the shape of the wavefront is deviated from the reference spherical wavefront at the exit pupil
-Measured using wavefront aberrometers

Question 10

measuring wavefront error - Shack Hartman principle

Answer 10

-Laser emits a plane wavefront (invisible infrared light), which is reflected by beam splitter into the eye
-Wavefront reflected from the eye passes through beam splitter to a lenslet array and to a digital sensor
-The individual lenslet collects and focus the light to discrete spots on the digital sensor. The digital sensor measures the spot positions to the reference spot field.
-If the wavefront reflected from the eye is distorted, the spot gets displaced from their reference positions
-Measured spot positions from the distorted wavefront are compared to the reference spots
-Spot data is finally used to reconstruct and analyse the wavefront shape using Zernike polynomials.

Question 11

Zernike Polynomials

Answer 11

-The shape of the distorted wavefront with respect to the plane wavefront is displayed as an infinite series of small polynomials known as, Zernike polynomials.
-Zernike polynomials represent the total wavefront error in an optical system, and are illustrated as wavefront aberration map.
-They progressively get smaller with increasing radial order.
-Each polynomial is one component of total wavefront error.
-Radial orders 0,1 and 2 represent lower order aberrations.
-All other aberrations are higher order aberrations.

Question 12

diffraction

Answer 12

-Bending of light rays around the edges of the aperture, including the eye
-Depending on the phase of the wavefront emitting from the centre of the aperture vs the edge of the aperture, the two wavefronts can either undergo constructive interference or destructive interference

Question 13

Point Spread Function

Answer 13

• PSF is the light intensity in an image following diffraction through a circular aperture
• Wavefront from the centre of the aperture interferes constructively and destructively with the wavefront from the edge of the aperture and forms a series of rings with varying intensities
• The intensity distribution of the image after diffraction is normalised to value of 1.0, which corresponds to a diffraction limited image (free of aberrations)
• All optical systems have an aperture so diffraction cannot be eliminated

Question 14

Airy Disc

Answer 14

• Diameter of the first diffraction minimum
• Airy disc is directly proportional to the wavelength and indirectly proportional to the pupil size

Question 15

Strehl Ratio

Answer 15

• An image quality metric that defines the effect of the optical aberrations in PSF
• Includes the effect of both aberrations and diffraction on the image quality
• The peak of PSF reduces in the Prescence of aberrations
• Strehl ratio is typically 0.2-0.4 in the human eye

Question 16

Rayleigh Criteria

Answer 16

• The maximum resolution of an optical system occurs when the maximum of one PSF coincides with the first minimum of another ( or separated by half an airy disc diameter)

Question 17

optimal pupil size

Answer 17

• Optimal pupil size is 2.5-3 mm
• Larger pupils result in smaller airy discs, but effect the image quality due to aberrations
• Smaller pupils result in larger airy discs and effect the image quality due to diffraction