Direct glare

The degree of direct psychological glare by luminaires in an indoor lighting installation can be determined using a formula:

Where...

• UGR
  is the glare value according to the uniform international method used to describe (direct) psychological glare,

• L_b
  is the background luminance in cd/m², calculated as E_ind/π with E_ind as indirect vertical illuminance incident on the observer’s eye,

• L
  is the average luminance in cd/m² of the light-emitting surface of each luminaire in the direction of the observer’s eye,

• Ω
  is the solid angle in steradian (sr) of the light-emitting surface of each luminaire in relation to the observer’s eye, and

• ρ
  is the position index according to Guth for each luminaire in relation to its spatial deviation from the main line of vision.

The UGR procedure (unified glare rating) is based on various similar glare rating methods. These, in turn, are based on studies in which photometric values of glaring light sources and their surroundings were systematically modified and their results on the glare assessment of observers were documented on a glare scale. The glare scale consists of seven degrees of glare:

The results of these studies are combined in the following glare formula which yields a UGR value. The procedure is described in detail in the Technical Report CIE 117 (1995) "Discomfort Glare in Interior Lighting".

The UGR method is applicable for most glaring light sources (particularly luminaires). A UGR rating is not possible for

• indirect-distribution luminaires,

• single-workstation luminaires and

• accent lighting spotlights.

With asymmetric-distribution luminaires, it is important to rate the illuminance values in the observer’s direction. A UGR reference value labelling is thus not sensible for such luminaires.

Psychological glare from windows has not yet been conclusively researched. Currently, there is no suitable glare rating method available for it.

Psychological glare cannot be described using precise figures (meaning with some positions after the decimal dot), instead it is described using a figure range (levels) which reflects the statistically recorded glare perception of a multitude of observers. For this reason, UGR values are divided up into levels with the standardised UGR limits 13, 16, 19, 22, 25 and 28.