This is a measure of total output for all sources does not have to be spherical

This is a measure of total output for all sources

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This is a measure of total output for all sources (does not have to be spherical since we are integrating) Because it is based in Candela, it is also a photometric measurement 28
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ELEN4193 Amal Ghosh/Ioannis Kymissis Lumen A lumen is a total unit of measurement (i.e. it measures all photons…it is not per unit area) One lumen is the perceived brightness of 1/683W of 555nm light A typical lightbulb puts out about 1000 lumens Remember that a sphere subtends 4 steradians, you can work the math from there…a 1 steradian cone is about 65 degrees
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ELEN4193 Amal Ghosh/Ioannis Kymissis Easy examples-Lumens Imagine a candle (point source) of 1 candela which emits totally uniformly in all directions—how many lumens will it have? How much energy is this if the candle is at 555nm? What about another 1 candela source (an LED) which only emits uniformly upwards (say, 180 degrees?) How about a lighthouse that emits a 10 degree cone?
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ELEN4193 Amal Ghosh/Ioannis Kymissis Lumens sometimes make a lot more sense Candela is an unusual unit…it tells you angular brightness, perceptually weighted. Candela are conserved from point sources, but not other emission patterns. Lumens represent total light, weighted by color. Lumens are conserved and represent a total value. Light bulbs, etc. are specified in lumens.
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Amal Ghosh / Ioannis Kymissis [email protected] Some useful units
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Amal Ghosh / Ioannis Kymissis [email protected] Illuminance is the total luminous flux incident on a surface, per unit area It is a measure of how much the incident light illuminates the surface, wavelength-weighted for the human eye Luminous emittance is the luminous flux per unit area emitted from a surface. Luminous emittance is also known as luminous exitance. What is Illuminance
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ELEN4193 Amal Ghosh/Ioannis Kymissis Illuminance If we have a light source shining on a flat surface, we can measure how many lumens are hitting per meter squared This is known as illuminance, and is measured in lux Wikipedia Imagine taking a 1000 lumen lamp (~60W equivalent lightbulb), and shining it on 1 square meter—this is 1000 lux.
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ELEN4193 Amal Ghosh/Ioannis Kymissis What about FPDs? For an FPD, we want to know “brightness” normalized per unit area None of these units are conserved in this way…candela are good for point sources (maybe), lumens are total output
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ELEN4193 Amal Ghosh/Ioannis Kymissis Luminance 1 nit = 1 candela/meter squared Why candela and not lumens? Because light from the surface will spread out! Near the surface you can assume 2 steradians for a plane emitter. Light emitted this way is measured in candela per meter squared or nits. It is known as luminance .
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Amal Ghosh / Ioannis Kymissis A Lambertian surface reflects light equally into all directions. Lambertian surfaces therefore are also called isotropic/uniform/perfectly diffuse reflectors.
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