22
24
4
Irradiance”
, known as
“E
ffective Energy
”
or dose, will
directly correlate with the amount of curing that took place.
Since the ILT1700 is capable of integrating, you can obtain
energy readings directly in absolute units, to control
exposures for industrial applications and for photo therapy.
We stock hundreds of glass and thin film filters to match
many different applications. Contact ILT for more
specific information on a custom design for your needs.
Lets get back to the classical Irradiance measurement.
You would have a
“flat”
response detector, such as a
thermopile, and a light source that is restricted to output
within the
“flat”
region of the detector. Some coatings for
thermopiles are
“flat”
from 200 to 60,000 nanometers, so
almost every light source will fall in that
“flat”
region. So
why doesn
’
t everyone use thermopiles? For one thing a
thermopile is limited to a low reading of about 20
microwatts per square centimeter, while quantum detectors,
such as silicon cells, can be used to measure less than 20
picowatts per square centimeter. The difference is a million
to one in sensitivity. For another reason, thermopiles
measure everything including the infrared output from your
hands and body as well as the room heating radiator, and
finally the thermopile measures all regions of the light
source, while you may not be interested in the output from
most of the emitted spectrum. By the way if you do have a
strong optical signal, and want flat response, we offer three
thermopiles for the ILT1700. Call the factory for details.
For the most part, your application will either need more
sensitivity or will require a selected spectral coverage.
After all this talk about spectrum we have not even
covered the units of measurement. Flux in the radiometric
field is normally measured in optical watts. Occasionally
some people use ergs per second, joules/sec., Langley
’
s/
minute, E-Viton
’
s, plus quanta flux in microEinsteins. Due
to the programmable nature of the ILT1700, we can handle
these different units. Our sales staff is very knowledgeable
in helping you with specific conversions for your
application. The most commonly used units are watts.
As mentioned for Illuminance, Irradiance is the
radiometric parameter for flux density measurement.
Therefore we must choose area dimensions to complete the
units. Most people agree on the centimeter, however occasionally
meters are required. When combined together we have watts per
square centimeter (W/cm
2
or W/ m
2
.). The cosine spatial
response is also required for Irradiance measurements. See
“S
PA
TIAL RESPONSE”
, section 8.7, later on in this
document.
In addition, the reference distance is defined by the first
groove between the diffuser and subsequent optical
elements. Exceptions to that would be for applications that
use a flat opal, diffuse white plastic, or flat teflon diffuser, in
which case the first surface is the reference distance. For
some applications that either sense the light directly on the
surface of the cell, or on the cell behind a transmissive filter,
the reference distance is slightly in front of the cell surface.
Each piece of glass that is in front of the actual sensitive
surface, moves the reference slightly forward. It requires
some sophisticated measurements to accurately define the
effective reference plane for each particular combination of
elements. In most cases it is not necessary to calculate the
reference this closely , but if the source is less than 100
millimeters, you should precisely define that plane or use a
detector that establishes a well defined reference plane .
Consult the factory for help in this endeavor.
For both Illuminance and Irradiance measurements, it is
often important to baffle the measurement environment. For
example, an open lamp on an optical bench will radiate in
all directions. Anyone moving near this lamp, becomes a
secondary reflecting source of optical radiation which is
sure to change the reading. Baffles and black satin cloth
curtains, are very helpful in isolating the experimental area.
A hole down the optical axis, should have a sharp edge to
avoid reflections from the edge itself. Also square holes are
better than round holes, since the edge reflections are less
likely to be propagated down an array of multiple baffles.
Last, but not least, use plenty of flat black paint. If you are
working in the infrared, you might want to get some
“3M
Black Velvet
”
which is known to absorb all the way out to
60 microns.
8.3 Flux Measurements
We started out discussing flux density measurements,
rather than flux alone. Even though this seems backwards,
the measurement of flux has many more geometric
possibilities, so we are addressing this second.
8.3.1 Laser Measurements
You might measure lasers for two reasons. If the final
use of the laser is for humans to see, you probably would
choose photometric units. An example of this is for laser
projection television. On the other hand, the end use may be
a
‘poin
t of sale
’
scanner which is sensed by a
photomultiplie r or silicon cell, so radiometric measurements
are indicated (see below).
8.3.1.1 Radiometric Laser Power - The optical watt is
still the most popular flux measuring unit. This is almost
exclusively true for laser applications. Laser flux is often
easier to contend with since the beam is like a thread of
power, whose position, direction and spectrum are clearly
defined and known. Tunable lasers have caused more
difficulty in the spectrum determination, but simpler spectral
dispersive devices are available to come to that rescue. Here
it will be assumed that the user knows the wavelength of the
laser, or else a flat response detector and filter combination
is used, so the wavelength will make very little error. A
detector should be used that has a defined spatial response
for both angle and for translational sensitivity. An ideal
angle response would be isotropic, which is available by
using a large area silicon cell (SED033, SED100 and
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