Download BIOPHYSICS OF VISION

BIOPHYSICS OF VISION
GEOMETRIC OPTICS OF HUMAN EYE
PHOTORECEPTORS
MOLECULAR MECHANISM OF VISION
THEORY OF COLOR VISION
ELECTRORETINOGRAM
Two problems:
“All cows are black in dark!”
Playing tennis in dark with illuminated lines, rackets, net,
and ball!
GEOMETRIC OPTICS OF HUMAN EYE
Refraction media of the human eye
Deye= 63 diopter, Dcornea= 40, Dlens= 15+
Human eye has complicated lens systems
Normal eye,
Nearsightedness – myopia,
Farsightedness – hyperopia
Astigmatism – need for a cylinderic lens
GEOMETRIC OPTICS OF HUMAN EYE
•Cornea – the main focusing
element
•Lens – adjustable focusing
•Iris – adjust sensitivity and
depth of focus
•Retina – photosensitivity
and much, much more
lens
GEOMETRIC OPTICS OF HUMAN EYE
GEOMETRIC OPTICS OF HUMAN EYE
The human eye has a complicated lens systems
Photoreceptors
(rods, cones)
Normal eye
Myopia
nearsightedness
corrected
Hyperopia
farsightedness
(Astigmatism -cylinderic lens)
corrected
GEOMETRIC OPTICS OF HUMAN EYE
LENS ABERRATIONS
Astigmatism
Simplified eye
tangential image
(focal line)
tangential plane
al
Optic
saggital plane
axis
lens
It can be applied when objects are farther than 5 meter
object
paraxial
focal plane
Sagittal image
(focal line)
PHOTORECEPTORS
PHOTORECEPTORS
Localization of rhodopsin
Structure of photoreceptors
cones ~(30 × 2-4) micrometer (color vision)
rods ~(60 × 1-2) micrometer (light sensing)
Distribution of photoreceptors
Sensitivity (1-2 photons for rods) (3-5 receptors)
Adaptation (10-9 – 105 lux)
Resolution (70 micrometer at 25 cm)
(two different receptors, between them one resting receptor)
PHOTORECEPTORS
PHOTORECEPTORS
Pigmented
epithelium
Outer
segment
Rods
~(60 × 1-2) micrometer
(light sensing)
Rods
nd cones
rods
cones
Inner
segment
Cones
~(30 × 2-4) micrometer
(color vision)
Bipolar
neurons
Müller-cell
Ganglion
cells
Optical
nerves
PHOTORECEPTORS
Distribution of photoreceptors
Receptor density
nasal
103/mm2
150
Rods
temporal
90°
90°
60°
60°
30°
100
sensitivity in
darkness
(%)
80
visus
1/1
60
30°
0°
blind spot fovea
centralis
50
cones
visus
90° 60° 30° 0° 30° 60° 90°
blind spot
fovea centralis
40
1/2
20
1/4
1/8
90° 60° 30°
0° 30° 60° 90°
blind spot
fovea centralis
PHOTORECEPTORS
Vavilov experiment
n=
E
, error =
hf
ΔE = hf ⋅ n = hf ⋅
ΔE
=
E
Ehf
=
E
n
(Poisson distribution)
E
=
hf
Ehf
hf
1
=
E
n
Rods are able to detect one or two photons
PHOTORECEPTORS
Rod
MOLECULAR MECHANISM OF VISION
Current (pA)
860 photon
2×
In dim light only rods can work.
There is no color vision.
2×
Light → retinal → opsin*→ transducin → PDE → cGMP↓
1:1
1:1
1:500
2:1
1:million
Na+ channels close → hyperpolarization → transmitter release
is decreased (glutamate –inhibitor) → stimulus
3 photon
Cone
36000 photon
2×
Response of cones is faster and
shorter. Cones are able to follow
fast movements.
(Hundreds of Na+ channels close,
a million Na+ ions will not enter.)
Elight = 1.5-3 eV, Eion = 6 x 103 – 6 x 104 eV,
190 photon
Amplification = 2 x 103 -2 x 104
Time (s)
PHOTORECEPTORS
Adaptation (10-9 – 105 lux)
a. Pupilla reflex (~16×)
b. Concentration of photopigment
(dim light, high pigment concentration)
c. Spatial summation
(dim light, many receptors per a single nerve cell)
d. Temporal summation
(dim light, longer time to induce stimulus)
e. (intracellular concentration of calcium ion)
MOLECULAR MECHANISM OF VISION
MOLECULAR MECHANISM OF VISION
Absorbed photon serves as a trigger
MOLECULAR MECHANISM OF VISION
Through the wizardry of biochemistry, sodium channels close
MOLECULAR MECHANISM OF VISION
MOLECULAR MECHANISM OF VISION
Light impulse
Resting potential
A= arrestin, GC= guanylate cyclase, PDE= phosphodiesterase,
Rh= rhodopsin, T= transducin
Time (s)
COLOR VISION
THEORY OF COLOR VISION
Different cones (blue, green, red)
(same retinal, different opsins)
Young-Helmholtz theory
X = rR + bB + gG
(Monochromatic color, mixed color)
(Color blindness)
COLOR VISION
ELECTRORETINOGRAM
Electric properties of human eye
Retina is at –6 mV potential compared to cornea.
Electrotinogram (ERG)
Early phase (ERP, Early Receptor Potencial)
Late phase ‘a’ ‘b’ ‘c’ waves
Dark adaptation (up to 30 minutes)
wavelength (nm)
Lack of vitamin A, night-blindness.
ELEKTRORETINOGRAM
BASIC PRINCIPLES OF GEOMETRIC OPTICS
incident
light ray
Biphasic wave
normal to
surface
reflected
light ray
Snellius - Descartes
ERP
Pigment layer
a
receptor cells,
hyperpolarization
ab
medium a
medium b
switch out peak
b
Müller cells
depolarization
Time (ms)
in
out
Light
BIOPHYSICS OF VISION
GEOMETRIC OPTICS OF HUMAN EYE
PHOTORECEPTORS
MOLECULAR MECHANISM OF VISION
THEORY OF COLOR VISION
ELECTRORETINOGRAM
Two problems:
“All cows are black in dark!”
Playing tennis in dark with illuminated lines, rackets, net,
and ball!
refracted
light ray
BASIC PRINCIPLES OF GEOMETRIC OPTICS
BASIC PRINCIPLES OF GEOMETRIC OPTICS
Image
formation of
thin lenses
• Monochromatic Aberration
7
1 1 1
= +
f i o
1
D (diopter ) =
f
1
= ( n − 1)
f
⎡ 1
1 ⎤
+
⎢
⎥
R2 ⎦
⎣ R1
–
–
–
–
–
i3
i5
i
i9
sin i = i −
+
−
+ − ...
3! 5! 7!
9!
Spherical aberration
Coma
Astigmatism
Field curvature
Distortion
• Chromatic Aberration
– Longitudinal chromatic aberration
– Lateral chromatic aberration
BASIC PRINCIPLES OF GEOMETRIC OPTICS
Paraxial light rays
Image
formation of
thick lenses
Major planes
Paraxial
focal
plane
⎡ 1
1
1 ⎤ ( n − 1) 2
d
= ( n − 1) ⎢
+
⎥+
f
R2 ⎦
n
R1 R2
⎣ R1
Real image formation
object
image
longitudinal spherical aberration
transverse spherical aberration
Spherical
aberration
LENS ABERRATIONS
LENS ABERRATIONS
Astigmatism
Object point
Coma
tangential image
(focal line)
Sagittal image
(focal line)
tangential plane
Image
saggital plane
al axis
Optic
lens
paraxial
focal plane
object
LENS ABERRATIONS
Coma
Coma is when a streaking radial distortion occurs
for object points away from the optical axis.
LENS ABERRATIONS
Astigmatism
If a perfectly symmetrical image field is moved off axis,
it becomes either radially or tangentially elongated.
LENS ABERRATIONS
LENS ABERRATIONS
Spherical focal surface
Cause: refractive index depends on wavelength
dispersion
Light is “bent” and the resultant colors separate (dispersion).
Red is least refracted, violet most refracted.
LENS ABERRATIONS
LENS ABERRATIONS
Distortion
Longitudinal chromatic aberration
white light ray
red light ray
red focal point
distorted images
object
“pincushion”
blue light ray
“barrel”
blue focal point
blue light ray
white light ray
red light ray
Longitudinal
chromatic
aberration
LENS ABERRATIONS
ELEKTRORETINOGRAM
Ocular electric potentials
Lateral chromatic aberration
Lateral color
red light
white light ray
blue light
object
focal plane
BIOPHYSICS OF VISION
BASIC PRINCIPLES OF GEOMETRIC OPTICS
LENS ABERRATIONS
GEOMETRIC OPTICS OF HUMAN EYE
PHOTORECEPTORS
MOLECULAR MECHANISM OF VISION
THEORY OF COLOR VISION
ELECTRORETINOGRAM
Two problems:
“All cows are black in dark!”
Playing tennis in dark with illuminated lines, rackets, net,
and ball!