I'm currently using LightUp's Lux Contour feature to test good(read as efficient) office lighting design, so I thought I should fill in some info on this process using LightUp. First I'll go through the basic methods of choosing appropriate luminaires given the data available for the room intended for lighting, then I'll go onto specific lighting evaluation using LightUp's Lux Contour feature. Sorry if this bores the pants off some people, but it can be helpful to know for the early stages of modelling your building.
For an analysis of the feature, I've used a testing room similar to the Classroom in LightUp's gallery. The room has no windows and is totally reliant on direct, artificial sources of lighting. The grid layout shown is 1m x 1m for every thick black line.
- Room Shape: The room has a length of 13000mm, a width of 9000mm and a height of 2700mm.
- Working Plane: 700mm above FFL (Finished Floor Level) - typical.
- Mounting Height: 2400mm above FFL - typical.
- Lamp Type: T5 flourescent lamp - typical.
As you probably know manufacturers provide data on the performance of their luminaires on their website. If the manufacturer is considerate enough to provide the appropriate data we can determine the effectiveness of certain luminaire types based on the proportions of our room, but what we first need to do is determine a Room Index (K) value.
Room Index (K) value
A Room Index value is a number that describes the ratios of the rooms length, width and height. These dimensions directly effect the performance of a specific luminaire, so determining your Room Index value is very important when considering luminaires for your room. The Room Index (K) value is determined by this formula;
K = [L x W] / [[L + W] x Hm]
- K = Room Index Value
- L = Room Length in Metres
- W = Room Width in Metres
- Hm = Distance between Mounting Height of luminaire and the Working Plane in Metres
So using this equation for the office space we get a value of 3.13 ([13x9]/[[13+9]x1.7]=3.128...), which is pretty good for office lighting. The next step is to choose an appropriate luminaire to light our room, and determine how many luminaires we will need. This can be done using the Utilisation Factor (F) for the luminaire to give a rough estimate of the effectiveness of the luminaire to light the working place, and the Lumen Method Caculation to determine how many luminaires we will need to light the workplane to a specific lux value.
Utilisation Factor (UF)
Considerate manufacturers provide this data so you can appropriately assess their products efficiency in your room. So from the Thorn Lighting US Indoor Catalogues CalcExpress feature for specific Luminaires (CalcExpress calculates your room index from the values you input for length, width, height) we can get a utilisation factor (UF) of 78% (or 0.78) for the Luminaire QUATTRO M 2x14w T16 HF L840 in the office space. This means that 78% of the light produced by the lamp will be useful on the workplane, and 22% will be lighting some other surfaces, which is pretty good imo. This utilisation factor is also determined by the reflectances of your rooms walls, ceiling and floor (noted sometimes as W, C and F respectively). Typical reflectance values are 0.7 (70%) for the ceiling, 0.5 (50%) for the walls and 0.2 (20%) for the floor. I would suggest hunting down the reflectance values of highly reflective materials from the manufacturers website if you intend to use them, but the typical values are often the best to go with if you haven't yet decided on materials.http://www.ndlight.com.au/lighting_calculations.htm wrote:This is a value between 0 and 1 that represents the percentage of total lamp lumens in the room that fall on the work plane. It takes into account the room reflectances, room shape, polar distribution and light output ratio of the fitting.
Interpretting Manufacturer's Data
There are a few calculations that you can make using data direct from the manufacturer which help in designing your luminaire layout on the ceiling grid, but the first calculation you should consider is the Lumen Method Calculation.
Lumen Method Calculation
The Lumen Method Calculation is an equation used to determine how many luminaires are required. It functions by first deciding on an appropriate level of illumination required for the working plane, and then using data obtained from the luminaire manufacturer, estimates an appropriate number of luminaires required to achieve this level of illumination. The equation is as follows;
So we've already worked out our Ultilisation Factor (UF) from calculating our room index (K) and obtaining the appropriate UF for our luminaire relative to our room index from the manufacturer. What is required is the Maintenance Illuminance (E), the Total Flux in Lumens (F), and the Light Loss Factor (LLF), so lets talk a bit about those;http://www.ndlight.com.au/lighting_calculations.htm wrote:N = [E x A] / [[F x UF x LLF]
- N = Number of Fittings
- E = Lux Level Required on Working Plane (Maintenance Illuminance)
- A = Area of Room (L x W)
- F = Total Flux (Lumens) from all the Lamps in one Fitting
- UF = Utilisation Factor from the Table for the Fitting to be Used
- LLF = Light Loss Factor. This takes account of the depreciation over time of lamp output and dirt accumulation on the fitting and walls of the building.
Maintenance Illuminance (E)
The first is the illumination required for the working plane. A good starting point to defining this value is to check with your local or regional (state/country) building codes for recommended maintenance illuminances for various types of tasks. The Australian Standards set out maintenance illuminance values similar to this;
- 40 lux: Movement and Orientation, areas where visual tasks are rare
- 80 lux: Rough Intermittent, areas where visual tasks involve movement, orientation and coarse detail
- 160 lux: Simple workplace tasks, continuously occupied area where tasks include occasional reading for short periods
- 240 lux: Ordinary/Easy workplace tasks, continuously occupied area where tasks have high contrast or large detail
- 320-400 lux: Moderately Difficult, continuously occupied area where tasks are moderate and have low contrasts
- 600 lux: Difficult, continuously occupied area where tasks have small detail
- 800 lux: Very Difficult, very small detail tasks
- 1200 lux: Extremely Difficult
Also I should mention that some Green Accreditation orginisations such as Green Star here in Australia assess light levels provided by luminaires on the working plane. In Green Star - Office Design v3:
So for our office space, we'll be aiming for an average of 320 lux of illumination on the working plane, and i'll talk a little about achieving a uniformly lit working plane a little later.Green Star - Office Design v3 wrote:One point is awarded where:
The office lighting design has a maintained illuminance level of no more than 400 lux for 95% of the Class 5 Commercial Office NLA [Net Lettable Area] as measured at the working plane (720mm AFFL).
Total Flux in lumens (F) of all Lamps per Fitting
Rather than bore you to death with further calculations and definitions, the total Flux in lumens (F) for a luminaire is fancy way of saying the value for the initial lamp lumens multiplied by the number of lamps in a luminaire. For instance, our previous example from the Thorn Lighting website, the 'QUATTRO M 2x14w T16 HF L840' uses two T16/14W lamps ('2x14w T16'). Each of these lamps outputs 1200 initial lamp lumens, so the Total Flux in lumens (F) for the luminaire is 2400 lumens. This represents the initial output in lumens of the luminaire.Some manufacturers provide this data, some don't. For the manufacturers that don't publish the flux in lumens, try tracking down lamp manufacturers that make the lamp that your luminaire uses. For our luminaire, this would be a 14 watt T16(or T5) flourescent lamp. An example is from the osram site;
Now its quite easy to assume that this value 2400 lumens for each luminaire won't be achieved for the entire life-span of the luminaire, so the Lumen Method Calculation features the Light Loss Factor (LLF) to ensure that at the time when this luminaire outputs the lowest amount of light (just before cleaning/replacement of the lamps), the required Maintenance Illuminance level of the working plane is still achieved.
Light Loss Factor (LLF)
To put it simply, as the hours of use a particular lamp increases, its lumen output decreases. This is due to the accumulation of dirt and the general reduction in efficiency and functionality of the lamp. So how do you determine your light loss factor? Well as a rule of thumb for dirt accumulation, the following factors can be used;
So our Light Loss Factor (LLF) for our office is 0.8.http://www.ndlight.com.au/lighting_calculations.htm wrote:Air Conditioned Office: 0.8
Clean Industrial: 0.7
Dirty Industrial: 0.6
Calculating the Amount of Luminaires Required
So with the above information we can get an approximate estimate of how many luminaires we should need to light our working plane to the appropriate maintenance illuminance;
N = [E x A] / [[F x UF x LLF]
- N = [320lux x 117msq] / [2400 lumens x 78% x 0.8]
- N = 25 luminaires
Determining the Grid Layout
There are two simple equations for determining the grid layout of your luminaires. They are;
NL = Sq Root[N x [L/W]]
NW = Sq Root[N x [W/L]]
- NL = Number of Fittings required for the length of the room
- NW = Number of Fittings required for the width of the room
- N = Total Number of Fittings
- L = Length of Room
- W = Width of Room