LED Flickering: A Devil hides under the Light

January 2015


Invented by American Inventor Nick Holonyak Jr in 1962, Light Emitted Diode (LED), as commonly believed, is a completely new energy saving lighting technology with series of advantages, for example, a much lower energy consumption and a much longer lifespan. However, there are still shortcomings within LED products, while flickering is the most common one (Chen L and Zhan WX, 2014)[i] . Over the last few years, a large number of LED luminaires have been found to exhibit serious visual flickers. Especially for retrofit MR16 or GLS lamps, which can have various types of flickers, due to the poor compatibility of commercial and domestic dimmers. Graph 1 and 2 had illustrated the speed camera test results of two LEDs. It can be seen that the experiment downlight of Graph 1 had displayed a strong 100Hz flicker, which may be from an inadequate DC converter. In comparison, the luminaire of Graph 2 had exhibited no visible flicker. (Hammarb ck, 2013)[ii]

Typically, flickers of LED lights are in a high frequency flickering (>1000Hz). Although it is normally invisible ripple for most people, it can still trigger a series of ocular illness. That is why I call LED flicking a devil that hides under the bright light.

 IMG_3503  IMG_3502
Graph 1 Speed camera photo test for flickering LED downlight Graph 2 Speed camera test for non-flickering LED downlight


According to the research from Nantong University, China (2011), under flickering lights, pupils in human eyes needs to adjust frequently for the visualisation on the macula, this will likely cause eyestrain and myopia(Yu et.al., 2011)[iii].  Even worse, researchers from the State of Ohio University, USA claims that long term expose in low frequency flicker lights can cause severe vision-threatening diseases such as detachments, choroid atrophy, cataracts and glaucoma (Walline et. al., 2011) [iv].Besides, if the flickering is in a frequency as high as several kHz, human eyes cannot be able to adjust fast enough to adapt the variation of irradiance. Thus, the excessing amount of light spectrum from the Led light will damage the retina, and causing photomechanical damage (Chen L and Zhan WX, 2014)[v].

On the other hand, it is indicated by some other researches that the impact of flickering can be varied, as some people are naturally more sensitive to ripple of lights. Even for the same person, the flicker threshold and the critical flicker fusion can also varied in accordance of time of day, mood, stress, hormone levels, etc. (Wilkins,2010)[vi]. Nonetheless, against all the negative effects above, there is no doubt that LED flickering is not acceptable.


Frankly, there is no single solution for LED flickering. Although IEEE PAR1789 had initiated a series of measures for safe flicker levels from LED lights. Unfortunately, these works are still in progress, and some time will be taken for it to formalize into specific standards, and additional time is also needed for these new technologies to be commercialized. Thus, to tame this devil, there is still a long way to go (Hammarb ck, 2013)[vii].

However, as a consumer, it is not necessary to get headache by considering all of the technical issues, as there a simple measure to avoid LED flickering, which is to find out the flickering index of a certain LED luminaire. This data can be found on the website of LED Benchmark®, an independent testing laboratory for LEDs. Graph 3-1 and 3-2 had shown the result of flickering test for two different LED products. It is obvious that luminaire A has a lower flickering than luminaire B, as it is indicated by to the much lower value of flickering index and percentage.

 3-1  3-2
Graph 3-1 flickering test result of Luminaire A Graph 3-2 flickering test result of Luminaire B

(Source: LED Benchmark)


Even though we are not yet able to resolve LED flickering, we can still easily get away from this devil by paying more attention when choosing an LED product. So, next time when you are looking for an LED light, please keep an eye on its flicking data, instead of only looking at the prices.


[i] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3949479/

[ii] Hammarb ck. P (2013), LEDs and Return of the Flickering, Lighting Magazine (October/November 2013), page 36-38

[iii] Yu Y, Chen H, Tuo J, Zhu Y. Effects of flickering light on refraction and changes in eye axial length of C57BL/6 mice. Ophthalmic Res. 2011;46(2):80–87

[iv] Walline JJ, Lindsley K, Vedula SS, Cotter SA, Mutti DO, Twelker JD. Interventions to slow progression of myopia in children. Cochrane Database Syst Rev. 2011;7(12):CD004916

[v] http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3949479/

[vi] Wilkins,A (2010) LED Lighting Flicker and Potential Health Concerns:IEEE Standerd PAR1789 Update. Energy Conversion Congress and Exposition (ECCE),2010 IEEE, pp 171-176

[vii] Hammarb ck. P (2013), LEDs and Return of the Flickering, Lighting Magazine (October/November 2013), page 39

Brief explanation of Clipsal C-bus system and its compatibility with dimmable LED lights

Nowadays, the “C-bus ” control  has been used widely for the HVAC and Lighting controls within commercial and residential buildings. Beside the convenience and effectiveness it brought, C-bus had also become one of the greatest headache for most of the electricians. As a lighting person, I have frequently encountered questions about C-bus from my clients. Thus I think it is it is worthy to share what I know about C-bus, and its compatibility with dimmable LED lights.

What is C-Bus

The “C-bus” system is a micro-processor-based control and management system for commercial and residential building, and it is patterned and owned by Clipsal® Australia.Be ware, it is not a dimming protocol.

This system can be used to control lighting and a wide range of other electrical devices, such as motor, pump and audio visual devices. For lighting, in particular, it can be able to manage from a simple ON/OFF control for a lighting circuit, to as complex as dimming a particular light accompany with sensors[1]. For a better understanding, please refer to chart 1, which is the simple flow chart for a typical C-bus system:

 C-bus flow chat 2

Chart 1: C-bus network flow-chart

(For more information, please refer to the C-bus Training manuals[2]. Special thanks for Ian Booker from Clipsal® technical support).

 Is my dimmable LED compatible with C-bus?

To find out the answer of this question, in my experience, there are two other things that need to be clarified:

1.    How my LED light is be dimmed?

Without doubt, this is the first thing you have to know. In the Australian market, most of the LED lights are power by electronic drivers, which means that they can be dimmed by trailing edge or universal dimmers. While for retrofit LED fixtures (for example some of the LED MR16 light bulbs), leading edge dimmers can be suitable as well. Of course, there are still lots of other different types of dimmable LEDs, you can always find my previous article “How to choose a dimmable LED?” for more information. Anyway, please ask your lighting supplier if you are not sure about it.

2.     Is my C-bus system on a dimmer?

Although it seems like a foolish question, the fact is, the reason of some problems regarding to C-bus dimming, is the system is not even on a dimmer. Regarding to this problem, there are basically two ways to find out if there is a dimmer in the system:

To begin with the most straightforward way, according to Ian Booker from Clipsal® technical support, you can always install a normal halogen lamp onto one of your C-bus output terminal, and try to dim it via your control interface. If the lamp can be dimmed, it means that the system is on a dimmer, and the way to fix the problem is either changing the correct LED light, or to change your C-bus dimmer unit. Otherwise, if the light cannot be dimmed, then it is no doubt that the system is not on a dimmer.

Moreover, there is also a not-that-easy way, which is, check through the PC interface. As a control network, the entire C-bus system is controlled through the network bridge, and it can be programmed and inspected through the PC interface. All you need, is to connect it to a laptop with the “C-bus toolkit” software installed. This software is a free application that can be downloaded from the Clipsal technical support website[3]. Through this software, you can easily find out if a dimmer is installed within the system network. However, it is strongly advised that you should find a licensed technician to do this job for you, at an incorrect programming may exert potential risks.

3.   Is my dimmable LED matches my C-bus dimmer ?

OK, if you have a clear idea of the dimming protocol of your Led light, and sure that you C-bus system is on a dimmer, it is time to figure out if your LED and your system matches with each other. To sort this out, in my experience, you can find out the part number of your C-bus dimmer, and figure out what protocol it is under. Table 2-1 to 2-3 in the Appendix section illustrate the part number and the dimming protocol for all the conventional C-bus dimmers, hope it can be a reference while you checking your C-bus dimmer. They are:

  • Table 2-1 : High power dimmer
  • Table 2-2: Analogue dimmers (Din-Rail and 0-10V )
  • Table 2-3: Digital dimmers (DSI, DALI and DMX)


Frankly speaking, although C-bus system has sometimes given us a great amount of trouble, if you can spend some time on it, it will eventually become an effective tool that make your life more convenient and colourful.


Part number Name Dimming Protocol No. of Channel Current Rating
L5112D10UAR6 L5112D Range 12 channel Infinity Architectural Dimmer All dimming protocol 12 40A 3 phase
L5112D5UAR6 L5112D Range 12 channel Infinity Architectural Dimmer All dimming protocol 12 20A 3 phase
L5106D20UAR6 L5106D Range 6 Channel  Infinity Architectural Dimmers All dimming protocol 6 40 A, 3 Phase
L5106D16UAR6 L5106D Range 6 Channel  Infinity Architectural Dimmers All dimming protocol 6 32 A, 3 Phase
L5106D10UAR3 L5106D Range 6 Channel  Infinity Architectural Dimmers All dimming protocol 6 20 A, 3 Phase
L5106D5UAR3 L5106D Range 6 Channel  Infinity Architectural Dimmers All dimming protocol 6 10 A, 3 Phase
L5103D20UAR1 L5103D Range 3 Channel Infinity Architectural Dimmers All dimming protocol 3 20 A, 3 Phase
L5103D16UAR1 L5103D Range 3 Channel Infinity Architectural Dimmers All dimming protocol 3 16 A, 3 Phase
L5103D10UAR1 L5103D Range 3 Channel Infinity Architectural Dimmers All dimming protocol 3 10 A, 3 Phase
L5103D5UAR1 L5103D Range 3 Channel Infinity Architectural Dimmers All dimming protocol 3 5 A, 3 Phase
L5112D20LP L5112D Range 12 Channel Professional Dimmers All dimming protocol 12 60 A, 3 Phase
L5112D16LP L5112D Range 12 Channel Professional Dimmers All dimming protocol 12 60 A, 3 Phase
L5112D10LP L5112D Range 12 Channel Professional Dimmers All dimming protocol 12 40 A, 3 Phase
L5112D5LP L5112D Range 12 Channel Professional Dimmers All dimming protocol 12 20 A, 3 Phase
L5112D3LP L5112D Range 12 Channel Professional Dimmers All dimming protocol 12 36 A, 1 Phase; 18 A, 2 Phase
L5106D20LP L5106D Range 6 Channel Professional Dimmers All dimming protocol 6 40A 3 phase
L5106D10LP L5106D Range 6 Channel Professional Dimmers All dimming protocol 6 20 A, 3 Phase
L5106D5LP L5106D Range 6 Channel Professional Dimmers All dimming protocol 6 30 A, 1 Phase; 10 A, 3 Phase
L5106D3LP L5106D Range 6 Channel Professional Dimmers All dimming protocol 6 20A , 1 phase
L5103D20LP L5103D Range 3 Channel Professional Dimmers All dimming protocol 3 20 A, 3 Phase
L5103D10LP L5103D Range 3 Channel Professional Dimmers All dimming protocol 3 30 A, 1 Phase ; 10 A, 3 Phase
L5103D5LP L5103D Range 3 Channel Professional Dimmers All dimming protocol 3 15 A, 1 Phase

Table 2-1Clipsal C-bus high power dimmers

Part number Name Dimming Protocal No. of Channel Current Rating
L5504D2U 4 Channel Universal Dimmer with Power Supply Universal 4 2A
L5504D2UP 4 Channel Universal Dimmer Universal 4 2A
L5504D2A 4 Channel Dimmer with Power Supply Leading edge 4 2A
L5504D2AP 4 Channel Dimmer  Leading edge 4 2A
L5508D1A 8 Channel Dimmer with Power Supply Leading edge 8 1A
L5508D1AP 8 Channel Dimmer Leading edge 8 1A
L5504AMP 0-10V Analogue Output Unit  0-10V 4 25mA

Table 2-2 Clipsal C-bus Analogue dimmers


Part number Name Dimming Protocal No. of Channel Current Rating
 L5508DSI 8 Channel DSI Gateway with Power Supply DSI 8 200mA
L5508DSIP 8 Channel DSI Gateway DSI 8 18mA
5502DAL 2 Channel DALI Gateway for C-Bus DALI 2 32mA
5500DMX C-Bus DMX Gateway DMX 1 for 40 units 50mA

Table 2-3 Clipsal C-bus Digital dimmers

[1] http://www2.clipsal.com/cis/technical/product_groups/cbus

[2]  http://www2.clipsal.com/cis/technical/technical_support/training2/cbus_control_and_management_system/basic

[3] http://www2.clipsal.com/cis/technical/downloads/c-bus_toolkit

How to Choose a Dimmable LED ?

The rising cost of electricity is certainly driving consumers, retailers and business owners to seek for energy-efficient lighting technologies, and dimmable LED is considered as one of the most efficient choices. However, as people may know, there are a huge amount of different dimmable LEDs in the Australian market, and the dimming methodology for all these products are varied. When facing this massive amount of options, consumers are frequently encounter such question: Which dimmable LED should I choose?

How LED lights be dimmed?

Before answering this question, it is necessary to clarify that most of LED lamps are powered by electronic drivers, as they are more suitable for DC device. In general, there are mainly 4 protocols that are using in Australia:

1.     Phase-cut dimming

This method dims the output illuminance through cutting the phase of the AC current, which will decrease the power input to the light. There are mainly four types of phase-cut dimming methods:

a.    Leading Edge dimming:

This can be alternatively called as “TRIAC dimming” or “incandescent dimming”. It turns off the sinusoidal signal of the electrical current at the front of the AC input. It fits the traditional incandescent lamps or magnetic low voltage transformer. Nevertheless, it can cause buzzing noise when dimmed, and when it is used for an LED lamp, it can produce a high inrush current to its electronic component, and eventually the entire luminaire will be damaged. [1]

b.    Trailing Edge

This is also called “electronic dimming”, which can be considered as a reverse form of leading edge dimming, as it turns of the end of the AC input waveform. It can be used for electronic driver and low voltage transformers. In comparison to leading edge dimming, it will not exert high inrush current, while not causing buzzing sound when dimmed.[2]

Summary for phase-cut dimming

After comparing these two Phase-cut dimming methodologies, it is clear that for most LED lamps, trailing edge dimming is a better option comparing to leading edge dimming. However, because LED retrofits are mostly for the existing incandescent or halogen fixtures, which makes many manufacturers designed their LED lamps to be suitable for leading edge dimmer. So when you choosing a dimmable LED, it is better to clarify the capable dimming method for the product before getting a dimmer for it, or you can use a “Universal” dimmer, as it is suitable for both leading edge and trailing edge dimming.

2.     0-10V analogue:

As one of the earliest and simplest lighting control, 0-10V analogue dimming is used for early fluorescent dimming and still being used today. Nowadays, some of the LED fixture is also designed for this dimming methodology due to its simplicity. This 0-10V dimmer is connected to the circuit in parallel, and dims LED simply by adjusting the input to the lamp via scaling the DC voltage at the dimmer side. Theoretically, the LED will have 100% output when the dimmer is scaled at 10V; while the dimmer is at 0V, the LED will have 0% output, or be turned “OFF”. However, in many cases, the light can only be dimmed down to 10% due to the limitation of the power supply, and a switch or relay is needed to completely turn the light off. [3] Moreover, for a 0-10V dimming system, each control channel requires one wire, and the excessive amount of cabling can cause a considerable voltage drop that lowered the energy efficiency. [4]

3.     Pulse Wave Modulation (PWM) Dimming

In a technical perspective, PWM dimming can be categorised as a type of Digital dimming. Instead of adjusting the DC current, PWM dimming achieve the same effect by varying the duty cycle of the constant current. It switches the input current of the LED lamp at a high frequency between 0A and the rated current, and this modification of on-off rate of LED will result as the change of output percentage. This suitable for LEDs that need to maintain colour consistency when dimmed down to lower than 40%, and lighting applications with mixed colour control. Nevertheless, in order to be capable with a PWM dimmer, the power supply should be able to run the LED at a high enough frequency, and this will greatly increase the technical requirement of an LED driver[5].

4.     Digital Dimmings

(1)  DSI and DALI Control

The reason that I put these two dimming protocol together is that they are both developed under the working principle of 0-10V analogue lighting control.

DSI( “Digital Series Interface”) dimming technology is uniquely owned by Tridonic®, and it has been used for the control of LED interface or electronic Fluorescent ballast. This is also the precursor to DALI dimming system. [6]

On the other hand, although DALI is developed based on DSI protocol, it is a non-propertied technology, so it is now the most common type of commercial lighting control system, which is available from many different suppliers. Each DALI device has a “non-violate” memory, which contains its own settings. This allows DALI to operate without a central control point. [7]

(2)  Switch dimming

This is a stand-along digital protocol. It is a cheap solution that is commonly used for residential and hospitality systems. No control system is required for this type of dimmer.

(3)  DMX Dimmer

DMX (or “DMX512”) is the digital control system that is designed for RGB and RGBW colour change lighting system.Different from other digital systems, it allows complicated programming to communicate DMX devices through a ‘Data Bus’. It sends signal in an 8/16bit code package, which can instruct fixtures on everything from dimming to movement positions. All this feature makes it an ideal option for stage lighting and effect industry[8].


Although there are lots of people asking questions about “C-bus dimming”, however it is not a dimming protocol. This is actually a building control system that is owned by Clipsal® Australia. Due to its complexity, I will talk about it in another separate article.


To sum up, the conventional dimming method can be summarized as table 1below. Hope this can give you a bit of idea when you choosing a dimmable LED.

Category DimmingProtocol Control system needed? (Y/N) Capability
Analogue Leading Edge N Incandescent Lamp; Fluorescent lamp with Iron-core ballast; LED with Leading edge dimming driver
Traling Edge N Halogen lamp;Fluorescent lamp with electronic Ballast;  Dimmable LED with electronic driver
0-10V dimming N halogen lamp;Fluorescent lamp with 0-10V Ballast;  Dimmable LED with 0-10V driver
Digital PWM Dimming N LED driver with high enough output frequency
Switch dimming N most of the single fixture dimming
DSI Y LED or other device with dimmable Drivers with DSI or 0-10V functions
DALI Y Most of major 0-10V ballast. LED or other device with dimmable Drivers with  DALI ,DSI or 0-10V functions
DMX Y RGB and RGBW LED control; Stage and effect lighting
 Table 1 summary for dimming methods
p.s.: One of the discussion of the C-bus community forum has been cited for the information above: 


[1] http://www.leapfroglighting.com/trailing-edge-or-leading-edge-led-dimming/

[2] http://www.leapfroglighting.com/trailing-edge-or-leading-edge-led-dimming/

[3] http://www.lightology.com/index.php?module=tools_faq_0_10v_control

[4] http://www.epanorama.net/documents/lights/0to10v.html

[5] http://www.lutron.com/TechnicalDocumentLibrary/048360a_PWM_vs_CCR_LED_App_Note.pdf

[6] http://lightproject.com.au/the-basics-dimming

[7] http://lightproject.com.au/the-basics-dimming

[8] http://lightproject.com.au/the-basics-dimming