This is the ratio of the luminous flux incident on an infinitely small flat surface to the area of that surface.
The surface can have any orientation.
The orientation is defined by the normal
vector on the surface. Y
d
X
γ
α
n
Z
Ip
P
The plane illuminance (from one light source) at point P on the calculation grid is given by:
E = I d Cos
p p
2 α
Variables: Meaning:
Ep Plane illuminance at point P
Ip Luminous intensity from the light source in the direction of point P d Distance from the source to point P (m)
α Angle between the normal n and the light incidence
This formula assumes that the luminaire is a point source. For fluorescent luminaires, of which the distance between the luminaire and the point P is short in comparison with dimensions of the luminaire, the above formula is not valid. Calculux has a built-in feature (luminaire split-up) which overcomes this problem. When the luminaire split-up feature is activated, the luminaire is considered to be made up of a number of smaller luminaires with the same light distribution but proportionally smaller lumen output.
The following types of surface orientation information relating to each point on the grid are recognised by Calculux.
a) The surface orientation of each point on the grid can be in one of the main directions of the XYZ coordinate system:
Hor +Z
Horizontal +Z grid point.
The surfaces in the grid points, used in the calculation, are orientated towards the positive Z direction.
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Z
Y
X
(
The surfaces are infinitely small planes (one in each grid point) on which the light calculations are being performed.Hor -Z
Horizontal -Z grid point.
The surfaces in the grid points, used in the calculation, are orientated towards the negative Z direction.
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Z
Y
X Vert +X
Vertical +X grid point.
The surfaces used in the calculation are orientated towards the positive X direction.
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Z
Y
X
Vert -X
Vertical -X grid point.
The surfaces in the grid points, used in the calculation, are orientated towards the negative X direction.
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Z
Y
X Vert +Y
Vertical +Y grid point.
The surfaces in the grid points, used in the calculation, are orientated towards the positive Y direction.
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Z
Y
X Vert -Y
Vertical -Y grid point.
The surfaces in the grid points, used in the calculation, are orientated towards the negative Y direction.
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Z
Y
X
b) The surface orientation is parallel to the plane that passes through the grid points.
This enables the illuminance to be calculated on two sides of the plane through the grid points:
Surface +N
Surface +N grid point.
The surfaces in the grid points, used in the calculation, are orientated parallel to the plane which passes through the grid points in positive N direction.
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Surface -N grid point.
The surfaces in the grid points, used in the calculation, are orientated parallel to the plane which passes through the grid points in negative N direction.
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3.13.2 Glare
Glare is the condition of vision in which there is a reduction in the ability to see details or objects due to an unsuitable distribution or range of luminance, or to extreme contrasts.
Glare can occur in one of two possible forms:
• Disability glare glare that impairs the vision;
• Discomfort glare glare that induces a feeling of discomfort.
For indoor Lighting the measure for discomfort glare is called the UGR factor.
UGR is explained in the following section.
UGR
The Unified Glare Rating, UGR, is a measure for the amount of discomfort glare in an indoor lighting installation. A lower glare rating results in a better glare restriction. The practical meaning of the range of the glare assessment scale is from 10 (unnoticeable) to 30 (unbearable).
For the calculation of the UGR the CIE formula is used:
UGR = 8 log{0.25/Lb å L2
ω
/p2}Variables: Meaning:
UGR unified glare rating.
Lb background luminance, determined by the lighting, the room size and the reflectances. It is calculated from the vertical illuminance caused by interreflections on the observer's eye.
L luminance of the luminaire in the direction of the observer's eye.
ω solid angle of the luminous parts of the luminaire as seen by the observer.
p position index of the luminaire (a value given by CIE typical for the displacement of the luminaire from the line of sight).
To get insight in the overall effect of glare from a lighting installation, an UGR calculation for reference conditions as specified in the CIE tabular method is most suitable.
The resulting single value (called in Calculux UGRCIE), is the value against which specifications can easily be checked.
For detailed glare rating evaluations, Calculux has also the possibility to calculate UGR values for non-reference conditions. In this case the observers are situated in a given grid at each grid point. For four mutual perpendicular viewing directions with parallel and crosswise view, individual UGR values can be calculated. The line of sight is always horizontal and
the eye height is given by the grid parameters.
CIE tabular method reference conditions for UGR
CIE specifies reference conditions (according to the tabular method) for the calculation of UGR. The resulting single UGR value is called in Calculux UGRCIE tabular method. It is the most typical value for the overall effect of glare from the total lighting installation.
The reference conditions are:
• Luminaire geometry: luminaire spacing in both directions 0.25 H, with H being the vertical distance between the observer eye and the luminaire.
• Observer position: against the middle of the respective walls at 1.2 m above the floor with horizontal viewing directions perpendicular to the wall.
(
CIE's tabular method is not defined for complex situations. As a consequence also UGRCIE is not defined for such complex situations. Calculux will therefore only calculate UGRCIE for the following situations:• Rectangular rooms
• All luminaires of the same type and at the same height
• All luminaires positioned parallel to the walls
• Luminaires not asymmetrical or tilted.
In all other situations Calculux will print 'UGRCIE not defined'.
UGR in a calculation point
For each luminaire in the room, it's contribution to the Sum å in the main formula is calculated.
The calculation uses the formula:
UGR contribution = L2
ω
/p2The Luminance (L) and the solid angle
ω
are calculated by Calculux.P is taken from the Guth Position Index Table:
Background luminance
The background luminance, Lb, is defined as that uniform luminance of the whole
surroundings which produces the same illuminance on a vertical plane at the observer's eye as the visual field under consideration excluding the glare sources. It may be obtained from the formula:
Lb = Ei/π
where Ei is the indirect illuminance at the eye of the observer (lux).
The indirect illuminance is the illuminance on the eye caused by the luminance of the walls (direct illuminance from the light sources is not taken in to account).
Output
The output format of the calculation of point values will be presented in a textual grid, analogous to Calculux's vertical illuminance output.
The values will be presented as rounded whole figures. The average, min/ave and min/max values are not calculated and will not be printed in this output.
At the top of the output page the UGRCIE value will be presented, if defined.
Also in the summary the UGRCIE values will be output per switching mode, that is, if they are defined and applicable.