Физика/1. Теоретическая физика
Vyrezkova A. V.
Irkutsk National Research Technical University, Russia
Luminous flux and photometer
Luminous flux (Φv) is energy per unit time (dQ/dt) that is radiated from a source over
visible wavelengths. More specifically, it is energy radiated over wavelengths
sensitive to the human eye, from about 330 nm to 780 nm. Thus, luminous flux is
a weighted average of the Radiant Flux in the visible spectrum. It is
a weighted average because the human eye does not respond equally to all
visible wavelengths.
The sensitivity of the eye peaks at
555 nm and falls off to approximately 10-4 at 380 and 750 nm. This
constitutes the range of daylight sensitivity, or photopic vision. The eye's
nighttime sensitivity, called scotopic vision, shifts toward the blue end of
the visible, peaking at 507 nm and falling to 10-4 at 340 and 670 nm. This
weighting factor, or luminous efficacy (Vλ), allows for conversion of Radiant Flux to Luminous Flux at any
wavelength. In the photopic region, the peak at 555 nm is assigned a conversion
value of 683 lumens per Watt. The lumen is the unit of luminous flux,
and is defined in terms of the candela, an SI base unit like the meter or
second. 1 lumen is defined to be 1/4π candela, the SI
base unit of Luminous Intensity.
Since the eye does not see all
wavelengths equally well, the efficacy curve is a very important way to
determine the Luminous Flux from a source. The Luminous Flux from a
monochromatic source producing light at a single wavelength is easiest to
determine.
Φv = Φ *Vλ * (683 lm/W)
Determining the Luminous Flux from a
source radiating over a spectrum is more difficult. It is necessary to determine
the Spectral Power Distribution for the particular source. Once that is
done, it is necessary to calculate the Luminous Flux at each wavelength, or at
regular intervals for continuous spectra. Adding up the flux at each wavelength
gives a total flux produced by a source in the visible spectrum.
Some sources are easier to do this
with than others. A standard incandescent lamp produces a continuous spectrum
in the visible, and various intervals must be used to determine the Luminous
Flux. For sources like a mercury vapor lamp, however, it is slightly easier.
Mercury emits light primarily in a line spectrum. It emits radiant flux at
6 primary wavelengths. This makes it easier to determine the Luminous Flux of
this lamp versus the incandescent.
Generally, it is not necessary to
determine the luminous flux for yourself. It is commonly given for a lamp based
on laboratory testing during manufacture. For instance, the Luminous Flux for a
100W incandescent lamp is approximately 1700 lm. We can use this information to
extrapolate to similar lamps. Thus the average luminous efficacy for an
incandescent lamp is about 17 lm/W. We can now use this as an approximation for
similar incandescent sources at various wattages. Often times, the manufacturer
will list 'initial lumens' in its data for a lamp. This is the Luminous Flux
for that lamp. It is listed in this manner because as a lamp ages, its power
distribution shifts slightly and no longer radiates at precisely the
wavelengths it did at the time it was new. However, for all intents and
purposes, 'initial lumens' may be used for Luminous Flux for any necessary
calculation.
The basic purpose of a photometer is to
measure light. Since light comes in all different forms, many different types
of photometers exist. The measurement of
luminous flux from a light source is carried out using special instruments — a
spherical photometers, goniometers or photometric. Photometry has been around for many years,
though photometers have come a long way from their humble beginnings.
The first type of photometer wasn't an
instrument at all. In fact, before photometers were invented measuring light
was only possible by using the naked human eye. As time progressed, actual
photometers came into existence, and today photometers can be used to measure
all kinds of light ranging from fluorescence to light absorption.
The most common use of photometers today is within the
field of digital photography. When used in photography, the main task of
a photometer is to determine the right amount of exposure. Since
light is never even, obtaining an ideal photograph means taking light into
account. Photometers that are built into digital cameras measure the amount of
light within an area in order to produce the best possible photograph. While
photometers are widely used within the field of photography, there are other
uses for photometers as well.
In astronomy,
a photometer is used to measure the amount of light
contained within stars or other celestial points. In addition, some photometers
can accurately measure sunlight as it falls onto the earth. These instruments
can also be used in industrial settings such as the paint industry. For
example, within the paint industry, photometers are often used to determine the
actual color of an area accurately.
Photometers can also be used to measure the intensity
of a flame, and they can be used to measure infrared light. In almost
any instance that light needs to be recorded or measured, aphotometer is
often the instrument of choice. Choosing the right type of instrument for the
task at hand requires a basic understanding of the type of light that is meant
to be measured.
Photometers can measure illuminance, irradiance, light
absorption, scattering, reflection of light, fluorescence, phosphorescence, and
luminescence. Determining the kind of light that is to be measured is the first
step towards selecting the right instrument. To detect light, photometers use
photoresistors, photodiodes, or photomultipliers. In order to accurately measure
light, the light must first pass through a filter. Once the light has been
filtered, it can then be measured in wavelengths.
Some photometers measure light in photons, rather than
measuring light through a constant light stream. The main difference between an
instrument that measures light through photos and one that measures light
through flux is the actual read-out of the light analysis. A photon photometerwill
produce results in photon units.
Reference
list
1. http://www.ntpo.com/physics/
2. https://en.wikipedia.org/wiki/
3. http://electricalschool.info/