23
SED324) or our INS250 integrating sphere. They will
measure the beam from off axis angles equally. They both
also have a uniform sensitivity across the entrance aperture,
which eliminates translational errors. When high power
sources are used, it is necessary to attenuate the beam.
Attenuation inherently occurs with the integrating sphere.
Due to expense and size, most people choose a detector with
a narrow beam adapter on it, such as the SED033/F/HNK15 .
This is designed to have a cosine spatial response which
accepts an off axis angle of +/- 8 degrees with only a 1%
error, and translational error of +/- 5 millimeters off axis for
another 1% error. It is generally quite easy to keep within
these limits, without any special effort. Of course you
should try to keep the input beam normal to the detector and
on the center line of the detector axis. By the way, distance
should not matter when making laser power measurements,
as long as the beam underfills the detector input receiving
surface. The sensitivity factor that is dialed into the ILT1700
is in Amperes per Watt (A/W) at the laser wavelength.
Generally the wavelength should be specified at the time of
calibration, or picked from a chart of multi-wavelength
calibrations. It is possible to record 10 factors in the
ILT1700 for each of 10 different wavelengths. To make an
accurate reading, you select the correct factor from registers
0 through 9, and the instrument will read directly in optical
watts.
8.3.1.2 Photometric Lumen Flux - Everything applies
to photometric laser measurements as it does for radiometric
measurements, except that the detector must have a photopic
spectral response or a photometric monochromatic
calibration, to match the
‘CI
E standard observer
’
curve. The
photometric flux is measured in lumens. The calibration
will therefore be in units of Amperes per lumen (A/lm). The
same spatial considerations apply as for the radiometric
measurement in the previous section.
8.3.2 Wide Beam Flux Measurements
An integrating sphere is the ideal receiver for wide
beam measurements, especially if the beams are converging
or diverging. Wide beam sources are accurately measured
by catching all the light in the beam. The large opening of
the integrating sphere input port (which is 37.6mm
diameter) makes this an easy task. For diverging beams, it is
necessary to be close enough to insure that the outer edge of
the beam is still smaller than the input port diameter. The
uniform sensitivity of the port makes it possible to measure
the total flux entering the hole. In addition the sphere acts
as an attenuator and provides a uniform signal to the
detector. Large solid angles can be accommodated . In fact
one steradian of flux can be measured by establishing the
point source at 34.7 millimeters distance away from the user
port. This makes it very easy to make beam candela (lumen/
steradian) measurements, since you would be measuring
with a solid angle of one steradian. The receiving cone for
other solid angles can also be measured with a great deal of
accuracy, since the distances are large and uncertainties are
minimized. If the distance to the input port is large with
respect to the port diameter (37.6mm), then the calculation
reduces approximatel y to the area of the input port (11.10
cm
2
.), divided by the distance squared. In other words, if
you were 10 centimeters away from the rim of the port, you
would divide 11.10 by 100 and get the solid angle to be
0.111 sr.
If the sphere is calibrated to read optical watts, it can
still be used to measure irradiance by overfilling the input
port. By dividing the number of watts measures, by the
input area (11.1 cm
2
.), you get the irradiance in watts per
square centimeter. If it is calibrated in lumens, then by
dividing by the input area in square feet (11.95e
-3
), we will
get the number of lumens per square foot which is equal to
foot candles.
8.3.2.1 Radiometric (Total Flux/Watts) - A sensitivity
factor is required for a flat response detector and sphere
combination in units of Amperes per Watt (A/W). If a flat
response is not available, the calibration must be performed
at the wavelength or wave band of interest. The optical
radiation must under-fill the sphere port hole to be totally
quantized in optical power in the beam.
8.3.2.2 Photometric Flux (Lumens) - A sensitivity
factor is required for a photopically responsive detector and
sphere combination in units of Amperes per Lumen (A/lm).
The same spatial conditions apply as in 8.3.2.1 above.
8.3.2.3
Photometric Intensity (mean Spherical
Candle Power) - To properly measure the total flux from a
source, one must
“catch”
all the radiation regardless of the
emission direction. A sphere is the ideal choice for this
application, since you can put the lamp right inside the
sphere. Two of the standard intensity measurements would
be the candela and the watts/steradian for photometric and
radiometric applications respectively. An intensity
measurement is the best indication of total efficiency of a
lamp, since it indicates its ability to convert electrical power
to optical flux.
Isotropic intensity is equivalent to a point source that
radiates equally in all directions. This is not physically very
practical since most lamps require electrodes and a holder to
support the light, which tends to block some of the output
radiation. Many lamps produces a close approximation for
many applications. A lamp with a reflector behind it would
radiate a great deal in one direction, but the same intensity
units are often used so this combination can be compared to
an isotropic radiator. The units of beam intensity for these
applications, are beam candela (or beam candle power). For
this situation, the measurement is best performed with an
illuminance meter. Beam intensity is then calculated by
multiplying the illuminance by the distance in feet squared,
to get this equivalent intensity in one direction. If the output
of the source may be used in all directions, then the Mean
Spherical Candela (MSC) measurement is better indication
of performance. On the other hand, if the output from the
source is used in one direction, then beam intensity
measurement s would be more appropriate. These could be
expressed in lumens per steradian (or watts/steradian) in a
given orientation, or in units of beam candela as previously
mentioned. To make total flux measurements in an
integrating sphere requires either a
“flat”
response or a
photopic response, detector sphere combinatio n for the
radiometric or photometric measurement respectively. The
radiometric calibration would be in units of Amperes per
Watt per Steradian, and the photometric calibration would be
in either Amperes per Lumen per Steradian, or Amperes per
Candela.
.
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