Measuring Light Bulb Brightness

When we just had incandescent light bulbs, getting a good indication of  the brightness of the bulbs was quite straight forward. People just used to use the light bulb wattage as the brightness measure. So people would say, ‘this room needs a 60watt bulb’ or ‘this room needs a 100watt bulb’. Importantly, wattage is not a measurement of light output it is a measurement of power consumption, but as all light bulbs used the same technology this measure proved to be a good brightness indicator. In fact, it almost turned into the universal measure nearly achieving the level of accreditation that a British Standard would!

Now this is all well and good when you only have just one technology, but the plot thickened with the emergence of the fluorescent tube because an 18watt fluorescent tube produced far more light output than a 100 watt incandescent light bulb. Clearly we now need to understand light output in more detail and use more sophisticated light measurement techniques now that we have a spectrum of different light bulb technologies. As an example a 60watt incandescent bulb will produce the same light output as a 15watt CFL but it may only require a 40 watt halogen bulb or a 10watt LED to produce the same light level in a room! The other complexity is the light bulb cover. With traditional bulbs a clear bulb would produce more light than a pearl one and in a similar way, with modern CFL light bulbs a 15watt globe will produce less light output than a 15 watt spiral CFL. This is because the global bulb is manufactured by producing a spiral CFL and then putting a glass global cover on it to give the aesthetic design character. Obviously the cover will absorb some light a and as a consequence the brightness will be reduced.

The Lumens light output measure

The simplest and arguably the best standard measurement of light output is the lumens measure. This is a straight forward measurement of the light intensity as perceived by the human eye. This means that it is a very good light output measure as it is adjusted to take into account the different sensitivity the human eye has to different wavelengths. So in summary, the lumens is a measurement of the total light output from a source and generally the base way to compare the light output from from different light bulbs.

Maeasurement of Light Level in a Room

Quite often you need to specify a light level within a room which is needed for the purpose that that room is intended for. As an example in an office area or a workshop or indeed in a kitchen the level of light is clearly fundamental, This can be specified by stating the lux value which is a known as the illuminance. Lux is simply a measure of lumens flux on a particular area. So as an example if 500 lumens were concentrated onto an area of 1 square metre then this would correspond to an illuminance level of 500 lux. It is not a straight forward process to convert from lumens to lux as the lux illuminace level will be a function of a whole range of factors, for instance, the geometry of the light bulb, the wall material and  the colour of the room. Architectural lighting specialists or lighting suppliers  will typically have formulae to provide guidence on what type of light units and lumens output will be needed to provide a lux level within a room.

The Candela

MR16 CFL Bulb Certain types of bulbs like spot lights produce a directional output and certainly with reflector bulbs the intensity of the light will be a function of the angle of the reflector and the corresponding beam. This is where the candela (cd) unit assists as it is a measure of luminous intensity, which is basically the brightness of the beam in a particular direction. If a light source produces an output of 1 candela uniformly in all directions it will produce 4 Π , which is approximately 12, lumens output. By example if  this was restricted to only produce output across half the area, a hemisphere, but still at 1 candela it would only produce half the lumens output. This means that this measure is particularly useful in measuring the lighting effect produced by light bulbs with reflectors, LED MR16 Bulbs and the  halogen spot . If the angle of the reflector is reduced the light intensity and accordingly the candela (cd) value will increase, but the lumens value would remain the same.

3 thoughts on “Measuring Light Bulb Brightness”

  1. Nice post! On the subject of light bulb brightness don’t use incense bulbs as lamps or generally where they can be touched, you can get a nasty burn from the heat generated! In fact don’t touch them at all, use LEDs, they don’t get nearly as hot. Incense bulbs actually get so hot I’ve heard that if one falls shortly after/during them being on they can start fires! Another reason why energy saving light bulbs are brilliant! ;)

  2. This is a very good post. After reading it, I moved onto you other posts (lazy Saturday browsing) and they are all fantastic! After reading them I checked out your site and I love it! All that money I have been throwing away, solved just with efficient bulbs. I don’t meant to advertise (that the companies job!) but I recommend this the site; Saving Light Bulbs to anyone. Not just for the Energy Saving Light Bulbs, you can get them from other places, but for the low prices! Thank you for saving me money!

  3. OK; I get what you said : 1000 lumens at 1 meter = 1000 lux, and
    1000 lumens at 2 meters = 250 lux, and 1000 lumens at 4 meters = 62.5 lux. Consider a 100w flood that puts out say 1000 lumens. And the distance to a photographers subject is 4 meters; so the subject is getting 62.5 lux. Why are there low cost lux meters on Ebay that measure 50,000 lux ! and the good ones measure 200,000 lux !! Where on Earth would you need even a 50,000 lux meter !?
    I thought one of these $15 meters would be good for measuring lights (for purposes of changing to compatible LED lights); but when I see things like 50,000 lux, I wonder what I’m not getting here ? If I measure the light at 1 meter and get 1000 lux, am I not looking for a LED light that is rated at 1000 lumens ?


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