Ingineria Iluminatului - Journal of Lighting Engineering - Prof. Florin ...

cylinder. The innermost layer consists of a
metal strip extending the entire length of the
pipe and featuring extraction slots at intervals
that become closer together as the distance with
respect to the electric light source increases, in
order to compensate for light attenuation within
the pipe and ensure uniform extraction. This
extraction strip permits the downward diffusion
of light on the outside of the pipe.
The light rays emitted by the electric light
source are guided along the length of the light
pipe by internal reflection off the surface of the
prismatic optical film (see Figure 3) at certain
incidence angles below a given maximum
angle.
Figure 3 Prismatic film placed inside a light pipe
to pipe the light.
This maximum angle is given as a function
of the refractive index (n) of the prismatic
optical film material. In the most commonly
encountered case where n = 1.5 (polycarbonate),
the maximum angle at which total internal
reflection is ensured is 27.6°.
2 2
1/
2
⎛1
− n ⋅sin
22,
5°
⎞
−1
θ ≤ cos ⎜
⎟ ≤ θ = 27,
6°
⎜
2
cr ⎟
(1)
⎝ 1−
sin 22,
5°
⎠
The light pipe diffuses those rays of light
whose incidence angle exceeds the maximum
angle as a result of their change in direction
caused by the metal strip and its extraction
notches.
3. Characterization of a diffusing prism light
pipe
The light pipe taken both as a whole and
divided into 13 cm long extraction sections, and
the lamps, were photometrically characterized
using several types of measuring instrument (T-
10M luxmeter, LS-100 luminance meter). In
order to be able to perform a comparative study,
two types of lamp were used to inject light into
the light pipe, namely a 70 W HQI lamp (see
Figure 4) fitted with a 6400 lm bulb, and a
prototype lamp with 16 white LEDs (see Figure
5).
Figure 4 HQI 70 W
Halogen lamp
Figure 5 80 W LED
lamp
The characterization served several aims:
a) To produce the lighting distribution isolux
curves (see Figures 6, 7 and 8) as a function of
distance (plotted on the surface of a meshing
placed a certain distance from the lamp or light
pipe). The mesh is represented by a 5 cm x 5 cm
square. As the light pipe was symmetrical with
respect to its environment, we have shown only
half its isolux curves:
Figure 6 Isolux curves of the HQI70W Halogen
lamp (distance 1.2 m)
Figure 7 Isolux curves of the 80 W LED lamp
(distance 1 m)
INGINERIA ILUMINATULUI 15-2005 7