Quality No 1: Color Rendition

What is good color rendition? Good color rendition means that colors are rendered as we know it. But wherefrom do we know the "right" colors? What is the reference light source? Shure, it is the sun.

Let's see the sun as a very very large blob of hot material hanging around in space. The suns surface has a temperature of ca. 5700 Kelvin (about 5400 degree Celsius) emitting a continuous spectrum peaking at roughly 550 nanometers wavelength - for us this is green light.

How to produce artificial light? Just use a much smaller piece of material and heat it to 5700 Kelvin. This is a tricky thing because just the strongest materials melt at around 3500 Kelvin. So man invented a glas apparatus filled with a inert gas, containing a thin filament. An electrical current heats up this filament to 2500 Kelvin. O.k., this is far from 5700 Kelvin but it is the same spectral type: A continuous spectrum containing each wavelength (= color) within a range. Hence the sun and lightbulbs including halogen lamps produce similar spectra - in physics they are representatives of the same physical model: They are black body radiators.

Fluorescent tubes and white LEDs produce light which originates from a nearly monochromatic (= single colored) light production mechanism.

• Fluorescent tubes produce ultraviolet radiation which is shiftet to different colors by a special coating of the tubes (and the folded tubes of compact energy saving lamps).
• Mainstream white LEDs are ... blue LEDs. A coating with different fluorescent dyes around the semiconductor element convert the blue light into red and green light. Hence the near monochromatic light is "shifted" to longer wavelengths and adds to the blue light. 

Both, fluorescent tubes and white LEDs of higher quality have a combination of line spectra (with narrow "pipes" at some main emitting wavelengths) and some more ore less continuous components.

Characterization of the spectral quality is done with the Color Rendering Index (CRI) which is 100% for the sun (no surprise!) and 100% for incandescent lamps.
Fluorescent lamps of high quality reach a CRI of 80%, special types of roughly 90%.
LEDs CRIs are in the region of 80%.

Quality No 2: Spatial Distribution of Light Output

Light is not produced for its own beauty (with some nice exceptions!). In most casese light is used to lit up a working area, the whole room or the entry door.

Each application needs its own light distribution:

• diffuse light:
  Fluorescent tubes have large light emitting surfaces - its light is diffuse by the nature of the lamp type.
  LEDs and incandescent lamps have small light emitting surfaces - external diffusors allow diffuse light with theses lamps.
• directed light:
  Very compact fluorescent lamps can provide some directed light by the use of large reflectors.
  LEDs and incandescent lamps are easily focused to produce directed lights.
• spot light:
  This is the home of incandescent lamps and LEDs. The can be focused to narrow beams resulting in a spot of light.

The largest amount of light can be produced by incandescent lamps if you have only one light generating device. When it comes to arrays of single light emitters the amount of light can be provided incandescent, fluorescent and LED lamps.

Quantity No 1: Lumens per Watt, not Watts!

What ist the main criterium when it comes to pure energy efficiency? It is not something boring like Watts. The lumen output per Watt input power is the correct levelling rod to compare different light sources. A conventional incandescent lamp at 110/230 V provides around 8-10 lumen per Watt. A high efficiency low voltage halogen lamp with IRC coating provides around 20-30 lumens per Watt. LEDs with warm white spectra and high CRI provide 40-50 lumnes per Watt, high quality fluorescent tubes around 50-75 lumens per Watt.

Some producers of lamps provide the information of power consumption in Watt and light "power" in lumens. To compare different lamps just calculate

    lumens / Watt = (generated amount of light) / (power consumption in Watts)

Quantity No 2: Degradation with burn time

Incandescent lamps especially modern halogen lamps do not degrade distinctly during their lifetime of roughly 4000 hours. Flourescent tubes degrade and reach half their light output during their lifetime of 5000 -10000 hours (by initial power consumption!). LEDs degrade after 10000-50000 hours use halving their light output.

Conclusion: Deciscions are complex!

You have to evaluate your optimized "light production device" by the following criteria:

• Color rendition of the light source,
• spatial distribution of the light source(burner + lamp),
• efficiency in terms of lumen per Watt and
• your tolerance against the degradation of the light source.

Don't forget to choose the

• the right voltage of your lamp,
• the correct socket type
• the correct color temperature
• etc.

O.k., doing the choice is complex. But modern light sources last perhaps a decade or two. Doing the right choice means: Reducing costs for lamps and minimizing emissions and the demand for raw materials!

And good light is a major factor for comfort and health!