Lighting can use 25% of a household energy bill, so it is worth doing the numbers to save waste.

Incandescent lights, with the white hot tungsten filament, are only 2% efficient. 98% is wasted as heat. Compact fluorescent Lights CFL are 8-15% and the new LED lights are 8-22% efficient.

LED lights are claimed to last 50,000 hours, (25 years), 35 to 50 times longer than incandescent lighting, and about 2 to 5 times longer than fluorescent lighting.

As well as economy lighting can vary in quality. Some light sources show colours accurately, while some lights have colours missing.


Efficiency and quality

There are two challenges in lighting:

1: the efficiency of how much energy is needed to produce the light.

2: The colour and quality of the light.


Comparing Lights 

How much light do we need?

The design of the reflector and the distance make a dramatic difference to the size of the light and how much is wasted.

The table below gives the lumens a light needs to produce the desired lux.



Desired light level


Bare light

360 deg

2 metre

Light with reflector

180 deg

2 metre

Light with reflector

90 deg

2 metre

Downlight on  bench / desk

30 deg

1 metre

Study 1,000 lux       7 lum
Lounge 200 lux 10,000lum 5,000 lum 300 lum 8 lum
Kitchen 220 lux 11,000 lum 5,500 lum 330 lum 9 lum
Bathroom 150 lux 7,000 lum 3,500 lum 220 lum 6 lum

So bringing the light closer to the source and not spilling any, can dramatically save power.

We can produce these light levels with the following lights:



Compact Fuoro




Watts Watts Watts Lumens
40 10 8 450
 60 16 10 775
100 20 14 1500
100 27 18 1800+ 
150 38 27 2780

So a desk lamp with a 40 W incandescent globe shedding all it's light over 0.5 sq metre will give about 900 lux which is suitable for study. 10 Watts of CFL, or 8 Watts of a good LED, will do the same.

The new GE LED light due mid 2014 has different specs. 1,600 lumens, 27 watt of power, 3,000oK, dimmable, life 25,000 hrs, self cooling, cost $29-39, .

It is very difficult to get consistent data on this. The data in the table comes from the Aust. government and Cree LED.

Free sunlight indoors

If you look at the table on the right you'll see that sunlight is 100,000 lux. That is 500 times the illumination needed in a house. So a skylight need only be 1/500 th of the floor area to give the enough light to turn off the lights altogether. Allow for losses so it could be about 1/200 th the area. In a 4 x 4 kitchen = 280 mm square.

Light can be piped along a reflective tube, a hollow clear tube, or optical fibres. Thus dark areas in the depths of a building can be illuminated with beautiful sunlight for nothing.


Ref:  Wikipedia: Light tube,  Solartube



Lumens and Lux

The standard units of illumination we'll use here are lumens and lux.

Lumen is  a measure of the amount of light energy a source puts out.

E.g. a 100 Watt light bulb gives off 1,500 lumens of light.

1 lux =1 lumen / sq m. It is the brightness of a position, eg a surface .

E.g. we like to have 220 lux on a kitchen bench in order to see clearly. 


Light levels in lux produced by light sources at different distances.

This has been calculated assuming all light is radiated out in all directions. A mirror shade will recover wasted light, sending it in the desired direction.

Light power  Distance 1M dist 2M dist 3M  dist 4M
500 lum 40 lux 10 lux 4 lux 2 lux
1000 lum 80 lux 20 lux 9 lux 5 lux
1500 lum 120 lux 30 lux 13 lux 7 lux
2000 lum 160 lux 40 lux 18 lux 10 lux
3000 lum 239 lux 60 lux 27 lux 15 lux
5000 lum 399 lux 100 lux 44 lux 25 lux


Natur​al lux levels



Full sunlight 100,000
Full Daylight not full sun 10,000
Overcast Day / TV studio 1,000
Very Dark Day 100
Family living room 100
Full Moon in tropics 1
Quarter Moon 0.01
Starlight 0.001


Summary of information below​

30 year cost


5 hr / day

@ 48c/kWh

50 W do​wnlight




















gas mantle


100 W tungsten


Fluorescent tube


LED lamp


mercury lamp


sodium lamp


Comparing Light​s to 50 watt QI down light

Despite wonderful hype and promises on the various websites, a simple comparison can be quite surprising, if expensive. I bought a few lights and compared them with each other.

I used a camera light meter for measuring light level

Some lights produced a spot only, so score was reduced by a very rough visual estimate. I've taken a QI down light to be 100% and made a very rough judgment on the other lights. It is simply a rough and ready check on the manufacturers specs showing lumens.

LED = Light Emitting Diode

CFL Compact Fluorescent




Test results

Manufacturer's specs

Type Make and Mode. Watts Light approx $ ​Lumens oK Angle Life Hrs Light quality CRI
QI Quartz Iodine  2,700oK 50 100% 2 720 2700 23     y
LED Brightgreen  DR700   10 120% 50 720 3000 60 70,000 83+ y
LED Philips  master 10 75% 35 380 3000oK 24     y
LED Ecolightup 10 100%     5500oK        
LED Osram 10   32 350 3000 36    
LED CREE MCE - MX801 10 25% 50   2700 60     n
LED Nelson  (Bunnings)       540 3000 38      
CFL Nelson  240V 13 10% 12 290          

Downlighting - 60º wide flood beam angle version is ideal for general illumination. Suitable for Hotels, Apartments, Office & Sh​oplighting
Spot Lighting - 30º spot beam angle version is ideal for accent lighting. Suitable for Museums, Hotels, Apartments & Shoplighting

Brightgreen manual


Lux Level



100 Casual seeing Corridors, changing rooms, stores
150 Some perception of detail Loading bays, switch rooms plant rooms
200 Continuously occupied Foyers, entrance halls, dining rooms
300 Visual tasks moderately easy Libraries, sports halls, lecture theatres
500 Visual tasks moderately difficult Offices, kitchens, laboratories, retail
750 Visual tasks difficult Drawing offices, meat inspection, chain stores
1000 Visual tasks very difficult Assembly rooms paintwork, supermarket
1500 Visual tasks extremely difficult Fine work and inspection, precision assembly
2000 Visual tasks exceptionally difficult


Assembly of minute items fabric inspection

Efficiency of light sources

Very little light energy is produced by most light sources. Most of the energy is given off as heat.

Lumens are a measure of visble light.

A perfect light would produce 683 lumens of light for every watt of energy and it would be 100% efficient at producing visible light. We are most interested in how many lumens of light is produced per watt of electricity put in.

It is also interesting to know how efficient the process is. All real light sources produce Infra red, IR, (heat) and sometimes UV as well. If is not visible so is counted as 0 lumens.

Light from different sources

Lumens/ Watt

Energy of light/Energy input
Ref: Wikipedia   Luminous efficacy  luminous efficiency
Theory Green light at 555 nm (maximum possible luminous efficacy) 683 100%
Combustion candle 0.3 0.04%
  gas mantle 1–2 0.2%
Incandescent 100–200 W tungsten incandescent 15 2%
  tungsten quartz halogen 12–24 V 24 4%
  photographic & projection lamps 35 5%
Fluorescent Fluorescent tube 45-104 8-15%
Light-emitting diode LED lamp 46-100 8-22%
  Theoretical limit (white LED) 260–300 38–44%
Arc lamp xenon arc lamp 30–50 4–7%
  mercury-xenon arc lamp 50–55 7–8%
Gas discharge 1400 W sulfur lamp 100 15%
  metal halide lamp 65–115 9.5–17%
  high pressure sodium lamp 85–150 12–22%
  low pressure sodium lamp 100–200 15–29%
Cathodoluminescence electron stimulated luminescence 30 5%

So to work out how much light a 100 watt incandescent globe will produce:

15 lumens/Watt x 100 Watts = 1500 lumens.



Gravity light

Because LEDs require so little energy, they can even be run with simple wind-up mechanisms. One charity has designed a light to be run by a weight on a rope. Pull the loose end of the rope to lift the bag of stones and this stored potential energy can run the light for half an hour. Very similar in design to the old grandfather clock.

It can also charge batteries.

The light is not powered by gravity, energy is simply stored as potential energy.




Cost savings of LED or CFL lights

Cost of an LED light

A 10.5 watt LED in its 50,000 hour (25-30 years) lifetime will consume about 525 kilowatt-hours. At say 30c/KWh this will cost $160.  During that time, it will never need to be replaced.

The Cost of Incandescent Lamps

 To match the light output and longevity of an LED light it would consume 3,250 kilowatt-hours, costing over $1,150 at todays prices. As well it would need to be replaced about 25 times. 

A typical 65 watt incandescent light lasts for 2,000 hours. 

Compact Fluorescent Lights CFL

CFLs are simply miniature versions of full-sized fluorescents. They screw into standard lamp sockets, and give off light that looks similar to the common incandescent bulbs - not like the fluorescent lighting we associate with factories and schools. The will use a little more power than the LED and be replaced about 5 times.


Coloring Rendering Index (CRI)

CRI represents the quality of light and its faithfulness to render colors correctly, that is, to enable us to perceive colors as we know them.

The ideal CRI is 100, and some incandescent bulbs approach this level.

LED bulbs CRI ratings range from 70 to 95.

The best CFLs have ratings in the mid 80s. 


Black body radiation

If a block of black carbon is heated it will become red hot, then yellow hot, white hot, them blue hot. 

There is a different amount of energy given off at each wave length (colour) for each temperature.

The temperature is always given in degrees Kelvin. This is a temperature scale starting at absolute zero (-273oC). To convert Kelvin to Celsius subtract 273.

Energy given off at various wave lengths (colour)



Correlated Color Temperature (CCT)

CCT is the measure used to describe the relative color appearance of a white light source. CCT indicates whether a light source appears more yellow/gold/orange or more blue, in terms of the range of available shades of "white." CCT is given in kelvins (unit of absolute temperature). 2700K is "Warm" and 5000K is "Cool". The typical light color we are used to in indoor home lighting is "warm", 2700 - 2800K.

Emission spectra

There are other ways of producing light. They all involve exciting the electrons in the outer shell of an atom raising them to outer energy levels (orbital), then letting them fall back to their original energy level. The may happen in stages. Each fall down an energy level gives off a single pulse of light called a photo. Each energy difference produces a photon with an exact wave length.

So, if we excite sodium vapour, it will give off only two wave lengths.

Each element gives off it's own emission spectra and this is used to detect elements ina sample.. The process is also very efficient converting  energy into light.

The problem is that it looks awful. If we look at a loved one under sodium vapour light it will only reflect yellow. Any other colour will look black.

Here we can see the yellow of sodium vapour, the green of mercury, and the warm light for another source.

Each light has it's own spectra. Our eyes adjust to colour temperature by increasing sensitively to different ends of the spectrum, but can never adjust to light sources from which colours are missing.



A fire is a mixture of red hot carbon and gas, and emission spectra from various elements in the flame.

You can see that the remaining daylight on the snow is very blue in comparison to the fire. Our eyes normally adjust to this and we don't see it until the photograph is printed.

Sometimes the waste heat is welcome.

The colours in fireworks come from burning different metals which give off their characteristic emission spectra.

Red is strontium, or lithium, Orange - calcium, gold - iron, yellow - sodium, green - copper or barium,  More




(lux, lumen/m2)
Public areas with dark surroundings 20 - 50
Simple orientation for short visits 50 - 100
Working areas where visual tasks are only occasionally performed 100 - 150
Warehouses, Homes, Theaters, Archives 150
Easy Office Work, Classes 250
Normal Office Work, PC Work, Study Library, Groceries, Show Rooms, Laboratories 500
Supermarkets, Mechanical Workshops, Office Landscapes 750
Normal Drawing Work, Detailed Mechanical Workshops, Operation Theatres 1,000
Detailed Drawing Work, Very Detailed Mechanical Works 1500 - 2000
Performance of visual tasks of low contrast  and very small size for prolonged periods of time 2000 - 5000
Performance of very prolonged and exacting visual tasks  5000 - 10000
Performance of very special visual tasks of extremely low contrast and small size 10000 - 20000



Temperature Source
1,700 K Match flame
1,850 K Candle flame, sunset/sunrise
2,700–3,300 K Incandescent light bulb
3,000 K Cool White/Soft White compact fluorescent light bulb
3,200 K Studio lamps, photofloods, etc.
3,350 K Studio "CP" light
4,100–4,150 K Moonlight,[2] xenon arc lamp
5,000 K Horizon daylight
5,000 K

Fluorescent light tubes or Cool White/Daylight compact f​luorescent light bulb

5,500–6,000 K Vertical daylight, electronic flash
6,500 K Daylight, overcast
6,500–9,300 K

LCD or CRT screen


Calculating lux from lumens


Area of sphere = 4 pi R2

12.6 lumens will produce 1 lux at 1 metre.

After that divide by 12.6  (4 x pi)

e.g. 12.6 lumens will produce 1/9 lux at 3 Metres

Lux = Lumens/4 pi R2

         = 0.796 x Lumens /R2

New rese​ar​ch

Field-induc​ed polymer electroluminescent (FIPEL)

Scientists at Wake Forest University have developed a flicker-free, shatterproof alternative for large-scale lighting that they claim is at least twice as efficient as CFL technology and less expensive than LEDs. The lighting is based on field-induced polymer electroluminescent (FIPEL) technology, which uses a nano-engineered polymer matrix to convert the charge into light.

Induction light

An electrodeless lamp or induction light is a light source in which the power  is transferred into the lamp via electromagnetic fields. A normal electrical lamp uses metal connectors. There are advantages:

  • Extended lamp life, because the electrodes are usually the limiting factor in lamp life.
  • The ability to use light-generating substances of higher efficiency that would react with metal electrodes in normal lamps.
  • Improved collection efficiency because the source can be made very small without shortening life, a problem in electroded lamps

In plasma lamps, radio waves energise sulfur or metal halides. A microwave beam can light a lamp 1,000 KM away.

Fluorescent lamps can be driven by a magnetic field.

Theoretically they have high efficiency and life. However present models are not yet achieving either.