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
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
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?
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.
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
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.
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
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 .
Amal Ghosh / Ioannis Kymissis • A Lambertian surface reflects light equally into all directions. Lambertian surfaces therefore are also called isotropic/uniform/perfectly diffuse reflectors.
- Summer '19