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| Additive Color Mixing |
Beyond wavelength conversion technologies such as chip
level, volumetric, or remote conversion technologies, the third way to create
white light using LEDs is by color mixing.
Like an RGB pixel of any standard video monitor, color mixing uses the
principles of additive color to combine two or more colored LEDs to create
white light. By far it is the most
electronically complex method of creating white light, as it depends on some
type of control of the LED RMS current through either Pulse Width Modulation
(PWM) or Amplitude Modulation which dims LEDs of different color combinations
(such as red, green, blue, mint green, amber, even yellow or cool white) at
appropriate intensity levels to achieve white light of a desired color temperature. As with any lighting system, there will
always be advantages and challenges.
Top 5 advantages
of using LED Color Mixing to Create White Light:
1. Color uniformity
Capitalized upon by Cree through their TrueWhite™
technology, LEDs of different color are actively controlled via dimming through
either some type of optical, thermal, or electrical feedback system. This helps increase color uniformity from
fixture to fixture.
2. Easily customizable color temperature
Where chip level or remote conversion techniques require
different chemistries to achieve different CCTs such as 2700K, 3000K or 4100K, LED
color mixing often requires nothing more than a simple firmware change to set
the CCT of the system. This introduces a
higher level of simplicity on the manufacturing side, which helps lower the
overall cost of the system.
3. Advanced controllability during dimming
The PWM controls used in an LED color mixing system enable
the system to be inherently more controllable during dimming in application-
which opens the door for better color performance in a dimmed down state. A common complaint observed with dimmed LED
systems is the even further absence of a red component, which gives dimmed LED
lighting a faint, ghostly appearance, far from the warmer dimmed tone of an
incandescent bulb which is more generally preferred. By designing for increasing levels of red
and/or amber LED light while dimming, the system can more accurately
approximate the performance of an incandescent bulb.
4. Lower LED cost
Since issues such as color uniformity and dimming can be
effectively solved using the control techniques described above, LED binning becomes
less of a concern, which enables the usage of less expensive and larger
bins. This lower LED cost, however, may
be offset by higher costs in electronics and firmware.
5. High system efficacy and CRI
Luminous efficacy and CRI can be significantly increased
using LED color mixing with the use of red and green LEDs. Since the value of a “lumen” is based off the
human eye sensitivity curve and CRI is dependent on the blackbody spectrum of
Tungsten, adding strong green and red components can give a significant boost
to lumen and CRI performance.
As can be seen, LED color mixing in general does enable improved
color controllability and potentially superior CRI and efficacy performance as
well compared to the other two methods we have discussed. Of course, the types of systems mentioned
above do present some challenges. A
color mixing system will invariably require an array of LEDs which can limit
beam control options, and will most certainly require an efficient diffuser to
reduce shadowing on the application surface.
The inclusion of some mixing chamber or mixing distance into any system
using this technology is often desirable as well. Additionally, care must be taken during the
design to account for varying lumen maintenance of different color LEDs. Since Red LEDs often have a much longer lifetime
and shallower lumen maintenance curve than their cooler counterparts, care must
be taken to balance system color performance from a programming perspective as
well as heat sinking to make sure this higher lumen maintenance is accounted
for so the system does not experience a red shift over the course of its life.
Since all three conversion methods
to create white light discussed in this series- chip level (either volumetric
or true chip level), remote, or color mixing- all offer their advantages and
disadvantages, each may lend itself more pertinently to one application over
another. It is ultimately up to the
designer to determine which method to choose whether it is choosing the
appropriate light engine for a given fixture, or choosing the appropriate
fixture for a given application.
