product A: 19°
product B: 30°
-60 -90 30 60 90
Product A with
low diffusion system
and high clarity
Product B with
high diffusion system
and low clarity
quite easy and inexpensive to achieve a system with good diffusion and low light transmission, or vice versa a system with high
light transmission and poor diffusion. It is
much more challenging to achieve a system with both a high light diffusion level
and high light transmission which is as
close as possible to the original source light
level. Therefore, when testing optical performance of the system, both parameters need
to be measured: light transmission and light
diffusion. The optical quality of the system
can be specified only when both parameters
Light transmission measurements.
Of the two parameters, light transmission is easier and more straightforward to
measure. ASTM D-1003 is a standard test
method for haze and luminous transmittance of transparent plastics. This standard
has been adopted by most of the plastics
converters of transparent or semi-trans-parent products.
A light transmission meter or haze meter
is typically used to measure the light trans-
mission of a plastic film or sheet. An exam-
ple of a common laboratory haze meter is
shown in Fig. 1. The total transmittance is
the ratio of the transmitted light to the inci-
dent light. Total transmission or light trans-
mission (L. T) is dependent on the material’s
absorption and reflection properties.
L.T (%) = 100% – Absorption (%) –
L. T values range between 0% for opaque
systems and up to 97–98% for 3-mm glass.
Polycarbonate at the same thickness is typically 88–90% and acrylic is 92–93%. Absorption is a function of the material properties
and sample thickness. Embedded particles
or pigments may also increase the intrinsic value.
The reflection element is also influenced
by particles or pigments embedded in the
article surface, but not less important is
the surface quality. Rough surfaces will
result in high scattering while smooth surfaces will reduce scattering to a minimum.
Therefore, designing an optic cover or lens
with high L. T must take into consideration
as smooth a surface as possible.
A haze meter is primarily used to eval-
uate the semi-finished raw material at the
laboratory stage. Thus, this method is lim-
ited to the product-development stage. It
cannot be used to measure the light trans-
mittance or luminance level of a complete
Light diffusion measurements. When
we refer to light diffusion in optic covers,
we refer to the scattering of the light that
is transmitted through the optic cover. Our
optimal goal is to obtain high light transmission with high scattering, which yields
a “soft” light. To achieve scattering, we
can employ translucent materials such as
treated glass or treated transparent plastics. Such treated materials can be prepared
by incorporating small-scale structures on
the surface, or by embedding pigments or
small particles into the material.
When we want to report light diffusion
properties of an object, it isn’t always simple, as producers and designers use different terminology and measurement methods
to quantify this property. Even on a laboratory scale, measuring light diffusion properties is more complicated than L. T measurements. The main reason is the fact that the
standard laboratory devices, like the haze
meter discussed earlier, provide only partial information regarding the system’s diffusion performance.
FIG. 4. The graph depicts the results of
light-diffusion power measurements of
four different transparent systems: clear
glass (red), poor diffusion system (olive),
medium diffusion (green), and excellent.
Light intensity is measured between
-85° to +85°. The area of each graph is
divided by the integrated area between
-85° to +85°.
FIG. 3. The graph provides an example of the half-value angle (HVA) curves of two
diffusion systems measured by a goniometer. System A (blue) features a sharp
curve at the center, indicating lower diffusion performance and higher clarity.
System B (red) has a shallow curve indicating better diffusion performances and
lower clarity. The HVA is calculated based on the half intensity of the peak intensity
for each system.