Article: Preventing Dead Travel with Dimming Curves

May 3, 2024 Deborah Perkins

When a person operates a dimming control—typically a slider or rotating knob—they expect the change in the light’s brightness to match the motion of the control. If a slider set at full brightness moves one quarter, the user expects the light to dim to 75% brightness. Move another quarter, and the light should be at half brightness. The dimming should smooth all the way from full brightness to dark.

It seems intuitive that a 25% reduction of the driver current supplied to the LEDs results in 25% dimming, a 50% drop equals 50% dimming and so on. Here’s the thing—that’s not how the human eye works.

The eye’s responses to changes in light intensity is not linear. This said, you need to regulate the dimming system—the combination of the controller, LED driver and LED light source—to provide the perfect user experience and to avoid ‘dead travel’ of the dimming control switch.

With our eldoLED drivers, we meet this requirement through the most comprehensive set of dimming curves that can handle every type of dimming control. Lighting systems based on our drivers provide the user with perfect, smooth, linear dimming and ensure that the user doesn’t experience dead travel. This article explains why dimming curves are necessary for ideal dimming and how we implement dimming curves in eldoLED drivers.

What Is Dead Travel?

Dead travel occurs when there’s no perceived change in brightness when moving the dimming controller from 50% to fully-on.  This is problematic due to two reasons: 1) It appears non-linear 2) It makes the user believe that the system is faulty.

Fig. 1: An example of dead travel. No change in brightness is perceived when moving the dimming control from half-way to fully on.

There are extreme effects to dead travel. For example, users that turned a dimming control from 0% to 50% noticed the light intensified from dark to full brightness. In this case, the dead travel is fully half of the control’s complete range of movement (see Figure 1).

This behavior is undesirable since dimming controls should be intuitive and natural. The user should not have to calculate or learn how to make a light operate at half brightness. Half brightness should be at the half-way point of the control’s travel.  

Why Dead Travel Occurs

Dead Travel occurs when the dimming system fails to adjust for the logarithmic operation of the human eye. Scientific research established a model for the human eye’s logarithmic perception of changes in brightness. This model is codified in the 9th edition of the IESNA Handbook (see Figure 2). This model indicates when measured brightness is reduced to 10% of full brightness, the human eye perceives the light intensity as 32% of full brightness.

Fig. 2: The human eye’s perception of dimming has a logarithmic rather than a linear curve.

This logarithmic relation applies across the entire dimming curve. Move a dimming control for a quarter of its travel from full brightness and the user will barely notice the difference. On the lower end, a measured light output at 0.1% of full brightness results in a perceived light perception of light dimmed to 3%.

The Value Of 0.1% Dimming

This emphasizes the value of deep dimming to 0.1%. Most LED drivers only support down to 1% dimming or 10% of perceived full brightness. Switching fully off from 1% dimmed produces a sudden cut-off. Our unique eldoLED LED driver technology supports deep dimming to 0.1%—which enables us to offer Dim to Dark capability.

What does this mean for the operation of dimming lighting fixtures? Many dimming controls—including all DALI-based controls—supply a linear dimming signal to the fixture. As Figure 2 shows, an implemented linear signal + a linear change in the current to the LEDs + a linear change in measured brightness = a non-linear perception with a high probability of dead travel in 50% and 100% brightness of the dimming control operation.

Logarithmic + Linear Or Linear + Logarithmic Is Ideal

Applying a logarithmic curve in fixtures to the linear signal input of the dimming curve produces a logarithmic change in brightness—and vice-versa—is essential for a linear smooth change in brightness (see Figure 3).

Fig. 3: How to combine a dimming control with the appropriate dimming curve to give the perception of a linear change in brightness.

Figure 3 demonstrates that some sliders and other dimming controls implement a form of logarithmic control. This is most often the case with dimming controls which use the standard 0-10V control protocol or have a LEDcode interface. All of our drivers can support linear or logarithmic dimmers. Some of our eldoLED drivers also support soft linear and square dimming curves.

Logarithmic is preferred when it comes to deep-dimming. The lighting fixture needs to apply a linear response to the control signal when operating alongside a logarithmic slider or knob. This produces a logarithmic change in brightness which will be perceived as a linear change by the user (see Figure 4).

Fig. 4: Applying the correct dimming curve gives perfect dimming with no dead travel.

How Fixture Manufacturers Can Choose The Right Dimming Behavior

The easiest way for a fixture manufacturer to provide the correct dimming behavior for any installation’s choice of dimming control is to use an LED driver with programmable dimming curves. This means that a single unit part number can be used with any dimming control. When the specifier or installer orders the driver, they simply need to establish which kind of output their dimming control provides–linear or logarithmic –and then specify the appropriately programmed LED driver.

Programmable Dimming Curves In eldoLED Drivers

We supply our programmable eldoLED drivers with a range of built-in dimming curves so that specifiers can match a fixture’s dimming behavior with the chosen dimming control to give perfect dimming (see Figure 5).

Fig. 5: Most eldoLED drivers that support the 0-10V or LEDcode control protocols are supplied with a choice of four dimming curves.

All DALI-compatible drivers have linear and logarithmic dimming curves. When used with a DALI-compatible dimming control, the eldoLED driver’s logarithmic curve provides flawless and smooth dimming.

When To Use Soft Linear And Square Dimming Curves

Figure 5 showcases two additional curves: soft-linear, and square. These specific dimming curves are available in some 0-10V and LEDcode-compatible drivers. We provide this option for dimming controls that fail to provide a perfectly linear or a perfectly logarithmic dimming signal—but are somewhere between linear and logarithmic. The soft-linear and square dimming curves ensures perfect dimming with non-ideal dimming controls.

The provision of the soft-linear and square curves allows the specifier or installer to experiment with a test set-up. Through a small amount of trial and error in the commissioning process, the specifier can discover the best match of our eldoLED dimming curve to the dimming control in use.

Our driver’s unique choice of four dimming curves also enables commissioners to adapt a fixture’s operation to take full advantage of the drivers’ deep dimming capability. Gradation between dimming levels at below around 5% dimming with dimming controls that have 50 steps is too coarse in linear dimming mode. What should be a smooth dimming experience becomes a step-wise change at very low brightness.

In this case, a better user experience is possible by combining a logarithmic dimming control with a soft-linear, square or logarithmic dimming curve in the eldoLED driver. This gives fine-grained, smooth control of dimming at low brightness.

How To Build eldoLED Dimming Capability Into A Lighting Fixture

We can program your desired dimming curve into the driver either 1) at the factory by eldoLED or 2) by the fixture manufacturer during assembly of the product. It is not a user-programmable feature.

Tip For Fixture Manufacturers: Provide Your Default Dimming Curve In The Datasheet

For the convenience of the specifier or installer, it is good practice for fixture manufacturers to declare the default dimming curve the fixture implements within the datasheet. If the fixture has a choice of dimming curves, the datasheet should also show how the choice of dimming curve is reflected by the part number or ordering code.

Another Alternative For Dimming Curves: nLight® Digital Control Network

It is also worth noting that there is an alternative to the use of the dimming curves built into eldoLED drivers: the nLight® digital control network operates at system level. Obviating the requirement to manage dimming operations at the component level. In a system using nLight controls, our driver’s dimming curve is overridden and the nLight system produces a perfectly smooth dimming output independent of the driver.

What To Prepare

Here’s our recommendations for lighting designers and original equipment manufacturer (OEM) customers:

·  Specifier or Lighting Designer: Provide a mock-up of your control system and test set-up to ensure dimming behavior performs as expected. Validate that your control and luminaire provide your desired dimming behavior

·  OEM Customers: Clearly state in your product datasheet what dimming curve you’re using to eliminate the question on which dimming curve to use. Give customers the option to use either linear or logarithmic dimming curve—give the user the flexibility to make their own decision.

LED Drivers For Perfect Dimming

Our products in both the eldoLED SOLOdrive and ECOdrive series of single-channel LED drivers support the DALI, 0-10V and LEDcode control interfaces.

SOLOdrive and ECOdrive LED drivers also feature familiar and signature eldoLED LED driver technology for perfect dimming:

· Natural dimming down to 1% (ECOdrive)

· Deep-dimming down to 0.1% for Dim to Dark capability (SOLOdrive)

· Hybrid HydraDrive technology for flicker-safe dimming at all brightness levels (SOLOdrive and ECOdrive)

Perfect dimming in all installations and with all LED light sources is a hallmark of eldoLED drivers. By choosing fixtures with our eldoLED drivers, lighting designers, specifiers and installers can eliminate the curse of dead travel in dimming controls. Providing their users a flawless and intuitive dimming experience.

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