L- 25

Question 1

what is the definition of radiance ?

Answer 1

• the radiance of the source is defined as the intensity per unit area of the source in a given direction

Question 2

what is the first case for radiance ?

Answer 2

• the direction of interest is perpendicular to the plane of the source
  L0= I0/A [ units: lm/sr/m^2]

L0= at o degrees with respect to the normal to the surface
I0= intensity
A= Area

Question 3

what is case 2 for radiance ?

Answer 3

• the direction of interest form an angle θ, wrt the source
• Lθ= Iθ/[A cosθ ]
• where θ represents the angle between the normal to the surface and the direction of interest

Question 4

is Lθ ( radiance )independent of θ?

Answer 4

• for majority of sources and surfaces the radiance doesn’t change with the direction of measurement
• radiance is constant and independent of the direction of measurement
• this means that visually the source on the surface appears equally bright from whatever angle it is viewed

Question 5

how does Iθ have to change with the angle of measurement to ensure that Lθ remains constant?

Answer 5

• if Lθ is independent of θ, then Iθ must be equal I0 cos θ
• Lθ=L0 [ constant ]
• Iθ= I0 cos θ

Question 6

what is the definition for a Lambertian source or surface?

Answer 6

• the intensity of the source or surface is always proportional to the cosine of the angle between the direction of measurement and the perpendicular to the surface

Question 7

what is a formula of Lambertian surfaces ?

Answer 7

• total light flux emitted by a lambertian source= πLA

Question 8

what is the light flux formula ?

Answer 8

• the amount of light which this surface receives in the first place will equal to the illuminance level or irradiance level that fall onto to the surface multiplied by the area of the surface
• A x E {lm}

Question 9

what is the flux radiated by the surface if the reflectance of the Lambertian surface is R?

Answer 9

• if the reflectance of the Lambertian surface is R, the flux radiated by the surface equals R x A x E
• L=RE/π

Question 10

what is the radiance of the luminance of a surface?

Answer 10

the radiance of the luminance of a surface is its reflectance x luminance level/π
- L= RE/π

Question 11

what is the photopic vision ?

Answer 11

vision is mediated largely by cones

Question 12

where does maximum luminous efficiency occur in photopic vision ?

Answer 12

the maximum luminous efficiency occurs at 555nm and is equal to 683 lm/ watt

Question 13

how can we calculate the amount of luminous flux in photopic vision?

Answer 13

weighting radiant flux by the sensitivity of eye and multiplying by 683

Question 14

what is scotopic vision ?

Answer 14

vision mediated largely by rod photoreceptors

Question 15

where does the maximum luminous efficiency occur in scotopic vision?

Answer 15

the maximum luminous efficiency occurs at 510 nm and is equal to 1700lm / watt

Question 16

what are the equivalence of radiometric and photometric quantities?

Answer 16

1. radiant energy (J) = photometric energy (lm s)
2. radiant flux (J/s or watts) = luminous flux (lm)
3. radiant emittance ( w/m^2) = luminous emittance (lm/m^2)
4. irradiance (W/m^2) = illuminance (lm/m^2)
5. radiant intensity (W/sr) = luminous intensity ( lm/sr)
6. radiance ( W/sr/m^2) = luminance ( cd/m^2)

Question 17

how to convert from radiant flux to luminous flux ?

Answer 17

• radiant flux multiplied by the sensitivity of the eye for every wavelength of interest

Question 18

what is a filter/light modulator ?

Answer 18

• a modulator or filter is any material or device which attenuates or changes this beam
• the change can be either in intensity or spectral content

Question 19

how do we find the reflectance of a particular wavelength ?

Answer 19

• we eliminate the test chart with light, certain amount of radiant flux is put on the surface of test chart and some of it is returned back
• the amount returned divided by the amount of light which falls on the surface gives us the reflectance about particular wavelength

Question 20

what is transmittance ?

Answer 20

at any wavelength, the ration of the radiant flux transmitted and the incident flux on the surface of the filter is known as its transmittance

Question 21

what is reflectance ?

Answer 21

the ration of the reflected flux to that incident onto a sample is known as the reflectance

Question 22

what are neutral density filters/ test charts?

Answer 22

they have a spectral absorption that is independent of wavelength over the spectral range of interest ( usually the visible spectrum )

Question 23

what is the term filter used to refer ?

Answer 23

the term filter is used to refer to sheets of coloured glass or gelatine, interference filters, liquid filters and neutral density filters

Question 24

what is the transmittance of a filter or the reflectance of an object expressed as ?

Answer 24

the transmittance of a filter or the reflectance of an object is often expressed in log units
Log [T] or log [R] and described as optical density

Question 25

what does higher optical density mean ?

Answer 25

higher optical density = less light is reflected from the surface

Question 26

what is formula for OD(λ)?

Answer 26

OD [ λ ] = -log10 T [λ] T [λ] = 10^-0D(λ)

Question 27

what is formula for OD when light is reflected from a surface ?

Answer 27

OD [ λ ] = -log10 R [λ] R [λ] = 10^-0D(λ)

Question 28

how can we describe the transmittance of filter ?

Answer 28

decimal percentage optical density

Question 29

what does greater pigment mean ?

Answer 29

the greater the pigment , the greater the absorption , the higher the optical density and the smaller the transmittance

Question 30

what happens to light level on retina ?

Answer 30

- depends on how large the image of the source is in the plane of the pupil - when we reduce light level on the retina the pupil size would increase, the amount of light that would end up on retina also increases

Question 31

what is integrated filter transmittance ?

Answer 31

a measure of the total radiant or luminous flux transmitted by the filter

Question 32

what are neutral density glasses ?

Answer 32

don't change relative spectral composition of light which is transmitted colour of light will not change

Question 33

how to work out radiometric transmission factor ?

Answer 33

total radiant flux that comes out passes through lens and comes out from the lens multiplied by spectral transmittance of the filter divided by total radiant flux that falls onto the filter

Question 34

how to work out photometric transmission factor ?

Answer 34

total luminous flux which passes through and is transmitted by filter divided by luminous flux incident on the filter

Question 35

what do the photometric transmission factor and radiometric transmission factor depend on ?

Answer 35

both depend on the spectral transmittance of the filter and the spectral power distribution of the illuminant

Question 36

how should a test chart be illuminated ?

Answer 36

- test chart is illuminated with source of light - we try to achieve uniform illumination over the test chart so both letter and adjacent background receive same amount of luminous flux

Question 37

how to work out the contrast of the letters ?

Answer 37

a spectrally neutral test chart is illuminated uniformly and our task is to work out the contrast of the letters in terms of the optical density of the uniform background (ODB0 and that of the letters ( ODL)

Question 38

what is luminance of background proportional to ?

Answer 38

luminance of background is proportional will be proportional to the amount of light that we put onto background multiplied by the reflectance

Question 39

what is luminance of letter proportional to ?

Answer 39

luminance of letter is proportional to the amount of light on letter multiplied by reflectance of letter

Question 40

what is contrast independent to ?

Answer 40

- contrast is independent of amount of light put onto the test chart - contrast independent of the spectral composition of the light