25
8.8.1 Specular Reflectance
All reflectance measurement s require some special
fixtures. In this case you must have a detector holder that
can be swept in an angular arc of 90 degrees about a
rotational point which has a holder for the flat sample
reflector. A stable light source is directed across the top of
this rotational point to some distance (x) which hits the
properly filtered detector. The button called
‘SE
T 100%
’
is
pressed to establish the unattenuated condition for reference.
The reflector is then placed directly over this rotational
point, at an angle of 45 degrees to the source. The detector
is rotated, also about this same point, for an angle of 90
degrees or until a peak output is found by watching the
meter readings. It is important that the distance (x) is still
the same as it was before moving the detector. The sample
surface must be flat in order to reproduce the same beam
divergence as present without any reflector present. There
are many variations to this method, depending upon the
ultimate use of the reflector. Obviously if the reflector is a
curved surface this would not work. It may be necessary to
use a setup similar to the diffuse measurement in an
integrating sphere, or use a goniophotomete r to integrate the
output over the entire divergent reflectance angle. These
methods are beyond the scope of this manual however.
8.8.2 Diffuse Reflectance
In the previous paragraph, we mentioned two techniques
for measuring diffuse reflectance, namely by use of an
integrating sphere or by integrating the total reflectance
using a goniometer. Both methods require very special
equipment, and will be lightly discussed here, since it would
be impossible to do the subject justice in a document of this
nature. In the first case, a collimated light beam projects
through an integrating sphere, out an opposite port on the
other side. A detector, with the desired spectral response, is
placed in the sphere surface orthogonal to this beam, so as to
be blind to either of the other ports. The
‘ZERO’
button is
pressed with the sample port open. Then a white reflectance
standard is placed in the sample port, with a surface such as
barium sulphate, magnesium oxide etc. and the
‘SE
T
100%’
button is pressed. Now the standard is removed, and the
sample is replaced in the sample port. The display on the
ILT1700 then reads the diffuse reflectance relative to the
standard reflectance. If the standard had a reflectance of
98% then you must divide the answer by .98 to get the
absolute sample reflectance. This type of measurement does
not measure the specular component, since it is reflected
back out the beam entrance port and lost. Other angular
geometries are required to include the specular component.
The application dictates what is to be measured.
8.9 Spatial Response
Spatial response refers to the change in responsivity as a
function of translational displacement s (x or y), and with
angular displacements (pitch, yaw, and roll). Or the measure
of a detector’s relative sensiti
vity as a function of incident
wavelength.
8.9.1 Lambertian Response
Lambertian response is in reference to a particular
angular response proportional to the cosine. In other words
the cosine of the angle normal to the face of the detector is
one or 100%. As the angle goes off axis and becomes
parallel to the face of the detector, the reading goes to zero,
as does the cosine of the same angle (90 degrees). At 45
degrees the cosine is 0.707, which means that the detector
should read the rays with 70.7% of the value produced by
the same rays entering normal to the input device.
The reason this spatial response is necessary for
accurate measurements is that it matches the spatial response
of a perfect absorbing surface. Since Irradiance and
Illuminance, are measurements of light falling on a surface,
the cosine is compatible with these measurements. An
analogy of the perfect absorber might be considered as being
a small hole in a piece of sheet metal, placed over a well.
All the light that goes in that hole will be absorbed by the
deep well hole underneath. None will get reflected back up
out of the same hole. If we analyze the effect of a change of
angle, such as the sun moving from high noon to sunset, we
will see that less light can make it into the hole at sunset,
because the effective area of the hole is smaller as you view
it from an oblique angle. This reduction in area is directly
proportional to the cosine of the angle normal to this
surface. On polar plotting paper, the cosine makes a circle,
which is convenient when comparing the ideal response with
that of an actual plot.
8.9.2 Field Baffle
There are times when you should restrict the field of
view to delete oblique angles. In a lab environment, you
may be working with a light source on an optical bench.
The only light of interest is from that source, yet light
bounces off the people in the room and back to the detector,
creating errors in the readings. This means that you are
better off to restrict the field of view if you know there are
no sources to be measured, at the oblique angles. This can
be done with external baffles, or with our accessory hood
(H). Baffles can be made from sheet metal cut to form a
sharp edged hole in the middle. A square hole is actually
better than a round hole, since it is less likely to create
reflections in a multi-baffle array. Also, black velvet is
excellent for dividing off test areas from the rest of the
rooms lighting. If it is necessary to have light travel down a
tube, you can thread the inside of the tube to reduce the wall
reflections.
When making luminance or Radiance measurements, it
is absolutely necessary to restrict the field of view to one
that
‘sees’
only an intended test area of a reflecting surface,
or rear lit surface. Baffles can be used to implement this
kind of measurement without resorting to expensive optics.
8.9.3 Narrow Angle (Luminance/Radiance)
As just mentioned, there is a requirement for a narrow
field of view when making Luminance and Radiance
measurement s (see section 8.5). This can be accomplished
with lenses as in our Radiance barrel (R) accessory. In some
applications it is accomplished by using a telescope where
代理美国International Light辐照计http://www.testeb.com/yiqi/ilt/zhaoduji.html 深圳市格信达科技 电话18823303057 QQ：2104028976
