Lecture 3: Photometry
What is Illuminance? What are the units? What is the Radiometry equivalent?
Illuminance: luminous power falling on a surface Lumens/square meter Irradiance (watts/square meter)
What is the Inverse Square Law? Formulas: E = ? I = ?
Inverse Square Law: as a surface is moved away from a point source, the number of lumens falling on this surface decreases with the square of the distance, resulting in a decrease in its illumination (energy is equal to the intensity over the radius squared) E = I/d^2 I = S/(4 x pi x r^2) E = illumination falling on the surface I = intensity of the point surface (at the surface sphere) d = distance from the point source to surface
What does a Long Pass Filter do? What does a Short Pass Filter do? What does a Band Pass Filter do?
Long Pass: allows long wavelengths of light to pass through Short Pass Filter: allows short wavelengths of light to pass through (could block all infrared light) Band Pass Filter: transmit only a certain wavelength band, and block others
How do you calculate the number of Lumens using Luminous Efficiency? Where is the peak efficiency for all 3 cones? Where does the ability of your cone opsins to detect photons peak? For photopic vision, what is the number of Lumens with 10 watts of 600 nm light?
Lumens = (Luminous Efficiency) x (683 lm/W) x (W) Peak efficiency at 683 lm/W 600 nm Lumens = (0.6) x (683 lm/W) x (10 W) = 4216 Lumens
What is Luminance?
Luminance: luminous intensity per unit area projected in a given direction (cd/m^2) -how much light is going over a certain area
What is Luminance? What are the units? What is the Radiometry equivalent?
Luminance: luminous intensity per unit projected area of an extended source Candelas/square meter Radiance (watts/steradian/square meter)
What is Luminous Intensity? What are the units? What is the Radiometry equivalent?
Luminous Intensity: light power produced in a solid angle by a point source Lumens/Steradian or Candelas (1 lm/sr = 1 cd) Radiant Intensity (watts/steradian)
What is Luminous Power? What are the units? What is the Radiometry equivalent?
Luminous Power: total light power produced by a source Lumens Radiant Power (joules/seconds or watts)
What is the Scotopic threshold in terms of Luminance? What is the Photopic threshold in terms of Luminance?
Scotopic Threshold = 0.001 cd/m^2 Photopic Threshold = 3.0 cd/m^2
Illuminance: 1. What is Illuminance? 2. Is it affected by the surface on which the light falls? 3. What units is Illuminance measured in?
1. Illuminance: quantifies the amount of light coming off a surface (e.g. a piece of paper) in a specified direction or Illuminance: the luminous flux (lm) incident on a surface per unit area 2. NOT affected by the surface on which the light falls 3. Units: lux (lm/m^2) -Illuminance is essentially the illumination of a given area
Photometry Units: 1. What are the units for Luminous Energy? 2. What is Luminous Flux? (Units?) 3. What are the units for Luminous Intensity? 4. Candela = ? 5. What are the units for Luminance? 6. What are the units for Illuminance?
1. Luminous Energy = lumen second (lm x s) 2. Luminous Flux = luminous power (lm) 3. Luminous Intensity = candela 4. Candela = lm/sr 5. Luminance = candela per square meter (cd/m^2) 6. Illuminance = lux (= lm/m^2)
Radiometry Units: 1. What is Radiant Energy? (Units?) 2. What is Radiant Flux? (Units?) 3. What is Radiant Intensity? (Units?) 4. What is Radiance? (Units?) 5. What is Irradiance? (Units?)
1. Radiant Energy = Energy (Joule) 2. Radiant Flux = radiant energy per unit time (Watt) 3. Radiant Intensity = power per unit solid angle (Watt per Steradian) 4. Radiance = power per unit solid angle per unit projected source area (Watt per Steridian per square meter) 5. Irradiance = power incident on a surface (Watt per square meter)
At what wavelength is the peak Luminous Efficiency for the Scotopic system?
About 500 nm
What is additive color mixing? What is subtractive color mixing?
Additive Color Mixing: mix colors together to make a new color Subtractive Color Mixing: mixing in colors eliminates the number of possible colors you could make (e.g. when you mix too many colors it turns black)
Luminance Illusions (look at picture)
Circles have the same luminance but different brightness
What is Brightness? Is Brightness equal to Luminance?
Brightness: subjective attribute of light to which humans assign a label between very dim and very bright (not measured, but perceived) NO
What is Abney's Law of Additivity? What does it allow us to calculate? What is the only way to increase the number of lumens hitting the retina?
Most objects emit (or reflect) a mix of wavelengths of light, not just 1 wavelength *The Luminous Power for such a stimulus is determined by calculating the number of lumens produced by each wavelength and then adding these values together Allows us to calculate the total amount of lumens our retina is receiving Increase the power (Watts)
What is a Neutral Density Filter?
Neutral Density Filter: affect all wavelengths of light equally, allow certain percentages of light to pass through Decrease transmission but do not alter wavelengths transmitted
Define Photometry Give wavelength range Define Radiometry Give wavelength range
Photometry: measurement of visible light (only the EM spectrum) 360 - 830 nm Radiometry: measurement of optical radiation (includes EM, UV, and Infrared) 10 nm - 1 mm (3x10^11 - 3x10^16 Hz)
Does Scotopic or Photopic vision have a higher luminous efficiency? Scotopic Efficiency = Photopic Efficiency =
Scotopic Scotopic Efficiency = 1700 lm/W Photopic Efficiency = 683 lm/W
What is a Steradian?
Steradian = 3 dimensional radian
What does the Troland unit (td) tell us? What is the formula for calculating Trolands? (td = ?) What photometry measure is the Troland equivalent to?
Troland tells us how much light is getting into the eye (and then to retina) td = LA Equivalent to luminous intensity (cd)
