24
8.4 Health Hazard Measurements
Applications
that
measures
the
effect
of
light
on
human
beings
(or
a
chemical
process)
requires readings
in
effective
dose.
Light
at
different
wavelengths
must
be
weighted
proportional
to
the
effect
that
each
wavelength
has
on
the
tissue.
The
ACGIH
(American
Conference
of
Government
and
Industrial
Hygienists)
has
specifically
defined
several
hazard
bands
which
are
recommended
by
NIOSH
(National
Institute
of
Occupational
Safety
and
Health),
namely,
the
UV
Actinic,
Blue
Hazard,
and
IR
bands.
The
ACGIH
Actinic
curve
exactly
weights
the
hazardous
effect
of
doses
at
each
wavelength
in
the
UVC,
UVB,
and
UVA
bands.
Using
our
ACT5
filter
to
precisely
match
this
function,
you
can
read
a
direct
Threshold
Limit
Value
effective
dose
measurement
of
the
effective
hazardous
dosage.
Remember,
of
course,
to
place
the
detector
at
the
same
nominal
reference
distance
from
the
source
as
the
typical
human
subject
would
be
located.
For
more
information
on
the
Actinic
Hazard
function,
seeACGIH
website:
www.ACGIH.org
8.5 Radiance / Luminance Measurements
Our eyes interpret image details over a relatively narrow
angle. This is the zone of the fovea where we analyze an
image. For this reason it has become very important to
measure light in a similar fashion to relate to the visual
effect. This photometric concept is called Luminance or
brightness, and the radiometric equivalen t is called
Radiance. For Illuminance or Irradiance discussed in 8.2, the
magnitude of the measurement will drop off inversely
proportional to the square of the distance from a point
source to the detector. This is true because the light is being
spread out in two dimensions (area), as one backs away from
a source, hence the square function. In making Luminance
or Radiance measurements, we are determining the output
from a surface, as a function of flux per solid angle per area.
In other words, we are summing up the output from an
infinite number of Lambertian emitters over some test
surface area. We measure this by looking at the area with a
very narrow acceptance angle to intercept a small area inside the
uniform sample emitting surface. Changes in the distance do not
change the reading, since the area being measured increases
directly proportional to the square of the distance, which is in
direct opposition to the inverse square attenuation as a function
of distance. In other words the two functions cancel to give us a
constant reading. This is why luminance is a constant value for
a surface, no matter where it is measured. The units for
quantizing Luminance are, cd/m2, lumen s per steradian per
square meter (nit) or foot-lambert (fL), (1 fL = 3.43 nits) and
for Radiance they are watts per steradian per square meter.
To get the proper acceptance angle we offer
the “PIN”
probe or the Radiance Barrel
“
R
”
which has a 1.5 degree
field of view, or you must restrict the field of view with a
baffled tube. The baffles are required to remove the
reflections from the wall of the tube, and allow
measurement of only the
“lin
e of sight
”
rays . Be careful
that the detector is
“looking”
at an area, located in the
uniform part of the test surface. If you back away from the
surface too far, the input angle will eventually be bigger than
the test area, and errors will occur.
8.6 LED Measurements
Since most L.E.D. applications involve visibility by
humans, it is often better to measure the photometric
intensity in millicandelas, which more nearly relates to the
ability to be seen by a human observer. We have an
accessory called
‘LED’
which is specifically designed for
this application. It permits the measurement of beam
intensity on the optical axis of the L.E.D. source, which is
where most of the radiation is concentrated. It provides a
holder, custom designed, baffled tube, photometric filter and
calibration to read directly in millicandela s with an ILT1700.
We also offer the New SED 324/YK113 Photometric detector
to conform to CIE 127 standards which include condition A
and condition B LED measurements. Includes SAR scanned
calibration on a CD. Consult the factory for more
information.
8.7 Transmission Measurements
The ILT1700 has been designed to perform the
multiplying and dividing necessary to read in percent
transmission. By pressing the
‘SE
T 100%
’
button, the
present magnitude is normalized to read 100%. Any change
in this reading will show the relative percentage to that of
the original number. In addition to the ILT1700 and an
appropriate detector, filter, diffuser combination, you will
need a light source that is stable over the time interval used
for the measurements. Keep in mind that a 1% change in the
lamp current often produces a 3% change in the light output.
Regulation is therefore important. Also be sure to let your
lamp warm up before making measurements. Another
important requirement is an aperture and baffles. The
aperture is necessary to define the central optical area of the
sample filter. An exception to this rule occurs if using a
narrow beam from a source such as a laser. The beam
defines its own aperture. The next step is to select an optical
bandwidth that is of interest. The light source or the
receiving detector is filtered to this desired region. Now you
are ready to make everything physically stable for the
measurements. The full scale reading (100%) is taken
through the limiting aperture before the sample is placed
behind this aperture. Then the sample is inserted and the
attenuation is directly indicated in percent as a digital
readout on the ILT1700.
8.8 Reflectance Measurements
Reflectance is similar to transmission (see 8.7), with a
few more complications. The ultimate use determines if it is
important to measure specular reflectance, diffuse
reflectance, or both. Most objects around the room are
diffuse reflectors, or close approximations. So if the result
relates to how well a human can see something, then diffuse
would be appropriate. A mirror is a specular reflector
designed to bounce the light at an angle that is equal, around
the normal to the surface. Many surfaces (such as a coated
paper), have a specular component as well as a diffuse
component.
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