Lighting installations
Good lighting is not about putting up lots of lamps: it is about putting up the ones you need, of the right type and in the right place, so that every task can be done comfortably and without fatigue. And that is something you calculate.
01The quantities
| Quantity | Unit | What it measures |
|---|---|---|
| Luminous flux Φ | lumen (lm) | How much light the source emits in total, in all directions. |
| Luminous intensity I | candela (cd) | How much light it emits in a given direction. It depends on the luminaire's reflector. |
| Illuminance E | lux (lx) | How much light arrives at a surface: 1 lx = 1 lm/m². It is what you specify and what you measure. |
| Luminance L | cd/m² | How much light leaves a surface toward the eye. It is what is actually perceived, and the cause of glare. |
| Efficacy | lm/W | How many lumens the lamp delivers for each watt consumed. |
| Color temperature | K | Warm at 2700 K, neutral at 4000 K, cool at 6000 K. It has nothing to do with the amount of light. |
| Color rendering index | CRI | How faithfully colors look under that light. Below 80 it is not suitable for fine work. |
| Lamp | Efficacy | Lifespan | Current use |
|---|---|---|---|
| Incandescent | 12 lm/W | 1,000 h | Practically obsolete: almost all the energy is lost as heat. |
| Halogen | 20 lm/W | 2,000 h | Accent lighting where color quality is wanted. |
| Fluorescent | 60 to 90 lm/W | 10,000 h | Being phased out. Needs a ballast and contains mercury. |
| High-pressure sodium | 100 to 130 lm/W | 20,000 h | Older street lighting: lots of light, orange color and low CRI. |
| LED | 100 to 160 lm/W | 30,000 to 50,000 h | Today's standard for every use: high efficacy, instant turn-on and easy dimming. |
02The lumen method
| Space | Recommended illuminance |
|---|---|
| Corridors and storage rooms | 100 to 150 lx |
| Living room | 200 lx |
| Classroom and general office | 300 to 500 lx |
| Machine shop | 500 lx |
| Electronics lab and fine work | 750 to 1,000 lx |
An 8 × 6 m classroom (48 m²), effective height above the working plane 2.5 m, required level 400 lx. LED luminaires of 4000 lm each. Cu = 0.55 and Cm = 0.80.
- Room index: k = (8 × 6)/(2.5 × (8 + 6)) = 48/35 = 1.37.
- Total flux required: Φ = 400 × 48/(0.55 × 0.80) = 19,200/0.44 = 43,636 lm.
- Number of luminaires: 43,636/4000 = 10.9 → 12 luminaires, arranged in 3 rows of 4 so that the lighting comes out even.
- Checking the installed power: if each luminaire draws 36 W, that is 432 W over 48 m², or 9 W/m². It is a reasonable value; with fluorescents it would have been almost double.
Distribution matters as much as quantity: twelve luminaires bunched up in the middle give the same total flux and terrible lighting.
It represents the loss due to dirt and aging over the life of the installation. A Cm of 0.80 means that the calculation is made so that at the end of the maintenance period the required level is still met. Ignoring it produces installations that start out fine and two years later are clearly dim, with nobody knowing why.
03Lighting circuits
| Circuit | How it is wired |
|---|---|
| Single-point switching | One switch interrupts the line conductor going to the lamp. Never the neutral: if the neutral is interrupted, the lampholder stays live with the switch off. |
| Multiple-switch | Two or more lamps controlled by independent switches from the same box. |
| Staircase | Two changeover switches joined by two return conductors. Control from two points. |
| Intermediate switching | Two changeover switches at the ends and one or more intermediate (crossover) switches in the middle. Control from three or more points: long hallways, bedrooms. |
| Automation | Motion sensor, photocell, staircase timer or pushbuttons with a latching relay. Energy savings and convenience, with the advantage that the control wiring is light. |
Mandatory in places of public assembly, schools, industrial facilities and escape routes. It must switch on automatically when the mains fails and keep the evacuation routes lit for a minimum time —on the order of one hour—, at the level set by the local regulations.
It is tested periodically by cutting the supply and checking the real autonomy: a depleted battery is indistinguishable from a healthy one as long as the mains is present.
They supply motors and machines, and are designed separately from lighting: a different section, a different protection and, where appropriate, a different distribution board. Starting a motor causes momentary voltage drops that, when the circuit is shared, show up as flicker in the luminaires.
The criteria for motor control and protection are developed in electrical installations and in AC motors.
04In the lab
Using a light meter —or a calibrated phone app— measure the illuminance on the working plane at a grid of points in the classroom. Calculate the average and the uniformity, and compare them with the recommended level. Repeat by day and by night.
Apply the lumen method to a real room in the school: survey the dimensions, define the required level, calculate the flux and the number of luminaires, and propose the layout on the floor plan. Compare the result with the existing installation.
Measure power consumption with a clamp meter and illuminance at a fixed distance for incandescent, compact fluorescent and LED lamps of similar rated flux. Calculate the real efficacy of each one in lm/W and compare it with the one on the packaging.
Build a staircase circuit on a panel and extend it to intermediate switching with a third control point. Draw the functional diagram and the wiring diagram before wiring, and check that the lamp responds from all three points in any combination.
05Common mistakes
| Mistake | Consequence |
|---|---|
| Interrupting the neutral with the switch | The lampholder stays live even with the light off: a direct hazard when changing a lamp. |
| Calculating without a maintenance factor | The installation complies on the first day and falls below the required level after a few years. |
| Confusing lumens with lux | The lumen is what the lamp emits; the lux is what reaches the surface. Specification is done in lux. |
| Choosing by watts instead of lumens | With LEDs, watts no longer say anything about the amount of light. |
| Uneven distribution | Some areas brightly lit and others in dim light: uniformity is as important as the average. |
| Inappropriate color temperature | Cool light in a bedroom or warm light in a fine-work shop: discomfort and eye strain. |
| Lighting and motor power on the same circuit | The luminaires flicker every time a motor starts. |
| Emergency lighting left untested | The batteries run down silently and the system fails just when it is needed. |
06Self-assessment
What is the difference between lumen and lux?
The lumen measures the total flux emitted by the source; the lux measures how much of that light reaches a surface: 1 lx = 1 lm/m².
What illuminance level is appropriate for an electronics lab?
Between 750 and 1,000 lx: it is fine visual work, with small components and tiny markings.
Calculate the total flux for 60 m² at 300 lx, with Cu = 0.5 and Cm = 0.8.
Φ = 300 × 60/(0.5 × 0.8) = 18,000/0.4 = 45,000 lm.
What does the room index represent?
The shape of the room in relation to its effective height. It is used to enter the manufacturer's tables and obtain the luminaire's utilization factor.
What is the maintenance factor for?
To anticipate the loss of light due to dirt and aging, so that the installation still meets the required level at the end of the maintenance period.
Why does the switch interrupt the line and not the neutral?
Because if the neutral is interrupted, the lampholder and the lamp remain live with respect to ground even when the light is off.
What is needed to control a lamp from three points?
Two changeover switches at the ends and an intermediate (crossover) switch in the middle. Each additional point adds another intermediate switch.
Why is it not advisable to share a circuit between lighting and motor power?
Because the starting current of motors causes momentary voltage drops that show up as flicker in the luminaires, and because the protective devices must be sized with different criteria.
How often is emergency lighting tested?
Periodically, by cutting the supply and checking the real autonomy. A depleted battery cannot be told apart from a healthy one as long as the mains is present.
Why is choosing a lamp by its power rating no longer useful?
Because efficacy varies enormously between technologies: 12 lm/W in an incandescent and over 100 lm/W in an LED. What you have to compare is the flux in lumens.