OP - Measuring Light - Week 1 Flashcards
What is the difference between photometry and radiometry?
Photometry deals with measuring within the visible spectrum, while radiometry deals with the entire EM spectrum.
What is a primary source of light vs a secondary one?
Primary sources directly emit photons, while secondary sources reflect or scatter them
What are the four basic radiometric quantities?
Radiant flux
Radiant intensity
Radiance
Irradiance
Define radiant flux, and the units it uses.
Total amount of radiation emitted, or the rate of emission of energy
Joules/second = Watts
Measured in F
What is the formula for radiant intensity? What are the units?
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
Define irradiance, and describe the formula, and units.
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
How are radiant intensity and irradiance related? Describe the formula that relates them.
Inverse-square E = I / d^2 E - irradiance I - radiant intensity d - distance from source
Describe the formula for radiance.
Radiance = F / Ω A or Radiance = I / A I - radiant intensity A - area Units - Watts/steradian.metre^2
What is the equation describing the radiance of a black body?
Radiance = σT^4 / π
π - pi
T - temperature
σ - Stefan Boltzman constant
What are the two main types of light sources?
Incandescent source - if temperature is greater than 1.5K
Luminescent sources
Define the equation for luminous efficiency, and its units.
Amount of light emitted / amount of radiation emitted
Units - lumens/Watt
What are the four basic photometric quantities?
Luminous flux
Luminous intensity
Illuminance
Luminance
Define luminous flux, and how it is calculated.
Total light power emitted by light source.
Calculated using the spectral radiant power and the spectral sensitivity of the eye.
What is a candela?
One unit of luminous intensity
Define lux.
One unit of Illuminance
Define the formula for retinal illuminance, and its units.
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
Define and describe the difference between luminance and illuminance.
Luminance - light given off by a surface (emitted or reflected light)
Illuminance - measure of luminous power incident on a surface (incident light)
Radiometric and photometric calculations are comparable. Is it possible to convert between the two units?
Yes, but only if the luminous efficiency of the source is known
Define a lumen.
Total amount of light
