PCB240

reducing spherical aberrations.

1. only use centre of lens: disadvantage = lose intensity of image 2. careful choice of radius of curvature r1:r2=1:6 with more curved surface toward object 3. object distance = image distance S is minimum 4. doublet (converg. + diverge lens combination)

triple point of water

273.16K = 0.01C

reflection

Brewster's Angle.

3 methods of photometers. to measure photometric quantities.

Method 1: integrating sphere (measures luminous flux). a sphere with a uniformly diffusing white reflecting inner surface. Method 2: light meter (lux meter). uses photodiode to convert light to electrical signal. Method 3: luminance meter (for continuous source/surface). measures luminance and sometimes colour using three detectors: R G and B.

population inversion

a high number electrons are in the meta-stable state at any time.

scattering example

absorption of light ray and emission at non-resonant frequencies is called scattering. when oscillating electric field (around nucleus) nucleus will move slightly, lesser than electrons (since much heavier). scattering of light off air molecules produces linearly polarised light in plane perpendicular to incident light. glare in sky. photos look washed out as too much light to polarise in one direction. can be fixed with polaroid film.

what is a standard source, and what is the internationally used standard source (Hint: Th O).

aka black body. absorbs all light that falls on it, reflecting and transmitting none: only radiating (when heated). international standard source: thorium oxide.

colour subtraction combo of primary, y+c, c+m, y+m, y+c+m

any combo of R + G + B = black y+c = green c+m = blue y+m = red y+c+m = black

chromatic aberrations. why do they occur? what are they subdivided into?

because different wavelengths refract at different angles through a lens. larger the wavelength the smaller the refractive index. the larger the refractive index the more refraction occurs. longitudinal chromatic aberrations. (LCA) transverse chromatic aberrations. (TCA)

Munsell System - normal and description of paint

characterised by: hue, value and chroma/saturation hue: red, red-yellow, yellow, yellow-green, green, green-blue, blue, blue-purple, purple, red-purple value: relative to grey scale - white (10) to black (0) - 'brightness' chroma/saturation: vividness of hue - scale from 0 (not vivid) to <20 (very vivid). paint: tint, base paint and quantity of tint powder, respectively.

CIE colour system. whats the other name? how to describe perceived colour.

chromaticity coordinate system. z = 1 - (x+y), Q = x+y+z excitation purity: A/B where, A = white to sample, and B = white to locus.

correcting chromatic aberrations.

combine two lenses (converging and diverging) in contact such that dispersive effects are cancelled. called achromatic doublet. where red and blue focal lines have the same focal positions. requires two different materials in order to retain focussing power. (crown and flint usually). V1 cannot equal V2. can have two separated lenses to correct however, leaves small amount of LCA aberration. this scenario does not require different materials.

coma aberrations.

comet-shaped image - further from optical axis, more pronounced the comet shape made of a series of comatic circles - each circle made from refractions at particular zone of lens. described by: Tangial Plane and Sagittal plane.

rods and cones

cones: populate central vision area (around 0degrees). rods: distributed throughout peripheral vision (peaking around 20degrees).

Eye components: cornea, lens, retina, optic nerve, fovea.

cornea: provides most focussing in the eye. iris: automatic aperture, regulates amount of light entering (adjust exposure of the image). lens: automatic fine focus that adjusts focus for objects at various distances. retina: acts as a screen of film (converts light energy into electro-chemical energy). optic nerve: carries nerve impulses from retinal receptor cells to the brain (visual cortex) for visual analysis. fovea: point on retina at optical axis of the eye and position of most acute vision (if wanting to look at detail of object, look directly at it).

complementary colours. cyan, magenta and yellow

cyan: absence of red, complementary red. magenta: absence of green, complementary green. yellow: absence of blue, complementary blue.

spontaneous emission

electron in excited state (meta-stable state) E_2 can relax and return to ground state E_0. photon may emit in any direction. between absorption and emission only ~10^-8s.

curvature of field

even in the absence of previous aberrations, the focussed image will still not lie in a single plane.

monochromatic aberrations.

five primary aberrations: spherical aberrations (SA) coma (C) curvature of field (P) oblique astigmatism (A) distortion (D)

Which light would appear brighter of the primary colours: red green or blue, at the same power?

green - much more intense vs red or blue of the same power

what is an aberration? what are the two main groups?

if rays from object are not near the principle axis and are at angles >5degrees to the lens the images will form at different points not in the same image plane. chromatic and monochromatic aberrations

stimulated emission

in meta-stable state (E_2) electron stays for 10^-3s before decaying. if in this time a photon collides with the atom, it stimulates the electron to decay so 2 photons are emitted with the same polarisation, phase and direction as the incident photon. results in light amplification creating a laser. requires population inversion

define unpolarised light + four ways of producing it

light radiating in more than one plane. 1. dichroism 2. reflection 3. scattering 4. birefringence or double refraction

distortions

magnification at different distances from the axis of a lens. if stop is placed behind lens - pincushion effect (image smaller) if stop is placed infront of lens - barrel effect (image larger) stops stop aberrations from marginal rays, however creates distorted images. this can be reduced using two lenses with a stop in the middle (camera)

birefringence

means two refractive indexes (Snell's Law)

longitudinal chromatic aberrations (LCA)

measures the distance between the blue line focal point (FF') and red line focal point (FC'). appears as separated colours displaying the same image. C - the circle of least confusion sits at the plane of intersection between the red and blue rays. be able to draw converging and diverging.

characteristics of laser light

monochromatic light. closest will get. naturally coherent directionality - small angular spread intensity - very high. focusability very high - short focal length, and small divergence of beam. - can be decreased further with beam expander as beam divergence fixed in creation.

oblique astigmatism

occurs when image formed by a lens in one plane and does not correspond to images formed in a different plane. spreads image along the axis.

transverse chromatic aberrations (TCA)

occurs when image-off centre wrt to optical axis blur of colour and magnitude not in a single plane, therefore, different magnitudes for each colour. described by: linear TCA - measured difference in heights of red and blue rays. angular TCA - measured angle between emergent red and blue rays.

spherical aberrations.

produces image in which centre of field of view is in focus when periphery may not. 2 ways of describing SA: a. S - the separation between Fp' and Fm' - longitudinal SA b. T - radius of circular patch of light at Fp'

elements that make a laser and how they work (3)

pump: external power supply to excite electrons and cause population inversion. wrapped around material in a coil to dilute light as laser light is not multidirectional. laser medium: element that must have a meta-stable state that can be readily reached via pumping process. determines wavelength and gives name to laser. resonator: optical feedback device which directs photons back and forth through laser medium to create more photons and more stimulated emission. usually using aligned plane or curved mirrors along the optical axis of laser (100% and ~25% reflectance) - lets some out one side to let light through.

reducing coma.

reduce by shape factor (choice of radii) can eliminate coma but wont be good to minimise spherical aberration.

Purkinje effect

shift visual sensitivity max. from 555nm in photopic vision to 500nm in scotopic vision.

how laser works (4)

step 1: pumping excites electrons to high energy level (E3 from E0). due to pure absorption or stimulated emission. step 2: electrons decay spontaneously from E3 to lower levels - many to E2 (meta-stable state) staying ~10^-3s so N2 increases. N~0 majority of time due to 10^-8s speed. step 3: electrons on E2 may relax to E1 producing photons by spontaneous emission. these photons may interact with other atoms and stimulate emission: electron drops E2 to E1 as photon emitted. step 4: spontaneous emission from E1 to ground in ~10^-8s so population in E1 is ~0. population is achieved because N2>N1.

perks of Nd:YAG - little water absorption and almost no haemoglobin absorption.

therefore, useful in deep eye surgery to still be able to operate through fluid and blood in eye. - glaucoma treatment (fluid pressure increase) - blocked ducts or new canal creation for better drainage between chambers - neovascualisation (growth or rupture of unwanted blood vessels) - diabetic retinopathy (burn or weld at retina) - weld retinas that detach - posterior capsulotomy - rupture unwanted opacified membranes alone optic axis - low absorption means can be used after cataract surgery when membranes become opacified and block light.

how laser works in (3) steps: Ruby material

this system omits the lower laser level (E1) so stimulated emission occurs between the meta-stable state and ground state (E2 and E0) as N2>N0. as E0 is ground level, 3 step process requires much more energy in pump to achieve population inversion.

dichroism example

wire grid. (Malus' Law) polaroid (PVA) thin film on sunglasses to protect from glare by polarising light. - film has to be heated and stretched in one direction (vertical) allowing the hydrocarbons to align and create a wire like grid. then the film is dipped in iodine solution to form a conducting grid to polarise a component of light (horizontal). - at everyday angles of viewing (30-60degrees) reflective surfaces have a high % of light polarised in direction parallel to reflecting surfaces. therefore polarised sunnies allow a transmission axis aligned vertically to reduce glare.

complementary addition. y+m,, m+c,, y+c,, y+m+c

y+m = white (red tint) m+c = white (blue tint) y+c = white (green tint) y+m+c = white

3 lines labelled when describing dispersive power of optical material. corresponds to yellow, red and blue.

yellow - d line (580nm) red - C' line (650nm) blue - F' line (480nm)