Visual Science: Photometry 1: Chapter 4 Flashcards
Electromagnetic Spectrum
- Visible Radiation: Wavelength range?
- 400 nm to 700 nm
Define Radiometry
Power produced by a source of electromagnetic radiation, irrespective of its effect on vision
Define Photometry
Deals with the effect that this radiation has on the visual System (radiometry)
Electro-magnetic Radiation
- How do we calculate Frequency of Light?
- v = c/lambda
v = frequency of light
c = speed of light
lambda = wavelength of light
Amount of Energy in a QUANTUM of light is given by what relationship? (eqn)
E = hv
E = energy per quantum
h = planck’s constant (6.626 x 10^-37 J/Hz)
v = Frequency
of
E = hc/lambda
c = speed of light and lambda = wavelength
- how much energy per quantum in UV light is there?
a. What happens when it’s absorbed by the skin? - Excessive exposure to what type of Radiation will cause a GLASS BLOWER’s CATARACT?
- Why is LONGER WAVELENGTH Radiation not visible?
- A LOT
a. can cause a lot of cellular damage, including mutations that can lead to malignancy - to Near-Infrared Radiation (1450 nm)
- Because it’s ABSORBED by the RETINAL PHOTOPIGMENTS
- What’s the basic unit of Photometry?
a. What is it a measure of? - What is Luminous Power used to specify?
- Certain wavelengths are more efficient at what?
- The LUMEN
a. Measure of Luminous Power - The TOTAL AMT of Light that EMERGES in all directions from a source
- at Stimulating the VISUAL SYSTEM than others
- How many Lumens per Watt is there at 555 nm?
- How would you figure out lumens at other wavelengths?
- What is the WATT?
- What is Luminous Power used to specify?
- 680 Lumens (the peak of the curve)
- Look at the curve, and see what the Luminous Efficiency is, then multiply that by the Peak (680 lumens) and you will get the Lumens/W for that specific wavelength!
- It’s the BASIC UNIT of RADIOMETRY, and is a measure of RADIANT POWER
- The Total Amt of light that emerges in ALL DIRECTIONS from a SOURCE
Abney’s Law of Additivity
- What does this Law state?
- Why does this work?
- That the PHOTOMETRIC System is ADDITIVE!!
2. Because most Objects EMIT or REFLECT a mixture of several different Wavelengths.
Luminous Intensity
- What does it refer to?
- What does Luminance Quantify?
a. What’s the Perceptual Attribute associated with Luminance? - What are the units for Luminance?
- The number of Lumens Produced in a Given direction by a Point light source
- The Amt of light coming off a surface (like a piece of paper) and in a SPECIFIED DIRECTION
a. BRIGHTNESS - CANDELAS/Area of light source. (cd/m^2) and foot lamberts
* 3.426 cd/m^2 per foot-lambert
Luminous Intensity
- If the ratio of Candelas to Image Surface area remains CONSTANT, what does this mean?
- What happens as Distance INCREASES?
- Luminance and Brightness DONT CHANGE
- Area of the image (retinal image or one formed on a luminance probe) DECREASES at the SAME RATE that the NUMBER of CANDELAS does that’s contained w/in the image.
Luminous Intensity
- What’s the unit for Luminous intensity?
a. What is that defined as? - What is a Steradian?
- Candela (cd)
a. 1 Lumen per Steradian - (Sr) is a SOLID ANGLE (It’s a CONIC Section of a SPHERE)
* omega = A/r^2
Illuminance
- What does it refer to?
a. Units?
b. Number per foot-candle?
- Luminous Power that falls on a Surface
a. Lumens per square Meter (LUX) and lumens per square foot (foot-candles)
b. 10.76 lux per foot-candle
*Look at chart on Page 69 of the book
Pg 69 of the book
illuminance
- Is analogous to rain. Why?
- Which will have HIGHER Luminance? A White piece of paper, or a black book?
- Intensity of rain is not affected by the surface on which it falls, the illuminance is not affected by the surface on which the light falls.
* Greater the density of raindrops (lumens), the harder it’s raining (greater the illuminance) - a white piece of paper, because it REFLECTS MORE LIGHT!!
Radiometric Units: Radiometric Equivalents
- Radiant Power
- Radiant Intensity
- Radiance
- Irradiance
- Luminous Power
- Luminous Intensity
- Luminance
- Illuminance
Cosine Diffusers
- What is a cosine diffuser?
- Luminance
- Illumination
- Eqn?
- Illumination of 1 lumen/ft^2, falling on a MATTE SURFACE w/a Reflectance Factor of 1, produces a Luminance of 1 Foot-Lambert.
- 1/pi candelas/ft^2 or 1 foot-lambert
- 1 lumen/ft^2
- L = rE
L = luminance in foot-lamberts
r = reflectance factor (nondimensional, no units)
E = illumination in foot-candles (lumens/ft^2)
Inverse Square Law
- As a Surface is moved away from a point source, what happens?
a. Eqn?
- # of lumens falling on it DECREASES with the SQUARE of the DISTANCE!! Causes a DECREASE in ILLUMINATIONa. E = I/d^2
E = Illumination falling on the surface
I = Intensity of the Point source
d^2 = distance from the Point source to the surface
E = (I/d^2)cos(theta)
E2 = E1 Cost(Theta)
E1 = Illumination falling on surface when it’s normal to light
E2 = illumination falling on the Tilted surface
Retinal Illumination
- What’s the basic unit?
- Eqn?
- Why is this important to calculate?
- The TROLAND
- T = LA
T = retinal illumination in Trolands (td)
L = Luminance of the surface that’s viewed
A = Pupil Area
- Because if it’s specified in Trollands, then all subjects will have the SAME AMT of light falling on their retinas REGARDLESS of Pupil diameter
Luminous Power
- Define
- Common Units
- Radiometric Equivalent and Units
- Total light power produced by a source
- Lumens
- Radiant power (Joules/second or Watts)
Luminous Intensity
- Define
- Common Units
- Radiometric Equivalent and Unit
- Light power produced in a solid angle by a point source
- Lumens/steradian (Candelas)
1 lumen/steradian = 1 candela - Radiant Intensity (Watts/Steradian)
Luminance
- Define
- Common Units
- Radiometric Equivalent and Unit
- Luminous intensity per unit projected area of an extended source
- Candelas/square meter (Foot-lamberts)
- Radiance (Watts/steradian/square meter)
Illuminance
- Define
- Common Units
- Radiometric Equivalent and Unit
- Luminous Power falling on a surface
- Lumens/square meter or lumens/square foot
- Irradiance (watts/square meter)
