Units of Measure

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Units of Measure

• Radiometry and Photometry
• Electromagnetic energy is launched into free
  space by a radiating source.
• The study of the properties and characteristics
  of this energy field through radiometry and
  photometry define the units and quantities that
  define it.
• Radiometric measurements are performed
  objectively by electronic instruments.
• Photometric measurements were established earlier
  in the scientific community with the human eye as
  the mechanism to describe the psycho-physical
  effect of radiation on the human eye.

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Units of Measure

• Human vision is a subjective and complex process.
• A variation of perception can exist over a range
  of observers viewing the same source.
• In order to neutralize this problem a highly
  defined relationship between photometric and
  radiometric units has been established.
• The Standard Observer Curve has been established
  by the International Standard Organization
  (C.I.E.). It represents the characteristics of
  average human vision and with the photopic curve
  serves as a mechanism to convert from
  radiomentric to photometric units.

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Units of Measure

• Radiometry unit of measure
• gt power measured in watts gt measures power
  density gttherefore watts per square meter (
  ).
• Photometry unit of measure
• gt power measured in lumensgtmeasures power
  densitygt unit is lux (lx).

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Units of Measure

• Angular Range
• The spread of electromagnetic energy in space is
  a behavior
• of study in radiometry and photometry as well.
• Angular range (solid angle or space angle) is a
  geometric quantity that defines this behavior. It
  is used to calculate the coupling between a
  source and a receiver.
• The radian is the practical choice for measuring
  angles in this application.
• A radian is 57.29578 degrees with a full circle
  (360 degrees)corresponding to ( 6.283185)
  radians.
• The unit of measure for solid angle is the
  steradian.

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Arc
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Area
Angular Range
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Spherical Area
Plane Area
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Units of Measure

• The terminology flux refers to field condition of
  an electromagnetic wave .
• gtThe unit of measure for Radiometric Flux is the
  watt(W).
• gtThe unit of measure for Photometric flux is the
  lumen (lm).
• Square Law Effect
• The density of the flux from a source decreases
  as the square of the distance increases.

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Units of Measure

• Lumen due to the nature of the psycho-physical
  properties of the human eye a standard reference
  was required to define the lumen relative to the
  radiometric unit.
• gtIt is taken at the peak of the photopic
  response curve (555 nm) that 1 watt of
  radiometric flux produces 683 lumens of
  photometric flux.
• 1 lumen 1/683 W at 555 nm

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Units of Measure

• From a quantum mechanic perspective of photon
  flow rate and photon energy (E) at 555 nm

This is what 1 lumen is equivalent to.
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Units of Measure

• Efficacy the ratio of photometric (luminous)
  flux to total radiometric flux from a source.

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Black body radiation spectrum
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Efficacy of blackbody radiator
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Radiometric Energy the product of power and time.
Photometric Energy same
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Units of Measure

• Radiometric and Photometric Intensity
• The intensity referred to here describes the
  distribution of flux in space.
• Radiometric Intensity is the radiometric flux
  density (watts) per steradian (W/sr)
• Photometric (luminous) intensity is the luminous
  flux density per steradian expressed in
  candellas.

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Units of Measure

• Presenting the Intensity Profile of a source
• Defines the radiation pattern of the source by
  characterizing the flux distribution of the
  source.
• Requires measurement of intensity in all
  directions.
• Two types
• 1.Polar radiation profile
• 2. Linear radiation profile

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Units of Measure

• Three general profile types
• Point Source Radiates equal intensisty in all
  directions.
• 2. Lambertian Source this profile is generated
  when the light travels through a transparent
  material and experiences diffusion or is
  reflected from a rough surface. The radiation
  profile takes the shape of a circle.

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Units of Measure

• Exponential Intensity Source profiles radiation
  patterns of LEDs, both wide and and narrow beam
  widths.
• Half Power Angle
• The width of spread of the radiation pattern for
  a source is defined by the angle.This can
  also be considered the half power point.
• Point Source even distribution.
• Lambertian Source
• Exponential Intensity Source

Where n is the radiation exponent
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Radiation profile
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Units and Measure

• Flux and Radiant Intensity
• gtMeasuring flux over a wide area can be
  difficult.
• gtMost manufacturers quote intensity.
• gtUnlike R.F. field patterns where the shape will
  tend not to be symmetric, flux can be calculated
  from a slice of radiation intensity pattern due
  to rotational symmetry.

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Units and Measure

• Optical Transfer Function (OTF)
• gtDescribes the efficiency of coupling flux from
  a source to a receiver.
• gtThe ratio of total flux (W or lm) to the
  flux coupled into the receiver.

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Units and Measure

• Optical Transfer Function and Numerical Aperture
• Exponent n source
• is the half of the cone angle to the receiver.
  
• NA is dimensionless ranging in value from 0 to 1.
• For small angles
• When NA is to be calculated from a circular area

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d
A area
D
Source
Receiver
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Units and Measure

• Lambertian profile
• Point source
• For a small angle

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Units and Measure

• Summary To maximize OTF, increase NA.
• Factors determining NA 1. Receiver area
• 2.
  Distance from source to receiver.
• OTF can be improved with the use of a lens
  between a source and a receiver.

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Units of Measure

• Radiant Incidence distribution of flux on a
  surface.
• ( Radiant flux)
• Incidence related to luminous flux is called
  Illuminance.

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Units of Measure

• 1 lux 0.09290 foot-candles
• 1 foot candle 10.764 lux

Units for illumination
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d
A area
D
Source
Receiver
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Units of Measure

• Correlating measurements
• Illumination on a surface

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Correlating measurements
Normal to the surface
I source intensity (w/sr or cd) d source to
surface distance
Point source with intensity I
Angular range
A
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Units of Measure
Incidence or illumination is directly
proportional to the intensity of the source and
inversely proportional to the square of the
distance between source and receiver.
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Units and Measure

• Radiant Sterance and Luminance
• gtFlux that is reflected or radiated from an
  object from a source and received by the eye.
  This is called radiant sterance or if visible
  luminance .
• gtThis reflection is what gives an object its
  signature of brightness, colour and surfaced
  texture.
• An optical system that uses a lens will respond
  to luminance or sterance. This includes the human
  eye.
• Ex. When luminance of objects are recorded on
  camera film or video.

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Units and Measure

• Luminance vs. brightness?
• Similar except that brightness refers to the
  psycho-physical action of the eye and brain as a
  response to luminance. This is a logarithmic
  function and not linear.