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.

#### You've reached the end of your free preview.

Want to read all 60 pages?

- Summer '19