Often, when dimming, the amount of light output doesn't change much in the first half of the dimmer's range and then drops fast when the dimmer is past halfway – leaving halve of the dimmer's range virtually unused. This white paper explains why this happens and how to use dimming curves to solve it.
An LED lighting system consists of a (wall) dimmer or controller, a driver and a luminaire. To dim the light, you move the slider or turn the wheel and the dimmer sends a signal to the driver. The driver translates that signal to an electrical current that powers the LED.
If the system is not configured properly, dimming performance can be unsatisfactory. In figure 2, the difference in light output between two dimmer settings is quite large at deep dimming levels (left side of the graph). And at the bright end on the right, the difference between two settings is barely noticeable. The cause of this behaviour lies in the combination of dimmer and driver, and in the workings of the human eye.
The result of a much better implementation of a lighting system is shown in figure 3. The light level changes evenly with the dimmer position. For example, when the dimmer's slider is at 25%, you will get 25% perceived brightness.
Lighting And The Human Eye
The human eye does not perceive changes in brightness in a linear fashion. That means that there is a difference between the actual, measured light intensity and the way we perceive it.
In figure 4, the horizontal axis shows light output measured by means of a light meter and the vertical axis shows the light output as we perceive it. At low measured values humans experience the light brighter than it really is. For instance, a measured light intensity of 1%, will be perceived as 10%. And 50% measured light, will have a perceived brightness of about 80%. This means that dimming to 1% will be perceived as dimming to 10%.
Linear And Logarithmic Dimmers
Both dimmers and drivers can be linear or logarithmic. In a linear dimmer, the signal that the dimmer sends to the driver is linear in relation to the position of the dimmer's slider. For instance, if the slider of the dimmer is at 25% of its range, then the signal to the driver also is at 25% – see figure 5A.
In logarithmic dimmers – the signal to the driver changes slower at deeper dimming levels and faster at the brighter end – see figure 5B.
Matching The Driver To The Dimmer
Like dimmers, LED drivers can also be linear or logarithmic. In LED drivers this is called dimming curves.
To achieve perceived linear dimming, the brightness of the LED must be matched to the way our eyes behave. With a linear dimmer, a logarithmic dimming curve should be used in the driver. And with a logarithmic dimmer, a driver with a linear dimming curve should be used.
Selecting Dimming Curves In eldoLED Drivers
By default, eldoLED drivers have a logarithmic dimming curve preselected and show good dimming behaviour with linear dimmers and controllers.
Soft Linear And Square Dimming Curves
In addition to linear and logarithmic dimming curves, eldoLED drivers offer soft linear and square dimming curves for more control over dimming behavior.
To configure an eldoLED driver, download and install eldoLED FluxTool software for PC or Mac, and connect a TOOLbox pro to this PC or Mac and to the driver's LEDcode connectors.
Choosing The Right Dimming Curve
DALI can have one of two standardized ways to behave. Typically, DALI controllers are linear. For these controllers, select a logarithmic dimming curve in the driver. In exceptional cases, DALI controllers are logarithmic and a linear dimming curve should be selected in the driver.
For 0-10V, dimming curves are not standardised. Most 0-10V dimmers are linear and therefore, a logarithmic dimming curve should be used in the driver. A small part of the market uses logarithmic dimmers. In that case, use a linear or soft-linear dimming curve. Download the compatibility sheet for a list of dimmer and controller brands with recommended dimming curves.
DMX is typically used for dynamic lighting applications. For best performance in these applications, a square dimming curve is recommended.