“When light strikes a physical material, any or all of three surface actions will take place: (1) it can be absorbed by the surface, normally transformed into heat; (2) it can be reflected back into space in a direction other than that from which it came; or (3) it can be transmitted (refracted) through a medium to continue onward on the other side” (Michel, 1996).
Moore (1991) also defined these three concepts as “When luminous flux strikes an opaque surface, it is either reflected or absorbed. Reflectance is the ratio of reflected flux to incident flux. Absorptance, conversely, is the ratio of absorbed flux to incident flux”. When surface is not opaque but is either transparent or translucent, some of the luminous flux is transmitted through the material and therefore “transmittance is the ratio of transmitted flux to incident flux” (Moore, 1991)
Reflectance
Egan (1983) gave the definition of Reflectance (P) as “the percentage of incident light that is reflected from a surface, with the remainder absorbed, transmitted, or both”. Reflectance is denoted by the Greek letter rho (p), and is a value between 0 and 1; p = 0 if the surface is perfect black and absorbs all light; p = 1 if all incident light is reflected. In lighting calculations
The direction of reflected light is affected by the surface textures; matte surfaces will reflect light equally in all directions while specular surfaces will reflect light in one direction only and real surfaces are not perfect diffusers and would reflect light unequally in all directions. Moore (1991) stated that “The reflectance (and thus the luminance) of such a surface is dependent on the angles of incidence and reflectance and the surface’s diffusion characteristics”. The thesis examines the impact of atrium reflectances of both diffuse and specular surfaces on the daylight in an atrium building.
The reflective surfaces of the dominant enclosures in buildings play a fundamental role in the perception of the space; illuminating high reflectance surfaces can create an illusion in terms of the physical boundaries of a space. Indirect lighting is a method of reflecting illumination off the ceiling or other surface. If walls are over lit and of a large surface area, they can be too bright for work environments and may cause visual discomfort (Michel, 1996). For general room lighting, IESNA recommend walls to have 50 to 70% reflectance and vertical interior partitions to be of 40 to 70% reflectance as shown in Table 2:2 (Egan, 1983).
Table 2:2 IESNA recommended reflectances for matte or diffuse reflecting surfaces and finishes in offices and educational facilities (Egan, 1983)
Surface Reflectance %
Classroom Office
Ceilings 70-90 >80
Walls 40-60 50-70
Partitions (e.g., partial height barriers) 40-70 40-70
Floors 30-50 20-40
Furniture and machines 25-45 25-45
Desk and Bench tops 35-50 35-50
Walls containing windows should have high reflectance of >80% to reduce the contrast between bright glazing and surround. Window frame, sash, and glazing bars also should have a light-coloured matte finish.
For floors, use surfaces with reflectance >25% for rooms where visual efficiency is a major concern but do not exceed 40% as reflected glare conditions would be critical.
Reflected light is a significant component of the light indoors, particularly further away from the window and provides all the illuminance beyond the no-sky line (discussed later). Research investigating which surfaces in a room are most effective in supporting task level illumination as shown in Figure 2:4 demonstrate that “the ceiling is the most important surface in controlling the daylight coming into the room and reaching the task (Evans, 1981).
Figure 2:4 Reduction of lighting at a point due to painted black surfaces in a room indicating those that are most effective in supporting task level illumination (Evans, 1981)
Higher wall reflectances are also important in small rooms as they will enhance the illuminance on the working plane and increase the inter-reflected component, thus improving uniformity. While ceiling and walls of light colours increase most of the reflected light, a light coloured floor nearer to the window would also reflect direct high level light that strikes it
The reflectances used in the experiments undertaken in Chapter Seven are chosen due to the daylighting opportunities presented by higher ceiling and wall reflectances as identified in this section and are as per those recommended by IESNA.
Transmittance
Tregenza and Loe (1998) gave the definition of transmission as the fraction of light that passes through a material; it is denoted by the Greek letter tau ( ) and is a number between zero and one. “Light transmittance is the ratio of transmitted light to incident light (less than 1.0). Measured in footlambert, transmitted luminance is the product of illumination on the reverse side of a surface (measured in footcandles) and surface transmittance” (Moore, 1991).
Diffuse transmittance is the fraction of a beam that is uniformly scattered while regular transmittance is the fraction that remains as a geometrical ray. With glass, the fractions that are reflected or transmitted depend upon the angle of incidence. When a beam strikes a glass surface at a glancing angle it is mainly reflected while when it is perpendicular to the surface most of it passes through.
Transmittance of glass to light differs from the transmittance of solar radiation because of the fact that the transparency of glass varies with wavelength. Therefore, daylighting and solar gain calculations are undertaken using separate values. Whatever is the wavelength of the incident radiation, all absorbed energy results in an increase in temperature of the material. All the light that enters a room is absorbed by the surfaces and results in thermal gain. For simple calculations of window performance an average transmittance is used, a weighted mean over all directions of incidence. Littlefair (1996) gave transmittance values for different glazing types as shown in Table 2:3.
Table 2:3 Transmittance values for different glazing types (Littlefair, 1996)
Table 1 Approximate diffuse transmittances for various glazing types (when they are clean)
Type of glazing Transmittance
Clear single glazing 0.8
Clear double glazing 0.7
Low-emissivity double-glazing 0.65
Double glazing and internal light shelf 0.55 Double glazing, internal and external light shelf 0.4
Double glazing with coated prismatic glazing 0.3
Double glazing with prismatic film 0.55
Double glazing with solar control mirrored louvres5 0.3 Extra corrections for dirt on glass
Horizontal glazing 0.7
Sloping glazing 0.8
Vertical glazing 0.9
The experiments undertaken in this study do not include window and atrium roof but consider an approximate loss due to them for the interpretation of the results and comparison of data obtained from real buildings.