© wesner, m. f. physical characteristics of light the basis of all vision is the presence of...
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© Wesner, M. F.
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Physical Characteristics of Light
The basis of all vision is the presence of physical light.
NOTE: An object is NOT a visual stimulus, rather the light emanating from a light source or reflected from the object’s surface is the stimulus.
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1. Light as particles of energy (e)
{ Isaac Newton (1687). Principia Mathematica. Publication included his theories on gravity, calculus and his observations from his Experimentum Crucis. }
2. Light as a waveform.
{ Christiaan Huygens (1690). Treatise on Light }
Two ways to think about light:
1. Light as particles of energy (e).
2. Light as a waveform.
NOTE: Both are appropriate depending on the application.
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l2
l1
1 second
n1=2.0 cycles/sec
n2=4.0 cycles/sec
Light speed is constant: c = 3 X 108 m/sec
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Because the speed of light c is constant..
1/l µ ne µ nthus..
n “nu” = c/l “lambda” ; where n (cycles/sec), c (m/sec), and l
(m [or cm, or nm]/cycle)
NOTE: Planck-Einstein relation: e = hn;
where e “epsilon” is the energy state of a photon (or energy contained in a quantum
packet), h is Planck’s constant (6.624x10-27 erg-
sec) & n is frequency (cycles/sec).
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Color Percepts
I
The measurement of the number of repeating units of a propagating wave (the number of times a wave has the same phase) per unit of space.
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Going from long to short , lthe perceptual interpretations are as follows :
“red”“orange”“yellow”“green”“blue”“indigo”“violet”
ROYGBIV
ROY G. BIV
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Sources of photons..
luminescent - electrical e is used to excite the electrons of an atom.
Hg+ gas
Common fluorescent lamp:
e-UV quanta
Visible phosphor
incandescent - thermal e is used to agitate molecules (e.g., high temperature of a tungsten coated filament will release visible photons).
Incandescent sources can give off a limited or broad range of frequencies depending on temperature and/or type of heated material(s).
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Color Temperature -where higher temperatures produce higher frequency photons (shorter wavelengths).
Black Body Radiator - an enclosure that has perfect “black walls” that absorb all electromagnetic radiation.
(Note: hypothetical. Bureau of Standards has something that comes close)Possible to measure spectral output of incandescent source based on °Kelvin.
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(tota
l ra
dia
ted
pow
er
per
un
it
are
a).
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20000 K
20000 K
5000 K
5000 K
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Correlated Color Temperature is used when measuring the chromaticity (l properties) of any light source (including luminescent sources). This assumes the spectral distribution of the source can be approximated to one produced by a black body radiator (i.e., usually used when measuring broad-band light sources).
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Narrow band - few wavelengths. Can be defined by half-bandwidth.
Spectral Distributions
Broad band - source composed of many photons (remember: the e contained in a photon identifies its spectral wavelength).
Total e(# of
Photons)
l (in nm)400 700550
Equal e “white” (hypothetical)
monochromatic (hypothetical)
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Spectral Distributions
Can have “near monochromatic” light (e.g., lasers and light passing through interference filters.
Total e(# of Photons)
l (in nm)400 700550
monochromatic (hypothetical)
half-bandwidth of ±12 nm
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Spectral Distributions
NOTE: Psychologically, you can perceive narrow- and broad-band lights as equal ! Two stimuli that appear perceptually equal are known as metamers.
Total e(# of Photons)
l (in nm)440 660550
+ + 440+550+660
Equal e
“white”
bipartite
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Light as a wave..
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Divergence
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6 meters or >Parallel or
Collimated light
¥
¥
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Can also have Convergence..
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Two Ways Light Rays Can Change Directions (Bend)
• Refraction - bending of light rays from one refractive media into another.
h term used to indicate refractive index
hvacuum = 1.0000 hglass = 1.2 - 1.520
hair = 1.0016 hwater = 1.3333
hdiamond = 1.6 +
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Refraction
i
ihair
hglass
q’i’
Willebrord Snel van Royen, or Snellius (1621)
hairsin i = sin qhglassSnell’s Law:
NOTE: Going from rarified to dense medium ( hair< hglass ) refracting ray bends TOWARDS the normal. Because hglass> hair, refracting ray bends AWAY from the normal.
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hairhglass
q
Refraction
Increase angle of incidence (i) get a proportional increase (Snell’s Law) in the angle of refraction (q) .
i’
Increase angle of incidence enough, you reach a critical angle.
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hairhglass
q
Surpass the critical angle, & you get reflection.
i’
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hv
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Image formation by lenses
TTop: Light emanates from a point source in all directions. When some portion of the rays passes through a lens, refraction causes the rays to converge back to a point. An image of the point is created on an appropriately positioned imaging surface. Bottom: An extended object can be considered as a spatially distributed collection of points. The lens produces a spatially distributed image of the object on the imaging surface.
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Properties of Lenses
Two base-in glass prisms:
hglass
hglasshair
This makes a positive (+) lens.
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Properties of Lenses
Two base-out (apex-in) glass prisms:
hglass
hglasshair
This makes a negative (-) lens.
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Power of a Lens - Diopter (D)
Diopter (D) = hair / focal length (fl)
D @ 1 / fl
fl
- fl
real image
virtual image
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Focal Position of an Extended Object
real image
virtual image
fl
- fl
Chief ray
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1/fl = 1/do + 1/di
It is this special case
fl that is used to
define the power (D of
a lens).
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Diopter (D) = hair / fl
~ 1.0 / fl in meters 0.5 D = 1.0 / 2 m -0.5 D = 1.0 / -2 m
0.33 D = 1.0 / 3 m -0.33 D = 1.0 / -3 m
23.0 D = 0.043 m (or 4.3 cm)
Cornea: +43.0 Diopters
Lens relaxed: +20.0 Diopters
Total eye: +60.0 Diopters
Typical myopic correction: -2.50 D
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Chromatic Aberration“w
hit
e”
hn
hglass
Paraxial ray shows no chromatic aberration
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Two Ways Light Rays Can Change Directions (Bend)
• Refraction - bending of light rays from one refractive media into another.
• Reflection (scatter) - light rays change direction at the surface of two different refractive media with the light coming back to original media (i.e., no penetration into other media (angle of incidence is > critical angle)
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Aqueous humor-synthesized and secreted from ciliary epithelial cells lining the ciliary processes.
Canal of Schlemm-found in angle between cornea and iris. Meets with trabecular meshwork that passes metabolically “used” aqueous through to a venous portal system.
Vitreous gel
- intraocular fluids
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Closed-angle glaucoma - a build up of pressure in the anterior chamber due to a blockage in the canal of Schlemm.
Open-angle glaucoma - a slowly developing glaucoma due to a noncongestive build up of pressure in the anterior chamber. Sometimes due to excess inflow (oversecretion) or lack of appropriate outflow due to metabolic problems.
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The Lens & Accommodation:
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Lens capsule slackens. Crystalline lens thickens yielding greater refractive power.
Lens capsule pulled tight – low refractive power.
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Note: Trying to focus on objects or lights sources composed of only short wavelengths are usually less distinct (more blurry).
Due to Rayleigh Scatter..
“wh
ite”
hn
hglass
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Rayleigh Scatter..“w
hit
e”
hn
hglass
hglass
retina& why the sky is blue..
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Lenticular (lens & cornea) & axial length..
Is there a miscorrelation?
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The axial ammetropias - refractive errors due to a miscorrelation of lenticular refractive power and axial size of the eye (as opposed to emmetropia or normal-sightedness).
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(hypermetropia or hyperopia)
(myopia)
Normal sighted (i.e., correlated) (emmetropia)
AXIAL LENGTH TOO SHORT
AXIAL LENGTH TOO LONG
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Focal Position of an Extended Object
real image
virtual image
fl
- fl
Chief ray
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Environmental Myopia?
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Astigmatism is a cylindrical aberration
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Cataracts..
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ordered array of crystallins
protein array folds and collapses
entangled mass not unlike neurofibrillary tau protein entanglements in Alzheimer’s brain
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Short wavelength light and cataracts?
Rayleigh scatter suggests that higher frequency photons (shorter wavelengths) are more susceptible to refraction and reflection (e.g., chromatic aberration). Therefore, the preretinal lenticular elements may be susceptible to these scattering wavelengths, particularly UV photons. The energy from these quanta could disrupt the protein elements in the lens ultimately leading to opacification.
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Claude Monet (1899)
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Cone Mosaic
Color added
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F
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F
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Photopic
Retinal duplicity
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Log
mm
LCentrally fixated
field
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Normalized curves
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FIG. 1. Spectral sensitivities (l/threshold) of dark- adapted foveal cones, peripheral rods, and peripheral cones (broken line). All sensitivities are expressed relative to the maximum sensitivity of the fovea. The relative positions of these functions on the ordinates are therefore those observed in the eye.
Peripheral Cones
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Figure 9.20
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Total e(# of
Photons)
l (in nm)400 550 700
X
=S
en
siti
vit
y
l (in nm)400 550 700
Inte
nsi
ty
l (in nm)400 550 700
VlRadiometric
Photometric
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steradian
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