For Lambertian emitters (which in practice means all emitters except high temperature radiant heaters where the radiation is beamed using reflectors) the view coefficient (C) is formally defined as
where
From this definition it can be shown that the contribution of the emitter to the irradiance (Watts/m2) at the observer position is
where W is the radiant power of the emitter per unit floor area. This relation is used in the calculation of the emitter's contribution to mean radiant temperature.
As a guide to the effect of the view coefficient in mean radiant temperature calculations the following relation, giving an approximation to the elevation of mean radiant temperature (in deg. C) due to a radiant heat source, may be used:
where W is the radiant component of the source in Watts/(m2 floor area).
Thus a lighting gain of 20 W/m2, with a radiant proportion of 0.48 and
a view coefficient of 0.457, causes an elevation in perceived mean radiant
temperature of approximately
The accurate calculation of a view coefficient for different emitter types and room geometries is complicated and involves numerical integration. However approximate formulae have been derived covering a range of situations and these are set out below.
This covers emitters distributed over a plane above the observer's head (e.g. ceiling panel heaters, chilled beams, ceiling lights) and floor heating systems. We assume the room is rectangular. For this case, the formula for C is
The factor of 0.9 in the expression for C allows for a statistical distribution of observers (occupants) over the room. Without this factor, the expression applies to an observer in the centre of the room.
Overhead radiant heaters direct the radiation downward in a narrow beam. Consequently an array of ceiling-mounted radiant heaters produces a radiation field similar to that experienced from non-beamed ceiling heaters in a room of large dimensions. Under these conditions equation 6.4-1 simplifies to
This covers emitters set into the walls of the room (e.g.. radiators, wall panel heaters, wall-mounted lights).
For this case, the formula for C is
The formula for C is an approximation based on numerical analysis of a range of cases. The calculations were done on the basis of a statistical distribution of observers over a region of the floor plan which excludes a 1m strip around the perimeter. The emitters were assumed to be distributed over a panel attached to one wall of the room and having a height of 1m; the results are similar when the emitters are distributed over two adjacent walls or when the panel height is varied by up to a factor of 2.
This covers emitters distributed over the room (e.g. computers, task
lighting).
The emitters are assumed to be distributed over a plane at the same height
as the observer.
For this case, the formula for C is
The formula for C is an approximation based on numerical analysis of a range of cases. The calculations were done on the basis of a statistical distribution of emitters over the entire floor plan and a statistical distribution of observers over a region of the floor plan which excludes a 1m strip around the perimeter. The calculations further assumed that no emitter is closer than 1m to an observer. The emitters were assumed to be distributed over a panel attached to one wall of the room and having a height of 1m; the results are similar when the emitters are distributed over two adjacent walls or when the panel height is varied by up to a factor of 2.