Watt? No. Think lumen!

Finding the right light is not so clear

By Morand Fachot

Domestic consumers, faced with the gradual phasing out of the incandescent light bulb that has lit their environment for well over a century, have some difficulty finding new energy-efficient bulbs that reproduce the same light and feeling. Their confusion is compounded by the wealth of information now given on lamp packaging. These days, finding the right bulb means looking at many different parameters.

Manufacturers give the watt / lumen equivalents on packaging (Photo: GE)

Watt is it all about?

For decades, choosing a new or replacement light bulb has been easy: consumers would look at the wattage (the amount of energy required to light the bulb), and decide on the product that best met their need. Everyone knew what light would be given by a 60 or 100 watt bulb.

The mandatory replacement of incandescent bulbs by energy-efficient products, initially CFLs (compact fluorescent lamps), introduced confusion for consumers as, to begin with, producers often only gave the watt equivalent of the new bulbs. Furthermore, other factors such as colour temperature and colour rendering (which relates to the way objects appear under a given light source), added to the complexity involved in selecting the right bulb.

The situation is no clearer now that LED-based lamps are becoming more popular and increasing in efficiency all the time.

Shedding light on watts and lumens

Watts and lumens are different units of measurement: watts indicate the power needed to light the bulb and lumens the amount of visible light emitted by a source.

The value of the bulbs required may differ according to the lighting effect desired. The following table gives average ratings for incandescent, CFL and LED bulbs. Consumers should compare the different values and types of bulbs and their intended use before making a choice.

Equivalence table

Typical product Lumens Efficacy (lumens per watt)
3 watt CFL 125 41
3,5 watt LED 150 43
15 watt krypton 185 12
25 watt incandescent 210 8
Ambient lighting
9 watt CFL 400 44
8 watt LED 450 56
40 watt incandescent 460 12
10 watt CFL 500 50
General room lighting
12,5 watt LED 800 64
60 watt incandescent 890 15
15 watt CFL 900 60
12,5 watt LED downlight 1 000 80
75 watt incandescent 1 180 16
20 watt CFL 1 200 60
Suitable for reading
100 watt incandescent 1 750 17
29 watt CFL 1 750 60

Source: EFI (Energy Federation Incorporated), USA

Perception matters

Consumers frequently complain that the new energy-efficient bulbs do not give the same "amount" of light as their incandescent equivalent. Actually it is not so much the amount of light emitted that differs, but that other factors affect users' perception of light. People are accustomed to the light supplied by incandescent bulbs in homes and a switch to the new bulbs requires some adjustment. One important element is the colour temperature. Lamps radiate colours of different "temperature". This is given in terms of the Kelvin (K), a unit of measurement for temperature defined by first IEC President Lord Kelvin, and named after him. In the colour temperature scale, the higher the temperature, the whiter the light. Light colours are divided into three or four groups that cover the following ranges:

  • warm/soft white is usually perceived as homely and comfortable; it corresponds to colour temperatures under 3 300 K, and appears yellowish to reddish
  • bright white, over 3 300 to 4 000 K
  • cool white, 4 000 to 5 000 K. Bright and cool white are more suitable for working environments
  • daylight white light corresponds to temperatures over 5 000 K and appears bluish

Incandescent bulbs fall into the first category; fluorescent tubes, CFLs, and LED lamps can be found in different colour temperatures ranging from warm to cool white. Another important factor in evaluating lamps is colour rendering, the ability of a light source to reproduce faithfully the colours of various objects under a reference light. The CRI (colour rendering index), also often described as colour "accuracy" on packaging, is the quantitative measure of that ability, with 100 representing the best possible accuracy. Incandescent bulbs have a CRI of 95-100, that of CFL and LED lights is lower: 80-85 and 80-90 respectively.

All these characteristics are important as they determine the kind of lamps suitable for a set environment. For instance, the wrong kind of light in a shop may make food look unappetizing.

Many factors other than lumens, colour temperature and CRI influence our perception of light in different environments and make comparisons between different types of lamp difficult.

Consumers will need time and some research to find the bulbs that correspond to their environment, needs and taste. Proper labelling should help ease the task. That being said, the efficacy (lumens per watt) of LED modules keeps improving and is expected to reach 160 lm/W in the next two years and forecast to increase gradually to reach 180-220 lm/W eventually, requiring adjustments to the equivalence table in the future.

ge_cfl_label_org Manufacturers give the watt / lumen equivalents on packaging (Photo: GE)
tech-4_light-bulb-chart_ftc_lrg Consumer and official websites provide charts to help consumers find the right product
philips-led-bulb_org To add to the confusion, equivalent values are different for LED bulbs (Photo: Philips)