C, D

Circadian system

Control of the biological (inner) clock, triggered by dedicated receptors on the retina, and responsible for the human sleep-wake rhythm. The melanopic action spectrum is C(λ). The maximum sensitivity of these receptors is at 480 nm for the blue portion of the visible spectrum. The figure shows a comparison with the luminous efficiency for photopic vision V(λ) and scotopic vision V´(λ).

Colour rendering index

( – general, Ra) Proximity of the seen body colour to its appearance under the respective reference light source. To determine the value of Ra, visually perceived colour variations (special colour rendering indexes) from 8 standardised test colours have to be evaluated, illuminated by the light source to be tested against a reference light source. The general colour rendering index is the average value of the special colour rendering indexes identified using this method. The lower the average variation value, the better the colour rendering characteristic of the tested light source, and the higher its Ra value. A light source with Ra = 100 renders all colours identically to the reference light source. The lower the Ra value, the lower the quality of the colour rendition. For colour temperatures up to 5.000 K the reference light source is a black body radiator (see below). For higher colour temperatures the reference light source is a standardised daylight spectrum at 6.500 K.

Constant light output

(CLO) Electronic control for balancing a lamp’s degradation within its rated service life Lx. Excessive lighting, which occurs in new (uncontrolled) lighting installations is avoided by restricting the electronic power consumption to a level where the produced luminous flux is reduced to its maintained value. With the degradation of the lamp, power consumption is increased, resulting in constant luminous flux over time. Maximum power consumption is reached at the end of the rated service life. Until then, the illuminance level is kept at the maintained value at any point in time. With constant degradation, this results in a relative energy savings potential ΔWrel of:

(with MF = x/100, Maintenance Factor)



Measure for the difference in brightness between an object (object luminance L1) and its surroundings background luminance L2), expressed through the luminance ratio  

Where an object is brighter than its surroundings, that is referred to as positive contrast; where it is darker, it appears in negative contrast (silhouette).

Control gear

The term “control gear” is generally used to refer to operating equipment for a light source. It is a device which is inserted between the power supply circuit and one or more light sources. In the case of LED operation, it provides the required constant current/constant voltage. In the case of discharge lamp operation it mainly serves to limit the lamp current to the required value. The control gear can consist of several components. For example, it can feature a transformer for the supply voltage, correct the power factor and – either individually or in combination with a starting device – create the required conditions for lamp ignition and operation.


DALI stands for “Digital Addressable Lighting Interface”. The interface internationally standardised in IEC 62386 facilitates a high level of compatibility of components utilised in lighting control. Besides the switching function and a standardised characteristic curve for dimming, its addressed operation of up to 16 luminaire groups through a joint control line is especially relevant (see also chapter "DALI").

The DALI logo indicates that the component fully complies with all requirements of the DALI standard.

Degree of transmission

(τ) Ratio of the luminous flux passing through a surface and the luminous flux incident on said surface.