Light measurements

Martin Vlietstra, one of our frequent contributors, has written an article which he says deals with the metric system in use rather that looking at metrication per se.

In the last few years, we have seen a big change in the way in which units of measure related to lighting are expressed. Although they have been expressed almost exclusively in metric units for almost a century, there is a general degree of ignorance as to what these units actually mean.
Those of us who have been responsible for replacing light bulbs in our own homes for some time are familiar with the terms “100 watt bulb”, “60 watt bulb” (with lesser power bulbs for use in torches). What does “100 watts” means? Most people are aware that light bulbs get hot when they are in operation. A “100 watt bulb” consumes 100 watts of electric power. Some of this power is converted to light but most to heat. During the last decade, there has been a move to using alternative technologies for light generation with the aim of reducing the amount heat (and also invisible infra-red light) produced by the light bulb. This has in turn led to an expansion of the detail used in the bulb’s specification.
The headline specification of a light bulb is the amount of visible light generated (measured in lumens – symbol lm) rather than the amount of power that it consumes. As a rule of thumb, a filament or incandescent light bulb that consumes N watts will generate about 15 times N lumens while a light-emitting diode (LED) that produces the same amount of light will only consume a seventh N watts. The additional power consumed by the filament bulb is given off as heat. Dimensionally lumens are identical to watts, but unlike watts are defined with reference to the sensitivity of the human eye to different colours.
Many lampshades have a warning such as “Maximum 40 W”. This refers to the actual power consumed by the bulb in question rather than the amount of light given off. If you are using a 40 watt filament bulb (no longer available), the maximum light that you can safely get will be about 600 lumens, but if you switch to LED technology, you can use a light bulb that gives off considerably more light as long as the input energy (and hence heat plus light) does not exceed the limit.
Anybody who has watched a blacksmith at work will be aware that the iron with which he is working is “red-hot” (about 1000 K which converts to 727°C). The temperature can be judged from the colour of the glow of the metal. This phenomenon is the same as in a tungsten filament bulb except that the filament bulb, which uses a tungsten filament rather than an iron filament, operates at 3500 K, giving a yellow-white light. If you dimmed a filament bulb, you might have noticed that it turns more yellow. The reason is that it is consuming less power and is not getting as hot. A brilliant white light would theoretically be generated by a filament bulb running at 6000 K. In practice it is not possible to manufacture a filament bulb that operates at this temperature as all known metals melt well below this temperature, but other technologies exist that can create light of almost any colour that is needed. You will often see the “temperature” of a light bulb quoted on the packaging – this tell you whether you a buying a “soft” light or a “harsh” light, but does not refer to the actual temperature of the light. For the benefit of readers who have not studied studied physics (or who have forgotten what they learned), temperature can be measured in SI units using either kelvins or degrees Celsius. “Zero kelvins” is better known as “absolute zero”, the lowest temperature that can theoretically be attained. It is equivalent to about minus 273°C.

8 thoughts on “Light measurements”

  1. A rummage in my store cupboard, produced the following:
    a dozen filament bulbs of assorted wattage (energy rating E?);
    two candle light bulbs (“40 W, 390 lumens, energy rating E”);
    some Philips energy savers (“11 watt energy, light 60 watt, 600 lumen, energy rating A”);
    an Osram energy saver (“8 W ->40 W, 400 lm, 2700 K warm white, energy rating A”);
    a box of GU10 halogens (“50 W, 400 lumen, colour temp 2700 K, energy rating D”);
    no LED.
    Thank you, Martin, for an explanation of what this all means.


  2. “A “100 watt bulb” consumes 100 watts of electric power. Some of this power is converted to light but most to heat.”
    This is wrong. All of the energy is turned into heat. There is no energy “consumed” or “used” to produce light as there is no energy used to produce magnetism. Light is produced as an effect of using an electric current to heat the filament. The filament gets so hot, it glows and emits light. Magnetism is produced as an effect of moving electrons in space or a conductor. When we say energy is consumed or used, it means energy changes from one form to another. The energy used to heat the filament results in the filament releasing photons. The energy in the form of the product an electric current and the voltage is changed to energy in the form of heat.


  3. Thanks Martin.
    Regarding lumens, I usually resort to conversion charts when trying to decide which LED lamp I need to replace an old tungsten lamp. The charts seem to vary depending on where you look so it was good to read that there is a formula for converting (tungsten lamp) wattage to lumens.


  4. Modernman, there is no direct correlation between watts and lumens. Depending on the manufacturer, different bulbs of the same power can have different brightness. A standard A60; 60 W bulb normally has a brightness of 800 lm. But, that isn’t written in stone.


  5. This is an interesting article and will be useful to many. One point that hasn’t been made is that the conversion from energy use to lumens varies over the lifetime of the bulb for the old incandescent types. So a so-called 60 W incandescent after a while appears less bright and the actual light rating should reduce over time. The 600 lumen figure that is often quoted is presumably some sort of average. By contrast an LED does not change its light output in time, and initially may seem quite dull compared to a so-called incandescent equivalent.


  6. @Daniel. From the point of view of the physicist, you are quite right when you say that energy is neither “consumed” nor “used” but is transferred from one form to another. However from an economic point of view it is useful to consider energy that is being taken from a supplier as being “consumed” by the person who buys that energy.
    If we consider a lamp to be a “black box” (ie something whose interior workings are of no consequence to use), then we see electrical energy going into the lamp electromagnetic radiation coming out of the lamp. Radiation that has a wavelength of between 400 and 700 nm is considered as visible light and radiation with a wavelength of greater than 700 nm is considered as heat energy. In the case of a filament lamp, the energy transfer is in accordance with Planck’s Law. In the case of LED’s the light that is generated is the result of excited electrons decaying back to their original levels within the atomic nucleus.
    For more information please visit:


  7. I had an e-mail from a reader who asked why I had not written anything about the candela. The answer in short is that I was trying to restrict the article to the units of measure that appear on the lamp’s packaging. I will now try to explain the difference between the two.
    The lumen is a measure of the total amount of luminous flux (or luminous energy) emitted by a lamp. The candela is a measure of the luminous intensity (amount of light flux emitted in a particular direction) and is normally qualified by specifying the solid angle (often in steradians) for which the figure holds. One candela is defined as one lumen per steradian. It should be noted that as one moves away from a lamp, neither the luminous energy emitted, nor the luminous intensity change (assuming no adsorption by the atmosphere).
    The lux is a measure of illuminance and is defined as one lumen per square metre. Unlike the luminous flux or the luminous intensity of a lamp, the illuminance decreases as one moves away from the lamp. This decrease follows an inverse square law.
    These and other definitions associated with photometry can be found in Wikipedia (


  8. Yet again, this is another example where a lack of direction on measurement and metrication over decades from successive governments has resulted in confusion to the general public.
    When we purchase a lamp (light bulb), it is a certain amount of ‘light’ that we require.
    When the first compact fluorescent lamps were sold in the UK over thirty years ago I wrote to the relavent government department to suggest that the information on packaging should be changed to make lumens the prominent unit and also to make the products in rational numbers of lumens, eg 250, 500, 750 & 1000. Unfortunately, nothing happened and the manufacturers were very timid in introducing the lumens unit on packaging and instead used phrases such as ’60 W Equivalent’. This mixed messaging resulted in the general public not fully understanding the potential savings in energy costs by using these types of lamp that had a higher initial cost. In the public’s mind, Watt is a unit of light and it is now proving very difficult to re-educate them to start to think about the amount of lumens they want from a device or lux they may need for a particular room or area.
    It’s just the old story of government not trusting people to be able to learn new and better ways of measurement.
    In my view, in this case, it delayed for many years the introduction of more energy efficient lighting resulting in a massive waste of energy/ fossil fuel, thus adding to the climate change problem.
    Martin refers to the maximum Wattage for lampshades and doubtless, the slow introduction of ‘cooler’ lamps would, over the years, have lead to large numbers of house fires that could have been avoided had we used these products earlier.


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