Non black surfaces do not absorb the entire radiant energy incident on them. However ,at thermal equilibrium absorptivity  of any surface equals emissivity  

Surfaces with emissivities nearly unity are good absorbers and hence poor reflectors of incident radiation. Thus, most highly polished, unoxidized metal surfaces are good reflectors of thermal radiations with total emissivities less than 0.1. A roughened or an oxidized surface has correspondingly higher emissivities for thermal radiation. Nonmetallic surfaces have emissivities and absorptivities above 0.8. An emissivity value of about 0.85 can be taken for furnace refractory and bricks at high temperatures. The slagging reduces the emissivity value to 0.6 - 0.7.

Consider rate of heat exchange between any two surfaces  and  at temperature and  respectively. The surface  is emitting radiation at the rate  and a certain fraction of this radiation is absorbed by the surface . After a while the surface  is heated and begins to emit radiation at the rate of ; the fraction of which is absorbed by . Net heat exchange between  and  can be expressed as heat loss of  to  as

(14)

F is a view factor which takes into account

• Geometric relationship of the two surfaces. Physically F can be visualized as the fraction of total radiation that is intercepted by the other, 
• The emissivity and absorptivity of surface
• Geometric relationship between two surface and or third surface e.g. a refractory surface.

The calculation of radiant heat exchange between any two surfaces involves the determination of view factor F. The following procedure may be of help to evaluate F.

• First  is calculated by considering the geometrical relationship between two surfaces.  is the value of  in equation 14 when the two surfaces  and  are black and are in direct heat exchange

• Next the effect of the third reflecting surface for example refractory on heat exchange may be considered and  can be determined.  is a view factor which takes into account both radiation and reflection, but assumes  and  are black surfaces

• Next the emissivity value of the surface can be included to determine F