Commercial Lighting Basics: Dimming with LEDs

The moment you enter a restaurant you notice how it is lit.  Bright lighting sets a more casual atmosphere and dimmed lighting sets a much more intimate mood. Restaurants and hotels offer great examples of the aesthetic qualities of dimming.  Dimming also is an important aesthetic feature in commercial offices, hospitals and educational facilities.

But aesthetics are not the only role dimming plays. The ability to control light with dimming is an essential feature in energy savings strategies.  We know that LED luminaires are energy efficient by design, but dimming offers additional energy savings at about a 1:1 ratio. Consequently, if you dim an LED luminaire to 50%, it will only need to use about 50% of the energy it would normally need to illuminate a space at 100%.  Additionally, dimming an LED luminaire will allow you to possibly extend its already long lifespan even further. Moreover, a dimmed luminaire creates less heat, so it runs cooler and thus less stress on the electronic components and driver enabling a longer life for those as well.

Couple these energy conservation benefits with the ability to achieve desired lighting effects and it is clear why dimming is such a critical component in commercial lighting systems.

Dimming Defined

Dimming refers to a reduction in lumen output and is typically measured as a percentage of the full lumen output capability of the fixture. For example, 10% dimming refers to 10% of the fixture’s full light capability.  It is an important feature of lighting control strategies such as task tuning, time scheduling, occupancy sensing, personal control and daylight harvesting.  Dimming drives the energy conservation and personalization of lighting for individuals.

Dimming has been successfully used for decades in incandescent lamps.  When LEDs first came to market, many thought the same wall dimmers that dimmed incandescent lights would work with LEDs. However, LEDs need a direct current (DC) rather than an AC power supply thus requiring a driver. Additionally, LEDs, drivers and dimmers must all be compatible to work.    

LED Dimming Techniques

There are a few common methods used by commercial LED drivers to dim LEDs: constant current reduction (CCR), pulse-width modulation (PWM), amplitude modulation (AM) and a hybrid approach.

• Constant current reduction (CCR) dimming, also known as analog dimming, is a somewhat simple method of controlling the amount of current supplied to the LED(s). Full LED brightness occurs when the maximum amount of allotted current is provided to the LED(s), and dimming occurs when the current is reduced continuously. The amount of current flowing through the LED is proportional to the light output.  CCR dimming can have issues with dimmable LED lamps at very low current or deep dimming levels.   
• Pulse Width Modulation (PWM) works by rapidly turning ON and OFF the LED(s).  As described in Power Electronics News, the pulse rate (approximately 200Hz, or greater) must be high enough as to prevent the eye from seeing the LEDs’ changes/flickers, and instead only observe the average LED light intensity. PWM works by using the exact amount of electrical current the LED requires. The process quickly switches between that current amount and zero. So either the LED is running on its required amount of power, or it's off. Because PWM has a wide dimming range and linear relationship between light output and duty cycle, it is used more broadly. However, PWM requires sophisticated driver electronics. 
• Amplitude modulation (AM) is a slow decrease of the drive current to attain a reduction in light output. AM is not as simple of efficient as PWM but the chance of visible flicker is evaded.
•  Hybrid approach – A more sophisticated method of dimming is where the attributes of both PWM and Amplitude Dimming are utilized. This approach has a low risk of flicker across the dimming range. PWM is used when lower light levels are needed and amplitude dimming is used at higher levels of light output.

Dimming Range

The dimming range required for an LED is directly related to the application.  For example, in an office lobby it might not be necessary for a product to dim to 5%.  However, a product that dims to only 25% would not be appropriate in a media room.  

The dimming range of a fixture is based on the driver.  The driver determines the achievable dimming range and the best possible performance of the lamp or fixture.

Flicker Happens

Like all electrical lighting sources, flickering can be a challenge for LED lighting.  Flickering (visible and invisible) can occur when the frequency of the light output from the source changes intensity rapidly. When a power supply converts mains electricity from AC to DC, and the current is then sent to the LED, it can cause the fixture to flicker because of high ripple in the output current.

IEEE PAR1789 cites that biological effects occurring from visible flicker include epileptic seizures as well as less specific neurological symptoms such as headaches and malaise. In addition, health effects have been reported as well with exposure to invisible flicker, including headaches and eye-strain.  Flicker also negatively impacts video and photographs creating color shifting and underexposed images.

A quality driver can mitigate these issues. In contrast, a poorly designed driver may deliver fluctuating current to the LED and not pure DC current, and this can result in flickering. 

LED Drivers

The LED driver is the primary component in the fixture that determines the dimming capabilities of the fixture and significantly affects the performance, reliability and lifetime of a luminaire. Unless it is a quality fixture leveraging a quality LED driver, the overall performance in light output and reliability could drop considerably resulting in an unpleasant experience or safety concern for the occupant.

Quality drivers can produce dimming levels as low as 1% without negatively impacting measured light output. On the other hand, low-quality LED drivers simply convert current from AC to DC, and can produce a poor quality of light resulting in flickering and stroboscopic effects.

Quality LED drivers can produce a steady state of measured light output across the entire lifetime of the fixture.  We covered the importance of quality LED drivers such as OPTOTRONIC LED drivers in a previous blog post.