36 JULY/AUGUST 2017 LEDsmagazine.com
ssl design | OPTICAL MATERIALS
Light diffusion level is typically reported
as haze. Haze is defined as the amount of
light that is subject to wide-angle scattering, at angles greater than 2. 5° from normal. Haze measurements are also conducted according to ASTM D1003, and like
L.T measurements, they are also determined by a light transmission meter or
haze meter. The main drawback to the haze
parameter is its low sensitivity. For medi-um-to-high diffusion levels, haze can be a
good indication of the system performance.
However, for highly diffusing systems, the
haze value reaches a maximum, and from
that level, the meter cannot distinguish
between different levels of light diffusion.
A different lab test based on ASTM D1003
is the clarity test. It is a complementary test
to the haze test. The clarity test is also conducted by the light transmission meter, but
the transmitted values are collected only
within the range of 2. 5° from the normal.
This measurement indicates how well small
details can be seen through an optic object.
The diffusion properties in this case are
tested indirectly. A value of low clarity means
that most of the incident light is shifted away
from the normal plane to an angle wider than
2. 5°. An illustration of wide-angle scattering
(low clarity) and narrow-angle scattering
(high clarity) is shown in Fig. 2.
The main advantages of the clarity test
are again the availability of the light transmitted and the simplicity of the test. This
test is also more sensitive in comparison
to haze. But the main drawback is the fact
that it is an indirect test. Clarity shows
how clear the system is (or is not) around
the normal plane, but it does not completely define the object’s light diffusion
Half-value angle test
One of the most proficient methods for characterizing quantitative light-diffusion performance is the half-value angle (HVA) test. The
test is based on measurements of the light
intensity at different angles with respect to
the incident light that is vertical to the object
surface. It is conducted from -90° to +90°.
The final half-value angle is the angle where
the light intensity is half of its intensity at
the 0° measurement. Wider angles indicate
improved light diffusion properties. An example of HVA is shown in Fig. 3.
The HVA measurements are conducted
with a goniometer. With appropriate detectors, the goniometer measures light scattering intensity at different angles. This device
is more of a research and development tool
rather than industrial lab equipment. Goniometers are used to measure angle dependence of various phenomena.
The advantage of the HVA test is its sensitivity, which enables it to distinguish
between systems with high light diffusion levels. This sensitivity is impossible to
obtain with haze measurements. The disadvantage is the fact that only a few laboratories are equipped to conduct this test due to
the high cost of the test equipment.
Light diffusion power
The light diffusion power (LDP) test is also
a proficient testing method for characterizing diffusion properties. In fact, the LDP
FIG. 6. A comparison of four optical samples in a Hot Spot chamber reflect the
complexity of selecting an optimal diffusion system.
FIG. 5. A Hot Spot chamber provides a simple way to observe diffusion and