Optical Aspects of Vision

• Point Spread Function:

  • to form a sharp image, light rays must reach the retina
  • the image of a point-like object always SPREADS light out
  • PSF = tell us everything of the image forming properties of an optical system

  ! Resolution = the smaller the PSF, the closer two objects can be placed and still resolved

  • Low psf = resolvable (can see that object consists of 2 points)
  • High psf = non-resolvable (cannot distinguish b/w the 2 points)
• What are the 3 factors that affect PSF? DAS

  • Diffraction = light behaves as a wave (the smaller the aperture/iris, the larger the spread/PSF)
  • Aberrations

  *Monochromatic = depends on angles

  *Chromatic = depends on the different wavelengths of light

  • Scattering = causes halo to appear around edges of PSF (from lens or cornea)

  Total PSF = PSF Diffraction + PSF Aberrations + PSF Scattering
• Colour Perception:

  • light is a wave and its wavelength determines colour - human eye is sensitive to visible light (b/w 400 - 720 nm )
  • the wavelength of light reflected (not absorbed) by the object determines its colour
  • light illuminates the object and passes through the optical components of the eye - cornea , lens and fovea
• 1)Diffraction:

  • occurs due to light behaving as a wave - spreads out light of the image
  • the smaller the aperture (iris) , the larger the spread (PSF)
  • Rayleigh Resolution Criterion - for an optical system w/ only diffraction , two points can be resolved if they are separated by at least the radius of their airy disk
  • Diffraction-limited Resolution - a perfect optical system can only be free from aberrations and scattering , which defines the maximum resolution it can reach

  ! diffraction cannot be corrected
• 2)Monochromatic Aberrations: unique to the individual

  • due to refractive error - myopia , astigmatism
  • higher order aberrations - even for an emmetrope , light rays don't actually meet at one exact point focus

  *effect isn't noticed

  • Keratoconus = aberrations are noticeable on cornea

  How are they measured?

  • spectacle prescription , defocus and astigmatism
  • Neural Adaptation = machine-measured prescription does not match the prescription found in the subjective exam

  ! Aberrations rapidly fluctuate - caused by the pulse and tear film changes

  
• 3)Chromatic Aberrations:

  • cause: shorter wavelengths being refracted more than longer wavelengths , giving blur (blue light being refracted more than red light)
  • solution: problem is reduced by the yellow pigment present in fovea which absorbs maximally at 460 nm

  
• 4)Scattering:

  • from lens or cornea - causes halo to appear around edges of the PSF
  • very severe in cases of cataracts - cloudy lens

  
• Nature of Light & Colour:

  • the range of wavelengths we are sensitive to depends on our light sensitive photopigments in our photoreceptors
  • sensitivity of photoreceptors can be measured with adaptive optics - each photoreceptor stimulated with a range of wavelengths of different intensities
  • Each photopigment has... (varies b/w us)

  *A peak wavelength it is most sensitive to

  *As you move away from this wavelength , sensitivity decreases
• Problems w/ Imaging the Retina:

  • Blur from the optical components of the eye becomes a​ problem when imaging the retina with high magnification - even zooming in will be unresolvable
  • This is due to monochromatic aberrations - in the fovea , cone photoreceptors are very small and very close together , image will be blurred
  • Adaptive Optics - correcting these aberrations allows us to image individual retinal cells - can correlate structure of the retina with visual perception (even if one cell has died , it will be seen)

  ! Allows early detection of disease

  
• Adaptive Optics:

  • How it works -

  *A deformable mirror continually changes its shape to redirect light rays so that they become parallel after reflection

  • What it corrects -

  *Monochromatic aberrations

  • 2 Main Purposes -

  *To image individual cells in the eye for early disease detection

  *To correlate structure and perception

  
• AO assisted camera can image 250,000 cells per square mm !
• Light Sources:

  • Natural = Sunlight

  *A lot of the Sun's radiation is eliminated by atmospheric scatter (short wavelengths) and absorption

  • Artificial = Tungsten and Fluorescent

  *Have very different spectra to the Sun