Methods for calculating illumination The problem of the lighting designer at the functional level is to determine how many lights and where to place them to get the correct level of illumination for a given activity. These calculations can be divided into two types • We want to determine the illumination at a small specific location in the room from a point light source. Accent or task lighting. This not much more complicated than exercises we have already been doing
Simply get polar plot for luminaire and work out the illumination on surface for a given lamp. Called the point illumination method.
The lumen method We want to estimate the illumination pattern from a set of diffuse lighting sources over a broad area in a room. This is a more complicated exercise. We will be using the total flux method, or lumen method to get rough and reasonable estimates of the lamps/luminaires needed. This method is used in areas where a uniform light intensity of the work area. Used for rectangular rooms with gridded luminaire pattern. Uniform: For the present discussion we require the minimum illumination level to be at least 70% of the maximum illumination level.
The lumen method
What we have to do is solve an equation of the type
Φ rec E= A
where E is the illumination level required at the work surface and A is the total area of the plane where the work is done. The factor Φrec is the flux of light received on the working surface. The factor Φrec is not the flux emitted by the lamps although it will be related to the flux emitted by the lamps. The main issue is to determine how much flux needs to be installed, i.e. Φinst, to get the required amount of received flux Φrec.
Methods for calculating illumination The first step is to determine how much flux is to be received.
Φ rec = EA
Multiply the illumination required by the surface area. The received flux is related to the installed flux by a simple formula
Φ rec = MF × UF × Φ inst
where MF = Maintenance factor (Light Loss Factor LLF) UF = Utilization factor The maintenance factor gives an estimate of how lighting conditions will deteriorate through use. Some factors are • Dust and dirt inside luminaire surfaces • Aging of light bulbs emitting less light • Cleaning of room surfaces, e.g. ceiling Without detailed knowledge of a maintenance plan, one sets MF = 0.80.
The utilization factor (UF) This utilization factor is somewhat more complicated to determine than the maintenance factor. The utilization factor will depend on the following The luminaire properties. One needs the light output ratio (LOR). How much of the light emitted by the lamps actually leaves the luminaire? An enclosed lamp in a luminaire with low reflectivity will have a smaller LOR than a naked lamp. The respective values of the ULOR and DLOR also have an impact. The light emitted downwards will probably reach the work area without reflections. However light emitted upwards can only reach the work area after reflection(s) from surfaces. A large DLOR usually means a higher UF.
The utilization factor (UF) The reflectance of the room surfaces. Are the surfaces (walls, ceiling) light or dark in colour? The larger the ULOR the more important this factor becomes. Some typical reflectance values are Colour White, Off-white, light shades of gray, brown, blue Medium green, yellow, brown or gray Dark gray, medium blue Dark blue, green, wood panelling Reflectance 75-90% 30-60% 10-20% 5-10%
(A colour with strong chroma does not reflect other colours). Reflectances are available from manufacturers of paints and furniture finishings. It is usual to make the reflectance of the ceiling highest, walls slightly lower and the floor darker. One typically recommends in offices Light 70-90% 50-70% 20-50%
Ceiling Wall Floor
Reflectances of desks should be 20-40%. Do not choose very dark wood, or bright surfaces.
The utilization factor (UF) The geometric...
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