methodology is similar to the HVA method. The main difference
is that in the LDP test, the sample is illuminated at an incidence
angle of 45°, not 90°. Like the HVA, the radiant intensity on the back
side is recorded by a goniometer or another measuring device. The
numerical or graphical data is integrated between -85° and +85°
and divided by the total area between those angles. An example of
LDP graphical data is shown in Fig. 4. At LDP values approaching
1, higher light diffusion power is indicated.
Among manufacturers of sheets, profiles, and lenses, the most
common and practiced test is the Hot Spot test. This is a quality test
that does not rely on any formal standard but is the most straightforward test that directly indicates the diffusion system performance. The test is conducted in a simple chamber with an array or
strips of LEDs located on the bottom of the chamber, as shown in
Fig. 5. The tested optical object is placed on the top of the chamber and then the samples are observed from the side opposite the
light source in order to determine to what extent the LED hot spots
Simple chambers are static, i.e., the distance between the light
source and the object is fixed. More advanced chambers allow for
changing the distance between the light source and the tested
object. In this way, the diffusion properties can be quantified by
measuring the minimum distance at which the hot spots are hidden by the object. The ruler on the chamber side indicates the height
of the LED strips. As the LED strips are moved closer to the tested
objects, it is more difficult to hide the hot spots. Different diffusion
systems can be compared visually simultaneously.
Fig. 6 shows a composite of four optic samples illuminated in
a Hot Spot test. Sample C best hides the LED point sources. But
when considering the tradeoff between diffusion properties and
light transmission, developers may select sample B due to the higher
Summary of Part 1
Communicating design criteria for light diffusion properties is not
a simple matter. Manufacturers, designers, and customers must
pay attention to the tradeoff between diffusion properties and light
transmission level. Considering only diffusion performance by
itself is meaningless since the material must also transmit light.
Among various methods to measure diffusion properties, the half-value angle is the most professional approach. On other hand, the
Hot Spot chamber is the most practiced method, if the manufacturers and designers know and have the benchmark that they are
trying to achieve. Selecting the optimal light diffusion system by
means of the right additives package will be discussed in depth in
a future article.
Considering only diffusion
performance by itself is
meaningless since the material
must also transmit light.