OV1001 Optics Flashcards
What is light?
A form of radiant energy
Wave particle duality
What are the rules of geometrical optics
- Light travels in the form of rays that are coming from a source of light
- A ray of light travels in a straight line until it meets an optical boundary
What happens to light rays as it passes a diffuse surface?
Light rays scattered in all directions
What happens to light as it travels through a more optically dense medium?
It travels more slowly
What do closer wavefronts within glass mean?
Indicates light is travelling at a slower speed
What happens to wavefront as it enters a more optically dense medium?
Wavefronts has been tilted because of the uneven slowing down
What does a higher refractive index mean?
More dense material
What can prisms help treat?
Muscular imbalance
What happens when objects are viewed through a prism?
They are displaced towards the apex of the prism
What does the amount of displacement depend on?
Angle of the apex
Thickness of prism
If a prism is thicker what does that mean?
Stronger amount of correction
Displaces objects to a greater degree
What is dispersion?
White light refracting into different colours when shone through a prism
What happens to rays as they go from a low refractive index to a high one?
They refract towards the normal
What is the ideal optical system?
All rays converging to a single point
What does the paraxial approximation do?
Ignores rays that refract too far
Key feature for n=c/v
Always greater than 1
What is the principal of reversibility ?
If we swap the location of object and image, the system behaves in the same way
What is vergence?
Measure of how much a set of rays are diverging or converging
How do you know if an optical surface has power ?
If it has the ability to change the vergence upon refraction
What does it mean if you get reflection instead of refraction? (TIR)
- Light is travelling from a medium with higher refractive index to a medium with lower refractive index (i.e. n > n0)
- The angle of incidence i is greater than the critical angle of incidence, ic (i.e. i > ic).
Define centre of curvature?
The centre of curvature of a spherical surface is a point representing the centre of the sphere the surface would be part of, had the whole sphere been present
Define the first focal point
The point on the optical axis such that the object rays that meet or originate from this point will refract parallel to the optical axis
Define the second focal point
The point on the optical axis where the image rays meet if the object rays are parallel to the optical axis
Why is the thin lens approximation used
Allows us to ignore this change in vergence due to the displacement through the lens
What causes focal points to be equidistant from the lens?
Optical media on either side of the lens is the same
Where is an image formed if the object is placed at the first focal point?
Infinity
How do you achieve a vergence of 0 in a focimeter?
Change vergence arriving at test lens
Move target until parallel rays are achieved
What are the conditions for H’ to e equivalent to a thick lens?
- Image it produces must be identical to that of a thick lens
- Same object rays
- Same image rays
What properties does a specular mirror have?
Flat
Does not change vergence of the beam
For transparent optical surfaces what does the values of R and T depend on?
The refractive idex change of he surface
Angle of incidence
Where are F and F’ for mirrors?
Coincident
What sign is given for the refractive index of image space?
Negative
What are purkinge images
The virtual image produced due to the reflected light from the first surface of the eye
What are purkinge images used to assess
Curvature
Separations of the surfaces in the eye
Eye tracking
What does purkinge images depend on
Refractive indices of surfaces - larger difference larger reflection
What does diffuse reflections in the eye allow?
Visualise internal structures of the eye
What do two lenses need to have to have identical cardinal points
Same:
- material
- radius of curvature
- thickness
How will the size of the lens affect the image produced?
Larger lens allows more light through the optical system giving a brighter image
What happens when you use a stop?
You reduce the aperture of the lens
Where is the stop located
Between the lens and image
What does the asymmetric use of lens introduce
Aberrations
Imperfections which degrade the image
What is a lens
An aperture that is defined by the diameter
What does a lens with a larger diameter have
Larger aperture
Define Aperture
Any clear region of optical surface through which light can be transmitted
Define aperture stop
The aperture of the optical system as a whole.
The largest cone of light that can be transmitted by the optical system
What does a chief ray show
The largest beam of light passing through aperture
stop
Define chief ray
Ray that passes through centre of aperture stop
What do extended marginal rays show
The size of the entrance pupil
The largest cone of light leaving the object that can go through the system
What is a field stop
A structure that limits the field of view of the system
Define field of view
Extended field that can be imaged by the camera
What happens when the field stop is not at the image plane
Image is not as bright and abberated due to asymmetric use of the beam
Gradual cut off in image
What determines aperture stop
Axial rays
What determines field stop
Off-axis rays
What happens in a direct opthalmoscope?
Some of the light leaving the patients pupil is lost and does not make it through the sight hole
What can you do increase the field of view in an opthalmoscope?
Move closer to the patient so you can get a larger view of the retina
What happens in an indirect opthalmoscope?
Px pupil is imaged onto examiners pupil using high powered lens
E and E’ is on the same plane
What happens when the examiners views the px more off axis in a direct opthalmoscope?
only part of the beam will go through the sight hole
What observations cannot be explained through geometrical optics
Light doesn’t always travel in straight lines
Light can interact with itself, not just with matter
Define amplitude
The height between the peak and trough
Represents how much energy is present in the wave
Define Phase
The position of the wave if two waves of the same wavelength are displaced w/ respect to each other
What does a light wave need to oscillate?
Electromagnetic field
What happens an object is emitting light ?
It creates an electric and magnetic field that coupled together and oscillate in sync
What happens to the wavefronts of light as it travels through a denser material ?
It becomes distorted
What happens in unpolarised light?
Electric field fluctuates in all directions
What happens in polarised light?
Electric field in only one direction
Where is polarised light used
Photo elastic stress analysis
Light microscopy
What does ‘by a fringes mean’
The material has a refractive index that depends on the polarisation and propagation direction of light
Where will light reflect
At the first surface at the active reflection coating
Surface of the glass
What will happen if the thickness of the medium is correct for a light ray passing through a medium?
Ray will end up with two waves out of phase, peak coinciding with a trough and will cancel each other out
Define diffraction
Property of light causing wavefronts to no longer be plane but slightly bent on the extremes on the wavefront
What causes diffraction
When light squeezes through an aperture
Light deviates from the straight line trajectory causing a bend in the wavefront
What does diffraction depend on
Size of the aperture
What happens to the effect of diffraction as the beam travels further away from the aperture
Effects becomes greater
Starts to affect the centre of the wavefront
What wavefronts give perfect objects
Spherical wavefronts
What is an airy disc
Image of a point object due to the effect of diffraction
What does a smaller aperture produce
Larger airy disc
More diffraction
What is Point spread function
The image if any optical system gives when an object is a point
What can the shape of PSF tell us
The diffraction and aberrations present in an optical system
What is a good measure of how good our image is
Deviation between reference and actual wavefront
What is a measure of aberrations
Distance along optical axis between both meridians
What makes up PSF
Combination of the effect of aberration and diffraction
What causes aberrations
Due to the imperfect shape of the optical system
What happens to PSF when aperture is at largest
PSF is dominated by aberrations with a very asymmetric PSF
What happens to PSF when aperture is very small
Non-paraxial rays removed
No aberrations
Only diffraction present
What does a larger PSF mean
The more blurred an image is
When are small/paraxial approximations valid
When ray height (h) is small compared to the radius of curvature of the surface (r)
Define ideal image formation
Lens produces a perfectly spherical wavefront surface
When magnification is the only parameter needed to restore image to original object
What is the wavefront aberration function
Corresponding difference between the paraxial and the real wavefront
What does it mean when the wavefront aberrations to be zero
System is free of aberrations
Real wavefront surface is matched the paraxial wavefront for every ray height
Define the paraxial optics region of an optical system
Rays that pass close to the centre of the lens and originate from object points that are close to the axis of the lens
What are the principal parameters that affect what we see
Stimulus size
Stimulus contrast
Retinal sensitivity to contrast
What is stimulus size
Its angular subtense at the eye
What is stimulus contrast
The luminace difference between the optotype and the adjacent background
What is weber contrast
Iuminance difference w/ respect to the background
What is michelson contrast
Increase/decrease in the contrast w/ respect to the average luminance
Define MTF
measure of the ability of an optical system to transfer various levels of detail from object to image.
What happens when light levels drop
Retina illuminance decrease
Retina is less able to resolve fine detail
Results in poorer acuity
Poorer functional contrast sensitivity
What are the conditions for MTF
Ideal eye
No aberrations
Only limited by diffraction
What are the advantages for using contrast sensitivity
- Sensitive to residual higher order aberrations, diffraction and increased scattered light
- Can reveal presence of subclinical retinal disease
What does a high spatial frequency mean
Large number of cycles per degree of visual angle
Width of bright and dark bars becomes very small
What does a forced choice procedure allow
Improved accuracy
What happens to mesopic Px when using contrast optoptypes
More affected by the increased scatter within the optics of the eye
Why do we measure Functional contrast sensitivity?
VA is not sensitive to early changes in the optics of the eye
VA not very sensitive to early structural changes in the retina caused by disease
Advantages of measuring FCS
Measurement variable - stimulus contrast
Fixed size optotype
One can use both positive and negative contrast polarity
What imposes the ultimate limit on the resolving power of any optical system
Likely to get diffraction around edge of aperture
Causes formation of airy disc
Define airy pattern
Diffraction pattern caused by a circular aperture
What does the rayleigh criterion require
The separation between two adjacent object points to produce an image separation equal to p (the radius of the first dark ring of the airy pattern)
What happens to the resulting power of the objective lens as the diameter of the aperture increases
Resulting power increases
Angle that can be resolvable becomes smaller
What key parameters affect aberration
Ray height
Meridian angle
Object field angle
What determines ray height
Size of the aperture
How is the object field angle formed
Formed when dealing with extended off axis objects
What are the effects of primary seidal aberrations
Expanded size for a point image
Curved image plane
Extended images are no longer geometrically similar to the objects
Why is spherical aberrations absent on axis
It doesn’t depends on the object field angle
What are the two image planes for spherical aberrations
Paraxial and marginal
What does the disc of least of confusion correspond to
The best image plane
What does the disc of least of confusion do
Divides between marginal and paraxial foci in the ration 1:3 for large aberrations
What is the difference in lens power between the marginal and paraxial focal planes proportional to
The square of ray height
What happens to coma when object field increase
Larger amounts of coma and more asymmetric distribution of rays
What do rays perpendicular to the wavefront surface generate
A distribution of intensity in the image of a point that as a comet like appearance
What is astigmatism in the eye caused
Due to the significance difference in the curvature of the cornea in one meridian.
Astigmatic like effects will be observed for on axis objects
What happens to an object with circular symmetry in the object plane
It will be images with good spatial resolution while a line that passes through the axis will be more blurred
What does field curvature represent
A variation in focusing distance that varies with the square of the field
Why is field curvature an advantage in the eye
Given the shape of the imaging surface
Why is a negative field combine with astigmatism
To ensure that the best astigmatic image plane forms a flat surface
How do you minimise the effects of field curvature
Use a curved image surface
Use a combination of positive and negative lenses
Introduce appropriate amounts of astigmatism
What does distortion represent
A displacement of focus for each image point in a radial direction that increases with the third power if the field
How does monochromatic aberrations arise
Due to the natural consequence of refraction and reflection in symmetrical optical systems
What seidal aberrations affect the quality of the image by distorting the shape of the wavefront
Spherical aberrations
Astigmatism
Coma
What seidal aberrations affect the position of the image without distorting the PSF
Field Curvature
Distortion
What aberrations cause image blurring and irretrievable loss of spatial detail
Astigmatism
Spherical aberrations
How do the refractive indices of most transparent material used in optical instruments vary in the same way
All show an increase in refractive index as the wavelength is decreased
What is v value
quantifies the change in power in a thin lens as result of dispersion
Why are the specific wavelength chosen for spectral lines
Easily produce from low pressure discharge lamps
They cover electro-magnetic spectrum to which the eye is sensitive
What is spectral luminous function of the eye
The sensitivity of the eye to different wavelength
What does longitudinal chromatic aberration in the human eye cause
Shifts in the image plane position with wavelength
What can a negative lens used to correct myopia also cause
A decrease in the chromatic aberration in the eye
What structure in the eye acts as ‘effective’ field stop for visual acuity
the entry pupil of the eye
What are the principal factors that affect the sharpness of the image and the resolving power of the lens?
High order aberrations
Scattered light
Diffraction
The Exit Pupil, E0
the image of the aperture stop seen from the exit of the system. It is therefore the image of the aperture stop formed by all the optical elements behind the aperture stop.
