Signs of the Times - June 2015 - 38

LED Update

ranges, adjusted for color gamut,
and balanced for viewing comfort
while maintaining color fidelity.
Contrastingly, low-quality LED
modules could cause dim lighting
with undesirable color variations.
Similar adverse effects may arise
due to inadequate electroniccontrol systems.
LED shortcomings, compared to
other applications, are more easily
solved in display and signage products, because both makers and
buyers better understand the brightness, glare and color parameters.
The signage and display industry is
also more mature and accustomed
to LED light measurements for
required parameters; it can also
implement electronic controls that
handle various, ambient-lighting
conditions.
Defining LED light
The general-lighting industry still
struggles to define and understand
LED lamps' glare. Essentially, glare
is related to luminance, which
translates to brightness in layman's
terms. Further, the lighting industry
hasn't quantified brightness properly because its leaders aren't
acquainted with the complications.
The problem is magnified for solidstate lighting because many lighting designers and engineers aren't
willing to measure LED luminance
because of inherent difficulties in
defining and measuring it.
LED lamps' luminance lacks
uniformity because the lamps are
constructed with discrete LED
chips or modules with gaps between them. Typically, lightemitting elements with periodic
gaps create a non-uniform luminance distribution at the lamp
surface that's difficult to measure.
The more recent, chip-on-board
(COB) LED technology permits
reduced gaps between LED chips,
but the luminance from these COB
constructs is extremely high and
directional. In addition, COB thermal
management is significantly more
difficult to accomplish.
In the lighting field, glare is
38 SIGNS OF THE TIMES June 2015

only empirically defined to some
degree; however, there is no consensus and, globally, the industry
hasn't set glare standards. Therefore, many LED lamps, including
car and truck lights, expectedly emit
excessive glare. As a correction,
some lamp units use thick, translucent covers to reduce glare, but
such practices decrease luminous
efficacy and, you've surely noticed,
the light doesn't have a uniform
distribution over any broad angular
range.
Light quality
In contrast to LEDs, incandescent,
fluorescent and neon lamps are
uniformly omnidirectional. Such
uniformity in the lamps' illumination provides a soothing appearance for the human eye. In addition,
such lamps and luminaire systems
uniformly span over broad angles to
produce natural object illumination.
Although rarely discussed by
consumers, acceptable light quality
requires uniform-light distribution
over broad angular spans, minimum
or no glare, dependability and
acceptable color properties for the
intended use.
Today's LED lamps lack such
quality characteristics. Dependability remains an issue because
many LED lamps are distributed
by manufacturers that lack the
proper, solid-state-lighting development platform required for producing
high-quality LED chips.
Today, achieving basic light
qualities from solid-state lighting
devices requires the manufacturer
to have a good understanding of
light and lighting science. Despite
remarkable LED technology improvements and product developments, achieving quality, LED
lamps suitable for general, humancentric illumination is conspicuously
difficult. To get past any barriers,
profit-seeking manufacturers unashamedly offer energy-efficiency
("green") marketing pitches, and
add various functionalities to overshadow the basic, lighting-quality
issue.

The most human-centric lighting
characteristics are lack of glare, a
good color-rendering index (CRI),
correlated-color temperature (CCT)
ratings and broad, light distribution in a homogeneous fashion,
so that surrounding objects look
natural. Can such functionalities
as variable CCT, dimming and
remote controls outweigh these
basic light qualities?
Not really.
Orthogonal rays
Inorganic LEDs are glarey compares to why the sun naturally
emits orthogonal rays from its
circular boundary (Fig. 1). Some
diffusion around the sun comes
from the aperture affect. But the
long and strong rays rising orthogonally to the sun's surface are
due to the Divergence Theorem,
which says energy contained in
an object with bounded volume
is released from its surface in the
direction orthogonal to the object's
surface.
How does this relate? Inorganic
LEDs are very flat and thin, and
the light energy is released orthogonally to the LED chip's surface,
which produces concentrated and
directional light with high-glare
characteristics. Light emitted from
such an LED source only has one
dominant direction - orthogonal
to the chip's flat surface.
To produce more human-centric
LED lamps for use in signage and
space-illumination applications,
the light-emitting surface must
be curved so the light emission is
less direct and glary. It needs to be
more broadly distributed. Manufacturers could accomplish this by
appropriately designing secondary,
optical elements that follow the
LED chips into a luminaire ensemble. An entire LED luminaire that
incorporates such a design, and is
also optimized for thermal management and other aspects, could
provide a complete, high-quality
and dependable light source. Nextgeneration LED lamps are expected
to bear these features. n

Signs of the Times - June 2015

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