lecture 24 - fundamental quantities and basic laws

Question 1

what is the solid angle Ω ?

Answer 1

• the solid (sr) subtended by a cone that marks an area
• size of cone is defined by A/r^2
  r= radius of the sphere

Question 2

what is a cone ?

Answer 2

cone defines a region of space within a sphere

Question 3

how can solid angle be expressed ?

Answer 3

Ω can be expressed in terms of the semi-angle of the cone

Question 4

how to calculate semi-angle of the cone ε ?

Answer 4

• dividing the length of cord AB by the radius of the disc
• this gives angle in radians
  angle (rad) ε = AB/r

Question 5

how to calculate cone of solid angle Ω ?

Answer 5

Ω = A/r^2

area A= formed by the intersection with the cone divided by r^2

Question 6

how to calculate solid angle Ω ?

Answer 6

2π ( 1 - cos ε )

Question 7

how else can the solid angle ( sr) of a cone of a semi-angle be measured?

Answer 7

Ω = π ε ^2

angle ε must be in radians

Question 8

what is radiant energy (Q) ?

Answer 8

• the total number of joules that might be produced in a flash of light
• units = Joules (J)

Question 9

what is radiant flux (Q)?

Answer 9

• rate of flow of energy which is the number of joules produced every second
• units= Joules / s (Watts)

Question 10

what is radiant emmittance (M) ?

Answer 10

• number of watts emitted( which is joules per second )per unit area of source
• unit= W/m^2

Question 11

what is irradiance (E) ?

Answer 11

• the amount of radiant flux received per unit area of a given surface
• number of watts received per a unit area of the source
• unit = W/m^2

Question 12

what is radiant intensity (I) ?

Answer 12

• measured in terms of the radiant flux per unit solid angle in a given direction
• described as intensity of source
• unit=Watts/sr

Question 13

what is radiance (L) ?

Answer 13

intensity over solid angle which is the amount of radiant flux per unit solid angle in a given direction and size of source that produces that intensity
- unit = Watts/sr/m^2

Question 14

what are the most important radiometric quantities ?

Answer 14

• Irradiance (E)
• Radiant intensity (I)
• Radiance (L)

Question 15

what happens if we place a the surface of the receiver which is perpendicular to the line in the direction of illumination which is line from centre of surface to the source?

Answer 15

• when we place the surface of the receiver
• the receiver will be irradiated
• irradiance level will depend on the intensity of the source but also on the actual distance between the source and the receiver

Question 16

what is the solid angle subtended by the illuminated surface ?

Answer 16

the solid angle subtended by the receiver at the source is the area of the receiver divided by square of the distance
Ω = R/r^2

Question 17

what is the light flux captured by the surface of the receiver ?

Answer 17

radiant flux captured by receiver is solid angle which receiver subtends at source multiplied by the intensity which is the flux per unit solid angle

radiant flux( Watts) = Ω *I

Question 18

what is the irradiance of the receiver ?

Answer 18

• radiant flux received per unit area of the receiver
• E=radiant flux capture/ area of the receiver (R)
  or E=I/r^2

Question 19

what are the basic photometric laws ?

Answer 19

. inverse square law

. cosine law

Question 20

what happens if angle ε is 90 deg ?

Answer 20

• plane of receiver will be along the direction of illumination
• so surface of receiver receives no light at all

Question 21

what is area of effective area as seen from source ?

Answer 21

A’B’ = AB COS ε

Question 22

what is solid angle subtended by the illuminated source ?

Answer 22

solid angle = R cos ε/r^2

smaller than it would’ve been if the receiver was perpendicular to direction of illumination

Question 23

what is the light flux captured by the surface ?

Answer 23

(lm) = Ω *I

Question 24

what is flux per unit of illuminated surface ?

Answer 24

E=I* cos ε/r^2

Question 25

what is the inverse square law ?

Answer 25

• the irradiance of a surface element by a point source is proportional to the reciprocal of the square of the distance between the source and the surface element?

Question 26

what is the equation the summarises the two photometric law ?

Answer 26

E(W/m^2)= I COS ε / r^2

Question 27

what is the cosine law ?

Answer 27

the irradiance level is proportional to the cosine of the angle between the normal to the surface and the line joining the source and the surface element

Question 28

what is the Maxwellian viewing system ?

Answer 28

• when we place our eye so that image is in centre of the pupil
• the eye will see the exit pupil of the lens which will have same brightness as LED when viewed directly
• this is known as maxwellien viewing

Question 29

example
subject sees exit pupil of lens
LED source changed with identical diode that generates the same amount of radiant flux at wavelength of 420 nm?

what will happen to the perceived brightness of source ?

if tunable diode was available which wavelength would generate maximum perceived brightness ?

Answer 29

• brightness will be much darker than yellow light and red light
• answer -
  555nm because that is the wavelength for which the eye has maximum sensitivity