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LED dimming for the light effects you really want

Advanced driver technology for dimming bridges the difference between the way LEDs work, and the way the human eye works.

 

Mark Cremer
Director, eldoLED Global OEM Channel Marketing


www.eldoLED.com

 


 

Dimming, and the user controls such as rotary switches or sliders which regulate it, is a familiar function, and dimming capability has been built into lighting systems for decades. This means it is easy to take it for granted. From a building operator's point of view, dimming capability, once considered a box to be ticked, is a standard functional requirement that's more technically demanding than ever before and requires careful specification      and implementation.

 

Prior to LED lighting, it might have been safe to assume that all dimming schemes functioned in more or less      the same way and fairly consistently. But with modern LED lighting systems, the function is more complex and    it's surprisingly common to achieve poor dimming performance.

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Moreover, to both lighting professionals and end-users, the difference between smooth, intuitive dimming and jerky, unpredictable dimming is instantly apparent. Inconsistency in the color of dimmed light from one luminaire compared to its neighbor is equally unmistakable.


It's critical to understand why we see poor LED dimming applications and how to specify dimming so that it meets expectations across an entire lighting installation.

 

The human eye: a non-linear photometric sensing device

When a lighting designer or a specifier of a new lighting scheme requests dimming capability, they commonly want to achieve one of two effects:

 

  • To create a cozy, snug ambience – perfect for diners on a romantic night out in a restaurant, or perhaps for the living room lighting at home where a family spends the evenings
  • To fill a space with the right amount of light for the activity taking place in it– for instance in a theatre's auditorium, where full brightness is needed before and after a performance, but very dim lighting is required during the performance

 

To achieve either of these effects, there must be correct alignment between technology – that is, the controlled flow of electrical current to a light source – and the human eye's response to changes in light intensity and color. Unfortunately, this alignment is rarely achieved in large-scale LED lighting installations, for two reasons.


First, LED light sources do not behave as the eye does. Cutting the current to an LED by 50% cuts the light output from the LED by 50% - a linear response to a control input.

 

 

Fig. 1: the human eye's response to changes in light intensity in non-linear. 

 

To the human eye, however, this 50% fall in measured light intensity is almost imperceptible. The human eye becomes more sensitive to changes in light intensity the dimmer the light becomes. As Figure 1 shows, reducing the measured light output of an LED luminaire to 10% of full intensity is perceived as around one-third of full brightness. At a measured light output of 1%, the eye fools the brain into thinking that the light is only dimmed to 10%.


Many dimming controls fail to take this non-linear behavior into account, so when a user turns a dial or moves a slider to request half-brightness, the dimmer cuts the power in half – and the user sees no change (see Figure 2). This often gives the user the impression that the dimmer is broken or malfunctioning. Further small turns on the dial at below 20% of full power can produce abrupt changes in brightness, followed by sudden darkness at the lowest dimming level. The user control experience produced by conventional LED dimming is jerky, unintuitive, and unsettling.

 

 

Fig. 2: linear control of an LED array produces almost no perceived effect on brightness between 100% and 50% brightness.

 

The second problem with conventional dimming implementations arises from the widespread inconsistency in the behavior both of individual LED light sources and of LED-based luminaires. So when a lighting designer specifies a 'warm dimming' capability to create that intimate, cozy glow when the lights are dimmed in a restaurant, the degree of warmth can vary substantially from one luminaire to another. This is true not only of the final color temperature at which the lights settle when fully dimmed but also of the dimming path from full brightness down to fully dimmed. Again, different luminaires might change from the initial color to the fully dimmed CCT value at a different rate from neighboring luminaires – a difference which will be easily visible to the user. Differences in operating temperature at the LED account for further color shifts between fixtures.

 

So, what is the solution to the problem? What does good dimming behavior look and feel like? And how can it be achieved across the whole of an installation?
To specify ideal dimming behavior, we have to know which of the two functions of dimming we want – the cozy glow of golden light – Dim-to-Warm, and / or the precise control of light intensity down to very low levels – Dim-to-Dark.

 

Consistent color change across multiple luminaires

Dim-to-Warm functionality provides for accurate control of the correlated color temperature (CCT) of a two-channel dimmed LED light source from full brightness down to minimum power via a smoothly graduated shift from a cooler CCT, typically between 3000K and 4000K to a warmer CCT, often between 2700K and 1800K or lower.


The lighting designer will want to specify that:

 

  • There should be no abrupt change in color temperature as the light source is gradually dimmed
  • he profile of the color temperature change as dimming occurs should be programmable, and the light should maintain accurate color temperature across the entire dimming profile
  • The color temperature should be consistent across every luminaire in the space when dimmed at the same rate

 

This Dim-to-Warm function requires that the system mix light from cooler and warmer LEDs to achieve the desired color at any given intensity. This color tuning function can be implemented with various degrees of success in the LED module, in a luminaire's controller chip, or preferably in a digital LED driver.

 

 

In a typical Dim-to-Warm installation, light output and colour temperature are adjusted together, based on the settings established in LightShape. As the light dims, it warms. These settings can be addressed by virtually any third-party control system.

 

 

Dim-to-Warm for a cozy aesthetic

Superior system-wide performance is achieved when Dim-to-Warm is implemented in a digital driver that includes technology like LightShape which takes the complexity out of creating, programming and distributing intuitive Dim-to-Warm lighting applications across an installation using standard controls and LED modules. It works equally well with DALI, 0-10V, and Bluetooth lighting controls and virtually any dim-to-warm LED.


LightShape technology enables a luminaire manufacturer, lighting designer, or specifier to easily configure the settings with the characteristics of the two LED channels, and then program a Dim-to-Warm profile specifying the shift in CCT across the desired intensity range.


Implementing the Dim-to-Warm function in the driver offers two crucial benefits to the specifier or lighting designer:

 

  • It assures the user of perfect consistency from one luminaire to the next – even across a population of luminaires of different types (spot or linear or wall wash, etc.) and from different manufacturers. The LightShape color profile and control technique will be applied equally across all fixtures. With the right digital LED driver specified, designers have the freedom to specify any luminaire from any manufacturer while maintaining uniformity of Dim-to-Warm behavior across an entire scheme.
  • The Dim-to-Warm function may be programmed into the lighting equipment at the factory, eliminating the need to perform programming and calibration of dimming functionality on-site. This further simplifies installation, minimizes programming errors, and reduces installation cost and time.

 

Dim-to-Dark for smooth, intuitive brightness control

Dim-to-Dark functionality gives precise, smooth control of light intensity from 100% brightness down to 0.1% brightness. The Dim-to-Dark function built into digital LED drivers, like those from eldoLED, alters the brightness in tiny, imperceptible steps for a smooth dimming experience.


The feature's name, Dim-to-Dark, is applied because the final dimming step, when the light is at minimum intensity, is almost totally dark. Most dimming schemes stop at 1% brightness, and as a result, the change from 1% to dark is an abrupt step, because – as Figure 1 shows – 1% brightness feels like 10% to the user. However, by controlling intensity all the way down to 0.1%, users get a smooth and real sensation of darkness.


Dim-to-Dark using a digital LED driver like those from eldoLED also gives intuitive and predictable user control of dimming. The lighting designer or specifier can choose from one of three 'natural dimming' curves: soft-linear, square, and logarithmic (see Figure 3). These dimming curves give the specifier the option to align dimming behavior with the characteristics of the human eye and preferences for the application. By selecting the right dimming curve, a specifier can ensure that physical dimming input, either through a dial, slider, or an application, matches the expected dimming response, e.g., 50% movement to achieve perceived 50% reduction in light output. The control device appears predictable and natural, giving the user the confidence to use the dimming functionality whenever required.

 

 

 

Fig. 3: eldoLED drivers give the specifier a choice of natural dimming curves.

 

As with the Dim-to-Warm functionality, Dim-to-Dark should be implemented in the same way across a digital driver portfolio, and the drivers should be compatible with third-party control technologies so that specifiers or lighting designers can use a single-source of drivers across an entire lighting scheme with a heterogeneous mix of luminaire types and manufacturers, and achieve consistent Dim-to-Dark performance across the entire scheme.

 

 

High-performance dimming for discerning users

Regardless of the scale of the project or the number and type of luminaires being installed, users of the space expect dimming to be smooth and consistent across the space, behave as expected in response to control inputs and to feel right. Digital LED drivers with technologies like those from eldoLED make it possible to achieve the highest dimming performance while retaining the freedom to choose virtually any type of LED fixture or control technology.

 

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