Dimming characteristic curves

The gateway converts KNX brightness values and brightness values configured in the ETS into DALI brightness values. On the DALI page, the brightness commands are transmitted in an 8-bit data value to the operating devices. This data value according to IEC 62386-102 is referred to as "Arc Power Level (APL)" in the DALI specification. The ideally dimmable brightness range on the DALI page is represented in the data value by the decimal values 1...254. The value "0" is interpreted by the operating devices as "OFF". A value "255" means "no change of brightness" ("MASK").

The DALI specification also describes a logarithmic dimming characteristic. This characteristic curve determines how the 8-bit data value is converted by operating devices (e.g. electronic ballast for fluorescent lamps or LED drivers) into an equivalent luminous flux at the physical output via the connected lamp. The luminous flux (physical device "Lumen [lm]") is a measure for the light output emitted by a lamp.
The dimming characteristic curve of the operating devices maps the 8-bit data values transmitted logarithmically on the DALI page onto the luminous flux within the range of 1...254. This range then corresponds to a physical light output of 0.1...100 %.

The logarithmic representation of the dimming values in the characteristic is used to adapt the emitted luminous flux to the subjective brightness sensitivity of the human eye. The human eye already perceives a far greater brightness at a luminous flux of 50 % (the lamp emits half of its light output into the surrounding space). Brightness changes in the upper physically possible dimming range (> 50 % luminous flux) are usually hardly perceived by the eye anymore.
By using a logarithmic characteristic curve, the dimming range of the DALI data value is mapped in large parts onto an area of the relative luminous flux where the human eye can optimally perceive brightness changes.

The actual brightness range that can be dimmed by a DALI operating device does not always have to correspond to the maximum possible DALI dimming range (0.1...100 %). On a DALI operating device, the least adjustable brightness is referred to as the "physical minimum level (PHM)". This lower brightness limit value is defined by the physical properties of the operating device or connected lamp and is normally specified in the manufacturer's datasheet. The lower physical brightness limit value of an assigned operating device is independent of the adjustable minimum brightness. It is quite possible to set a lower minimum brightness (e.g. 0.1 %) in the ETS than an operating device can actually set as a minimum (e.g. 3 %). In such cases, the gateway dims to the configured minimum brightness via the DALI data value. The operating device or lamp then already stops at the physical minimum, however. Ideally, the minimum brightness of a group or single device is configured to the physical minimum of the operating device.

A logarithmic dimming characteristic is standard in DALI operating devices and can normally be found in the delivery state. However, there are some operating devices, in which the characteristic curve can be adjusted - often by manufacturer-specific software tools. As a rule, it is possible to switch between a linear characteristic and a logarithmic characteristic. Such operating devices must always be set to a logarithmic characteristic curve in conjunction with the gateway! Otherwise, the mechanism of the configurable characteristic curves of the gateway will be incorrect!

When dimming, the logarithmic characteristic curve provided for DALI is not ideal for every control task. For this reason, the gateway allows the DALI dimming characteristic curve to be influenced for absolute dimming without having to intervene in the operating devices. The parameter "Characteristic curve in the value range" is available for this purpose on the parameter page "DALI system... -> Configuration groups -> [x] Group name -> Dimming characteristic" or "DALI system... -> Configuration single devices -> [x] Device name -> Dimming characteristic" for each group and single device in the ETS.

The dimming characteristic curve within the time range (relative dimming) is always linear.