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 Visible
Light
Photometry
Luminous
Flux
Intensity
Illuminance
and Luminance
V(λ)
Function
Colorimetry
 Glossary
A - Z
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Visible light is only a small section of electromagnetic
radiation which produces a sensation of brightness and color
in the human eye.
Electromagnetic radiation is a form of energy. The spectrum
of such radiation provides information on its energy composition.
The entire spectrum of electromagnetic radiation ranges
from X-ray radiation at the high-energy, short-wave end
to radio waves at the low-energy, long-wave end.
Radiometry is the measurement of optical radiation, which
is electromagnetic radiation within the frequency range
between 3×1011 and 3×1016
Hz. This range corresponds to wavelengths between 0.01 and
1000 micrometers (mm), and includes the regions commonly
called the ultraviolet (UV), the visible (VIS), and the
infrared (IR). Two of the many typically encountered units
are watts/m2 and photons/sec-steradian.
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Photometry
Photometry is the measurement of light, which is defined
as electromagnetic radiation detectable by the human eye
(daylight). It is thus restricted to the wavelength range
from about 380 to 780 nanometers (1000 nm = 1 μm). Photometry
is just like radiometry except that everything is weighted
by the spectral response of the eye. Visual photometry uses
the eye as a comparison detector, while physical photometry
uses either optical radiation detectors constructed to mimic
the spectral response of the eye, or spectroradiometry coupled
with appropriate calculations to do the eye response weighting.
Typical photometric units include lumens, lux, and candelas.
In order to have also a well defined photometer, an "artificial
eye" has been constructed to simulate the light sensitivity
of the human eye. The relative response of the normal human
eye to monochromatic light at the different spectral frequencies
was determined experimentally by the CIE and standardized
in 1924. This is known as the photopic luminous efficiency
function. The symbol of this function is V(λ)
and it is usually expressed as a function of the wavelength
of light (in air).
The following procedure was conducted to determine the
photopic luminous efficiency function: First, light of constant
intensity was emitted and its frequency was varied until
the lightness perceived by the observer was found to be
maximal. This occurred at a frequency of about 540 THz,
corresponding to wavelength λm = 555 nm. The wavelength
was then set to another λ and the power was readjusted until
the lightness was judged to be the same as at λm. V(λ) could
thus be computed as the ratio of the radiated power at λm
and λ, respectively.
Of course, this experiment has been conducted by many observers
and the resulting average was used to define the so called
CIE standard eye which is an optical sensor with sensitivity
corresponding to the function V(λ).
The photopic luminous efficiency function serves as a link
between the subjective response of the human eye and normal
physical measurement techniques. It thus provides the basis
for a group of photometric units (see Fig. 1).
Fig. 1: Radiometric and luminous quantities
| QUANTITY |
RADIOMETRIC |
PHOTOMETRIC |
| Power |
Flux: watt (W) |
Luminous flux: lumen (lm) |
| Power per unit area |
Irradiance W/m² |
Illuminance: lm/m² = lux (lx) |
| Power per unit solid angle |
Intensity: W/sr |
Luminous Intensity: lm/sr = candela (cd) |
| Power per area per solid angle |
Radiance: W/m²-sr |
Luminance: lm/m²-sr = cd/m² |
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Luminous Flux
The lumen is a derived unit for luminous flux. Its abbreviation
is lm and its symbol is Φv. The lumen is derived from the
candela and is the luminous flux emitted into unit solid
angle (1 sr) by an isotropic point source having a luminous
intensity of 1 candela. The lumen is the product of luminous
intensity and solid angle, cd-sr. It is analogous to the
unit of radiant flux (Watt), differing only in the eye response
weighting. If a light source is isotropic, the relationship
between lumens and candelas is 1 cd = 4π lm. In other words,
an isotropic source having a luminous intensity of 1 candela
emits 4π lumens into space, which just happens to be 4π
steradians. We can also state that 1 cd = 1 lm/sr, analogous
to the equivalent radiometric definition.
If a source is not isotropic, the relationship between candelas
and lumens is empirical. A fundamental method used to determine
the total flux (lumens) is to measure the luminous intensity
(candelas) in many directions using a goniophotometer, and
then numerically integrate over the entire sphere. Thereafter,
we can use this "calibrated" lamp as a reference
in an integrating sphere for routine measurements of luminous
flux.
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Intensity
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The candela is the basic unit in photometry. All
other luminous quantities can principally be derived
from it.
The candela is the luminous intensity, in a given
direction, of a source that emits monochromatic radiation
of frequency 540×1012 Hertz and that
has a radiant intensity in that direction of 1/683
Watt per Steradian.
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One steradian (sr) is the solid angle that, having its
vertex in the center of a sphere, segments an area on the
surface of the sphere equal to that of a square with sides
of length equal to the radius of the sphere.
The candela is abbreviated as cd and its symbol is
Iv. The above definition was adopted
by the 16th CGPM (International Committee of Weights and
Measures in Paris) in 1979.
Intensity sources are used to calibrate photometers beyond
the photometric limiting distance (the distance from which
the light source can be considered as approximated point
light source).
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Illuminance and Luminance
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Illuminance is another quantity derived from intensity
which denotes luminous flux density. It has a special
name, lux, and is lumens per square meter, or lm/m2.
The symbol is Ev. Most light
meters measure this quantity, as it is of great importance
in illumination engineering. Some examples for typical
illuminances range from 100,000 lx for direct sunlight,
or 500 lx on a working desk in office to 20-50 lx
for hospital corridors at night and 1 lx for emergency
lighting.
Luminance is analogous to radiance, differentiating
the lumen with respect to both area and direction,
and is measured in cd/m2. The symbol is
Lv. It is most often used
to characterize the "brightness" of flat
emitting or reflecting surfaces.
Luminance is the only photometric quantity that can
be visually seen by human beings (except starlight).
Lighting as well as illuminated surfaces (depending
on their reflectance) have a certain luminance. Examples
for luminance:
- Open window a little cloudy: 5,000-50,000 cd/m2
- Opal incandescent bulb 100 W: 60,000 cd/m2
- White sheet of paper, illuminated 500 lx: 130-150
cd/m2
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Fig. 2: Luminous Quantities
| Type |
Value |
Symbol |
Formula |
Name |
Unit |
| Radiation value |
Luminous flux |
Φ |
Φ = I x Ω |
Lumen |
[lm] |
| Sender-side value |
Luminous intensity |
I |
I = Φ/Ω |
Candela |
[cd] |
| Luminance |
L |
L = I/A |
Candela per square meter |
[cd/m²] |
| Recipient-side value |
Illuminance |
E |
E = Φ/A |
Lux |
[lux] |
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V(λ) Function
In order to have also a well defined photometer, an "artificial
eye" has been constructed to simulate the light sensitivity
of the human eye. The relative response of the normal human
eye to monochromatic light at the different spectral frequencies
was determined experimentally by the CIE and standardized
in 1924. This is known as the photopic luminous efficiency
function. The symbol of this function is V(λ) and
it is usually expressed as a function of the wavelength
of light (in air).
 |
The following procedure was conducted to determine
the photopic luminous efficiency function: First,
light of constant intensity was emitted and its frequency
was varied until the lightness perceived by the observer
was found to be maximal. This occurred at a frequency
of about 540 THz, corresponding to wavelength λm
= 555 nm. The wavelength was then set to another λ
and the power was readjusted until the lightness was
judged to be the same as at λm.
V(λ) could thus be computed as the ratio of
the radiated power at λm and λ,
respectively.
Of course, this experiment has been conducted by
many observers and the resulting average was used
to define the so called CIE standard eye which is
an optical sensor with sensitivity corresponding to
the function V(λ).
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The photopic luminous efficiency function serves as a link
between the subjective response of the human eye and normal
physical measurement techniques. It thus provides the basis
for a group of photometric units.
X-Rite - Optronik's proprietary photopic filters consist
of several elements designed to match the CIE photopic response
curve to achieve an f1 to better than 1.5% at
all wavelengths (f1 < 1.5 % defines the highest
accuracy class L according to DIN 5032 and CIE No. 69).
The sensitivity in the IR and UV range is reduced to a minimum
< 0.1%. The careful design of the detectors ensures best-of-class
equipment and repeatable measurement results, even for monochromatic
radiation sources.
Precision operation amplifiers convert the photocurrent
in nA resulting from the light sensation into a proportional
voltage. The voltage is converted by a precision AD converter
into a signal that is proportional to the expected illuminance
in lux.
Each Optronik photometer is carefully tested and calibrated
in our own calibration laboratories with intensity calibration
sources traceable to National standard (PTB); e.g., a WI41G
calibration bulb operated under stable conditions (25°C
ambient temperature), electrical values with a color temperature
corresponding with CIE standard illuminant A (2856 K).
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Colorimetry
Colorimetry is based on the fact that observers can match
colors with additive mixtures of three reference stimuli
in amounts known as tristimulus values. Using reference
stimuli at specified wavelengths, CIE has defined a standard
set of tristimulus values to match each different wavelength
of the spectrum. These data constitute the CIE 1931 standard
colorimetric observer. The reference-color stimuli are radiations
of wavelength 700 nm for the red stimulus (R), 546.1 nm
for the green stimulus (G) and 435.8 nm for the blue stimulus
(B).
The tristimulus values were chosen to match the typical
white color. There is a great imbalance in the three amounts
(the amount of green being the greatest and the amount of
blue being much smaller). As white is a color that is not
biased towards red, green, or blue, new relative units of
R and B were chosen so that the amounts are equal to the
amount of green.
Series of measurements have been carried out with the standard
colorimetric observer to find the different tristimulus
values for different colors. To make use of the huge resulting
data file, CIE has worked up a specific "map"
of colors. As three stimuli are assigned to each color,
a three-dimensional coordinate system would have been needed
to plot the actual coordinates. To simplify this representation
(at the expense of losing the lightness information), coordinate
transformation and some other calculations have been done,
resulting in a two- dimensional chart called chromaticity
diagram. In spite of this, the suitability of the diagram
for all colorimetric measurements without the need of the
related mathematical apparatus gives the chromaticity diagram
an outstanding importance (see Fig. 3).
Fig. 3: Luminous Color
| Value |
Symbol |
Unit |
| Color temperature |
Tcp |
[K] |
Color rendering index
Color rendering group |
Ra |
[1] |
| Trichromatic values |
X, Y, Z |
[1] |
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