OP - Measuring Light - Week 1

Question 1

What is the difference between photometry and radiometry?

Answer 1

Photometry deals with measuring within the visible spectrum, while radiometry deals with the entire EM spectrum.

Question 2

What is a primary source of light vs a secondary one?

Answer 2

Primary sources directly emit photons, while secondary sources reflect or scatter them

Question 3

What are the four basic radiometric quantities?

Answer 3

Radiant flux
Radiant intensity
Radiance
Irradiance

Question 4

Define radiant flux, and the units it uses.

Answer 4

Total amount of radiation emitted, or the rate of emission of energy
Joules/second = Watts
Measured in F

Question 5

What is the formula for radiant intensity? What are the units?

Answer 5

Radiant intensity = ∆F / ∆Ω
F = radiant flux
Ω = solid angle
Units - Watts/steradian
Measured in I
This also applies to luminous intensity, measured in lumens not Watts

Question 6

Define irradiance, and describe the formula, and units.

Answer 6

Irradiance is the measure of the radiation falling on a given surface.
Irradiance (E) = Radiant Flux (F) / Area
Units - Watts/metre^2
Measured in E
This also applies to illuminance, measured in lumens not Watts, lux rather than E

Question 7

How are radiant intensity and irradiance related? Describe the formula that relates them.

Answer 7

Inverse-square
E = I / d^2
E - irradiance
I - radiant intensity
d - distance from source

Question 8

Describe the formula for radiance.

Answer 8

Radiance = F / Ω A or
Radiance = I / A
I - radiant intensity
A - area
Units - Watts/steradian.metre^2

Question 9

What is the equation describing the radiance of a black body?

Answer 9

Radiance = σT^4 / π
π - pi
T - temperature
σ - Stefan Boltzman constant

Question 10

What are the two main types of light sources?

Answer 10

Incandescent source - if temperature is greater than 1.5K

Luminescent sources

Question 11

Define the equation for luminous efficiency, and its units.

Answer 11

Amount of light emitted / amount of radiation emitted

Units - lumens/Watt

Question 12

What are the four basic photometric quantities?

Answer 12

Luminous flux
Luminous intensity
Illuminance
Luminance

Question 13

Define luminous flux, and how it is calculated.

Answer 13

Total light power emitted by light source.

Calculated using the spectral radiant power and the spectral sensitivity of the eye.

Question 14

What is a candela?

Answer 14

One unit of luminous intensity

Question 15

Define lux.

Answer 15

One unit of Illuminance

Question 16

Define the formula for retinal illuminance, and its units.

Answer 16

E = Ltαcos(θ) / k
L = luminance in direction of viewing angle
t = transmittance of the eye
α = area of the pupil
θ = angle of incidence 
k = a constant

Question 17

Define and describe the difference between luminance and illuminance.

Answer 17

Luminance - light given off by a surface (emitted or reflected light)
Illuminance - measure of luminous power incident on a surface (incident light)

Question 18

Radiometric and photometric calculations are comparable. Is it possible to convert between the two units?

Answer 18

Yes, but only if the luminous efficiency of the source is known

Question 19

Define a lumen.

Answer 19

Total amount of light