RPC

Question 1

Define radiant energy

Answer 1

J
Radiant energy (Qe) = energy that can travel through space with no apparent vehicle

Question 2

Define radiant and luminous flux

Answer 2

W = J.s-1 
Radiant flux (Φe) = energy emitted, transferred or received over time  
lm 
Luminous flux = visible light emitted, transferred or received over time ie energy which stimulates a response in the eye

Question 3

Define radiant and luminance intensity

Answer 3

W.sr-1
Radiant intensity (Ie) = used for point source and is the radiant flux per unit solid angle
Radiant intensity (Ie,θ) = intensity along angle to the normal θ
m.sr-1
cd
Luminance intensity = used for point source and is the luminance flux emitted per unit solid angle

Question 4

Define radiant and luminance exitance

Answer 4

W.m-2
Radiant exitance (Me) = used for extended sources and is the radiant flux emitted per metre squared
lm.m-2
Luminance exitance = total luminance flux radiated by an area divided by the area

Question 5

Define radiance and luminance

Answer 5

W.sr-1.m-2
Radiance (Le) = used for extended sources, radiant intensity per metre squared per unit solid angle
lm.sr-1.m-2
cd.m-2
Luminance (Ee) = used for extended sources apparent brightness when viewed from a specified direction, it is a vector quantity

Question 6

Define irradiance and illuminance

Answer 6

W.m-2
Irradiance (Ee) = radiant flux incident per metre squared
lm.m-2
lux
Illuminance = amount of light incident onto a surface

Question 7

Define when to use inverse square law and lambert’s law

Answer 7

Inverse square law (Ee) = when irradiance reaches surface at an oblique angle θ
E=1.cosθ/d^2
Lambert’s law = radiance is independent of direction of viewing ie same radiance at all angles

Question 8

Define emissivity

Answer 8

Emissivity (E(λ)) = how much is released over how much is ideally released

Question 9

What are the 4 main reasons for the study of radiometry?

Answer 9

measurement of hazardous radiation eg UVR, assess risk to health and wellbeing, used for setting safety standards, can prescribe protective measures eg eye protection and shielding

Question 10

What are the 3 main reasons for the study of colorimetry?

Answer 10

It is used for specifying the colour of lighting, understanding colour vision and colour vision tests and colour in tints

Question 11

What is the industrially acceptable range for visible light and the practical range?

Answer 11

industry = 400-700nm 
practical = 380-780nm

Question 12

What are the 6 sources of optical radiation?

Answer 12

1. Line spectra
2. Band spectra
3. Luminescence
4. Fluorescence
5. Phosphorescence
6. Continuous spectra

Question 13

Define the Neils Bohr atom

Answer 13

electrons could only occupy certain orbits and the angular momentum of an electron is quantized where quantum = bundle of energy

Question 14

Define line spectra, 2 features, a practical example and the difference between bright light and dark light spectra

Answer 14

Line spectra = movement of an electron up a shell requires energy and movement down releases energy in the form of a wavelength

Practical examples
• Passing a current through a gas in discharge signs eg neon, argon, helium or in low pressure sodium discharge lamps (monochromatic yellow light 589nm)

Features
• They have exactly quantised orbits
• There are no interactions with nearby atoms

Bright line spectra = produced when an electron emits energy seen as a colour, when it moves down a shell
Dark line spectra = shine white light through a gas and you get the reverse effect

Question 15

Define band spectra and a practical example

Answer 15

energy is radiated (usually because of excitation of molecules) in discrete bands of wavelengths rather than at individual wavelengths

Practical examples
• Light emitting diode LED = A current is passed through a diode ie semi-conductor material with photon emitting colour, dependent on diode. Wavelength spread about 20nm = pure light

Question 16

Define luminescence and a practical example

Answer 16

Luminescence = emission of optical radiation when something non-thermal puts in energy to raise electrons to a higher, unstable energy state

Practical examples
• Chemiluminescence eg light sticks
• Bioluminescence eg glow worms
• Electroluminescence eg flat panel displays, LED

Question 17

Define fluorescence and 2 practical examples

Answer 17

Fluorescence = are molecules that absorb photos then undergoes stokes shift ie triggers emission of another photon with lower energy (longer wavelength)

Practical examples
• Fluorescein = after illumination with blue light, it absorbs and emits a green colour to assess cornea integrity eg fluorescence pooled to where the cut on the cornea is so it may be illuminated which otherwise, would have been invisible

• Fluorescent lamps = mercury vapour current passes through tube which emitting UV radiation thus, white coating on tube is a phosphate coating absorb the radiation from the mercury arc to convert UV to VL. The colour variation is dependent on the phosphate coating

Question 18

Define phosphorescence and a practical example

Answer 18

Phosphorescence = same principle as fluorescence, however the decay of energy is over a longer time

Practical examples
• Glow in the dark products = illuminated by VL and stores energy and over time, will give it off when dark

Question 19

Define continuous spectra, the limitations with the energy emitted, an example and the 3 types of radiators which emit continuous spectra

Answer 19

Continuous spectra = results when a solid body is heated and emits energy, which exerts forces on the closely packed atoms in a solid so discreet wavelengths are not emitted but rather all wavelengths (within the limits of the temp and nature of the solid) is emitted

Limitations with the energy emitted
• Imperfect transfer of energy
• Not all absorbed energy is emitted as electromagnetic radiation

Eg Tungsten Filament lamps/Incandescent lamps
• Thermal energy in is not equal to light energy out, due to imperfection in itself or in air
• Tungsten lamps are also called incandescent lamps because the metal heats up to release heat and light and stops glowing when the filament evaporates

Three types of radiators which emit continuous spectra

1. Full radiators (black body radiators) = energy in = energy out ideal for perfect transfer of energy fully absorbs all radiations at all wavelengths used by national standards bodies eg for calibrating the colour of light emitted by a lamp
   a. Black body radiators = when specific heated item produce optical radiation with a specific spectral shape that depends solely on the temperature
   b. Also called Planckian radiators as they obey Planck’s law
   c. How does it work? Equipment contains a cavity usually graphite, holding heat, with a small viewing hole to observe the colour of the light emitted or, if you know the temperature then a colour (wavelength) can be ascribed to it. More energy in = more radiation out, also shift to the shorter wavelengths ie colour will change from red blue
   d. Colour temperature = the temperature of a full radiator defines its spectral energy distribution, which also defines its colour. Maximum exitance wavelength is inversely proportional to colour temperature
2. Grey body (non-selective) radiators = energy in ≠ energy out. Spectral absorbance is constant for all wavelengths
   a. eg Tungsten filament lamp. Tungsten behaves closely like a non-selective radiator and the lamp can be used as a standard source for calibration purposes
   b. Emissivity = how much is released vs how much is ideally released
   E will be <1 because the measured energy emitted is less than what would be theoretically calculated (or what should have been emitted if it were a perfect system)
3. Selective radiators = energy in > energy out. Emissivity thus spectral absorbance is NOT constant for all wavelengths eg Quartz

Question 20

How do we combine spectral values to give a total from line spectra?

Answer 20

Addition of the energy (intensity, flux, irradiance, radiance) of each line, gives total flux

Question 21

How do we combine spectral values to give a total from full radiators?

Answer 21

When heated, the spectral exitance is a function of only temperature so it can be measured using waveband width of ∆λ between limits λ1 and λ2

Question 22

All wavelengths have the same heating effect (energy in to energy out) to increase temp but they are all not equally effective in?

Answer 22

• In the photoelectric effect
• In causing non-thermal effect
• In providing visible light

Question 23

Why must a weighting value spectral effectiveness S(λ) be multiplied to each wavelength or wavelength band?

Answer 23

to account for lesser or greater effectiveness at producing an effect

Question 24

What type of radiation do detectors measure?

Answer 24

A detector can only detect radiation if it is sensitive to those wavelengths

Question 25

What are the 2 main groups of devices to detect and measure radiant energy?

Answer 25

1. Thermal detectors = spectrally uniform and not wavelength specific, ie will collect ALL heat, not just the radiation (wavelengths) of interest eg passive infrared sensor in alarm where alarm goes off due to a change in temp which converts to an electrical current in a crystal 2. Photoelectric detectors = spectrally specific and wavelength specific, therefore we get accurate readings which rely on photoelectric effect, eg photovoltaic cells in solar panels which allow electrons to flow across a conductor/semi-conductor, to convert solar energy to electrical power

Question 26

What accounts for the variation in colour perception?

Answer 26

* Equal energy ≠ equal sensation * Some wavelengths are more efficient at stimulating a photopic response eg peak sensitivity of the human eye under photopic condition is 555nm

Question 27

Describe 4 applications used to measure relative luminous efficacy function, important in finding the link between radiant and visible light

Answer 27

Each application derived inconsistent results as they measured different physiological processes, thus the CIE standard observers were established * Heterochromatic brightness matching = colour is compared against a grey * Cascade method = for brightness matching by presenting 2 colours and asking px which one is brighter --> m-cells (magnocellular), +p-cells (parvocellular = many red-green colour coded), +k-cells (koniocellular = blue yellow colour coded) * Flicker method = the brightness is identical and ask px to adjust dial to stop flickering ie maintain constant colour assessing non-colour coded m-cells alone * Minimally distinct border method = the border becomes more distinct as the brightness of the colour increases so observer is asked to adjust the brightness of the variable field until the border between it and the reference field is minimally distinct --> possible m-cells and p-cells

Question 28

What are the CIE standard observers?

Answer 28

Standard observers = established to standardise how the colour of an object is measured because subjectivity to colour perception is different per individual * CIE 1951 symbol V’(λ) = scotopic (rods) conditions so disregarded * CIE 1924 symbol V(λ) = colour stimuli only subtended 2˚ at eye, photopic (cones) conditions * CIE 1964 symbol V10(λ) = for 10˚ stimuli ie larger viewing area, photopic sensitivity, used often in paint industry

Question 29

What do we need to consider when converting from radiometric to photometric measurements?

Answer 29

Need to take into account the eye's sensitivitiy to wavelengths, by multiplying the photopic luminosity function V(λ)

Question 30

What is luminous flux at any wavelength equal to and how can it be added?

Answer 30

luminous flux at any wavelength = radiant flux . photopic luminosity function . 683 (Kmax) Luminous flux can be added even though wavelengths are of different colours • Sum • by increments

Question 31

Define luminous efficacy, wavelength of maximum luminous efficacy and relative luminous efficacy

Answer 31

``` Luminous efficacy (efficiency) = is the efficiency of a wavelength to create a visual response = luminous flux/radiant flux (units = lumens/Watt) ``` Wavelength of maximum luminous efficacy = an observer finding that the spectrum is brighter at one wavelength than the other Relative luminous efficacy = standard observers data which eliminates the problem of differences between observers, wavelength and measurement methods V(λ) = maximum luminous efficacy/flux at wavelength that produces a sensation of brightness equal to that produced by max

Question 32

Describe 3 past instruments used to measure photometry ie measurement of light in terms of its perceived brightness to the human eye

Answer 32

1. Spectral measurements and calculations = used to be lab instruments 2. Visual methods = compare a known source with an unknown source using inverse square law 3. Broadband methods = make direct measurements by using a detector and filter to approximate V(λ) (photopic luminosity function)

Question 33

Define photometers and what photometers are used to measure illuminance and luminance

Answer 33

Photometers = measures the reflectance of a surface as a function of wavelength * Illuminance = measured with an illuminance meter = amount of light incident onto a surface so it doesn’t matter what the colour/reflectivity of the surface is * Luminance = measured with a luminance meter = apparent brightness of a surface measured from a specified direction, therefore depends on the amount of light reflected and viewing location eg light coloured objects will have higher measurements because it absorbs less light and reflect more

Question 34

What are things to remember when using an illuminance and luminance meter?

Answer 34

Illuminance meter = errors can be from measurement technique or calibration ``` technique • correct working plane and working height e • shadow so use cord or “hold” button • check display settings • measure using grid area ``` calibration • multiply calibration factor to measurements and lux reading should be the same luminance meter = held to the eye because it measures how bright an object appears to the human eye

Question 35

What are the 3 types of reflections, what does it depend on and what can be gathered using an integrating sphere?

Answer 35

Depends on angle of illumination and view therefore need to set an angle (CIE specifies 0 or 45 degrees) * Specular ie mirror like and glossy * Diffuse ie matte * Combination ie satin integrating sphere = diffuse and combination

Question 36

What 2 measurements can be taken when considering reflections and what does it mean when ρ = β?

Answer 36

1. total reflection factor/reflectance (ρ) ρ = reflected luminous flux/incident luminous flux 2. luminance factor (β) β = luminance of sample/luminance of white surface ρ = β when sample is perfectly diffuse

Question 37

What are the 3 types of transmission and what can be gathered using an integrating sphere?

Answer 37

* Direct * Diffuse * Combination integrating sphere = diffuse

Question 38

What 3 measurements can be taken when considering transmittance?

Answer 38

1. transmittance t and t(λ) t = transmitted flux/incident flux • Can be multiplied for multiple filters • total transmittance = surface transmittance . internal transmittance . surface transmittance 2. fresnel's law = amount of light reflected from a surface ρ(λ) = [n(λ)2-n(λ)1/n(λ)2+n(λ)1)^2 3. bouguer's law = shows relation between thickness of material and transmittance ti(λ)d2 = ti(λ)d1^d2/d1 does not apply for • Polarising filters • Interference filters (lens which absorb certain wavelengths of light) • Diffusing filters

Question 39

What is the effectiveness of anti-reflection coatings?

Answer 39

high index lenses have a high reflection off surface increase transmission through lens ρ≤0.5%

Question 40

Define metameric and isomeric

Answer 40

metameric = spectrally different but look alike under some sources isomeric = spectrally the same and look alike under all sources

Question 41

Because of the variation in how colour can be perceived we have sources to help predict colour matches but we cannot provide data for all possible light sources. Describe 3 traditional sources and the 4 CIE standard illuminants developed

Answer 41

Traditional sources • Incandescent tungsten filament lamps • Sunlight = not exacatly thermal and unclear • Sunlight and daylight = infinitely variable o Clear sky = Rayleigh scattering of light (scattering by particles smaller than the λ of the light) o Cloudy sky = Mie scattering of light (scattering by particles larger than the λ of the light) Standard illuminants = developed because we need a more standardised way to quantify light and colour • A = representative of incandescent sources thus can be achieved with a tungsten filament lamp o Use = colour measurement and specification • B = representative of direct sun o Hardly used as source B is achieved with source A plus a filter (Davis-Gibson) o Use = colour vision examination • C = representative of north sky on a cloudy day o Disadvantage ♣ Hardly used as source C is achieved with source A plus a filter (Davis-Gibson) ♣ not good for viewing fluorescent materials o Uses = non-fluorescent colour vision examination • D65 = representative of north sky on a cloudy day o One of a family of illuminants, D50 to D75 but only D65 is a standard illuminant o There is no D65 source so cant be achieved in practice but there are methods of assessing quality of the source eg the nearest stimulation is a Xenon arc and window glass o Use = paint industry, colour vision examination and universally used in colour measurement and specification

Question 42

Define the 3 types of colour vision and the statistics of dichromatic and monochromatic

Answer 42

Trichromatic vision = 3 different photoreceptor types with peak sensitivity to different wavelengths of light Dichromatic = 2 cone photoreceptors active and accounts for 2% of males and 0.02% of females Monochromatic = 1 cone photoreceptor active and accounts for about 1 in 15,000

Question 43

Define chromaticity

Answer 43

describes qualities of colour associated with hue and saturation but not brightness * Hue = wavelength of light in mixture * Saturation = purity * Lightness = luminance

Question 44

What was Young-Helmholtz trichromatic theory of colour vision and what is the equation used to define the basis of colour specification?

Answer 44

red + green + blue =- white Any colour can be matched by no more than the combination of 3 distinct primaries Q(Q) =- R(R) + G(G) + B(B) Where =- = matches, (R) = symbols to represent the primary and R = coefficient

Question 45

What are the 5 different ways colour can be mixed?

Answer 45

1. spatially additive = superimposing light from several sources so that the combination stimulates the same part of the retina eg stage lighting 2. spatially averaging = when the eye combines and averages the colours of juxtaposed colour stimuli that are too small to be individually resolved eg monitors 3. temporal averaging = when the eye combines the rapid alternation or flickering of colours as it cannot be distinguished individually eg via flicker or maxwell's top 4. subtractive colour mixture = specific wavelengths are subtracted as transparent or semi-transparent materials eg filters and dyes, superimpose each other, ultimately achieving black. used in photography and printing 5 . pigment mixing = mainly obeys subtractive rules eg water colour