© wesner, m. f. physical characteristics of light the basis of all vision is the presence of...

© Wesner, M. F.

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.

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.

l2

l1

1 second

n1=2.0 cycles/sec

n2=4.0 cycles/sec

Light speed is constant: c = 3 X 108 m/sec

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).

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.

Going from long to short , lthe perceptual interpretations are as follows :

“red”“orange”“yellow”“green”“blue”“indigo”“violet”

ROYGBIV

ROY G. BIV

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).

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.

(tota

l ra

dia

ted

pow

er

per

un

it

are

a).

20000 K

20000 K

5000 K

5000 K

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).

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)

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

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

Light as a wave..

Divergence

6 meters or >Parallel or

Collimated light

¥

¥

Can also have Convergence..

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 +

Refraction

i

ihair

qq

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.

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.

hairhglass

q

Surpass the critical angle, & you get reflection.

i’

hv

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.

Properties of Lenses

Two base-in glass prisms:

hglass

hglasshair

This makes a positive (+) lens.

Properties of Lenses

Two base-out (apex-in) glass prisms:

hglass

hglasshair

This makes a negative (-) lens.

Power of a Lens - Diopter (D)

Diopter (D) = hair / focal length (fl)

D @ 1 / fl

fl

- fl

real image

virtual image

Focal Position of an Extended Object

real image

virtual image

fl

- fl

Chief ray

1/fl = 1/do + 1/di

It is this special case

fl that is used to

define the power (D of

a lens).

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

Chromatic Aberration“w

hit

e”

hn

hglass

Paraxial ray shows no chromatic aberration

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)

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

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.

The Lens & Accommodation:

Lens capsule slackens. Crystalline lens thickens yielding greater refractive power.

Lens capsule pulled tight – low refractive power.

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

Rayleigh Scatter..“w

hit

e”

hn

hglass

hglass

retina& why the sky is blue..

Lenticular (lens & cornea) & axial length..

Is there a miscorrelation?

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).

(hypermetropia or hyperopia)

(myopia)

Normal sighted (i.e., correlated) (emmetropia)

AXIAL LENGTH TOO SHORT

AXIAL LENGTH TOO LONG

Focal Position of an Extended Object

real image

virtual image

fl

- fl

Chief ray

Environmental Myopia?

Astigmatism is a cylindrical aberration

Cataracts..

ordered array of crystallins

protein array folds and collapses

entangled mass not unlike neurofibrillary tau protein entanglements in Alzheimer’s brain

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.

Claude Monet (1899)

Cone Mosaic

Color added

F

F

Photopic

Retinal duplicity

Log

mm

LCentrally fixated

field

Normalized curves

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

Figure 9.20

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

steradian