Watt the..... a rough guide to LED output and colour.

Posted by Gordon Bush on

Once upon a time, light bulbs (or lamps as those of us in the industry rather pedantically at times, refer to them) were a simple thing. Incandescent (the ones with the curly filament) were available in 15, 25, 40, 60, 75, or 100 Watt (with a few exceptions) and we all knew what this meant!
Now bear with me please as things may seem a little confusing but I will do my best to pull it all together at the end.
Well, along came new regulations designed to reduce energy emissions and along with them the importance of Lumens and Kelvins came very much to the fore.
So, what does all this mean?
Let's start with Watts, and by start I mean forget everything you probably associate with the word. Wattage does not denote the brightness of your bulb,lamp,bulb (lets stick with lamp).
Watts are a unit of power and tell you how much energy your lamp is consuming. None of this really mattered with incandescent lighting or even the standard fluorescent tubes we are all so familiar with. But with the increase in Halogen, Compact Fluorescent and then LED replacements this has not remained the case.
For example if you look at three different brands of the same style of LED you will often find that the Lumen output varies. Quite often the same manufacturer can have variations at the same wattage depending on the colour temperature (Kelvins, but we will get to that in a minute). 
Lumens (luminous flux often shortened to lm) is how we measure the actual amount of light visible to the human eye. So, the higher the Lumens the brighter the lamp.
In addition to this we have useful Lumens, now this is a rather technical and jargon packed piece of legislation but in a nutshell in the early days of LED some unscrupulous manufacturers were making claims about their Lumen output that just didn't stand up. Yes, often the lamp was producing the stated number of Lumens but the light was being thrown in all directions meaning that at the level at which you were sitting trying to read your paper it was still dark.
Useful Lumens a requirement brought in by the EU in 2012 means that the quoted useful Lumens must fall within a  90° cone so that the Lumen output quoted is what you are getting on your newspaper and not being wasted lighting up the ceiling. 
There is a lot more to this but that is a basic outline.
On to Kelvins now, the Kelvin is a measurement of "colour temperature" which has it's origins in the temperature at which the metal filament of your lamp was heated and the colour it gave off as a result. Now while LED and Fluorescent lamps do not work in this way the colour temperature is still measured in Kelvins to offer a modicum of continuity in a constantly changing industry.
So, let's now try and pull all this together in a way that you can use when selecting the right light source for you.
Below is a table showing what you would be most likely familiar with in terms of watts and the Lumens you need to look for in order to best match it.

Incandescent Wattage  Equivalent Lumen Output Required
25 Watt 200-250 Lumens
40 Watt  400-450 Lumens
60 Watt  700-800 Lumens
75 Watt 900-1100 Lumens
100 Watt 1300-1600 Lumens
This is an approximate guide for you to best pick your Lumen output.
All good LED lamps will quote their Lumens the wattage is just for energy consumption and it's worth considering the money you can save on energy when replacing for example a 60 watt incandescent lamp with a 6 watt LED. Now as we have brought up the subject of money it's also worth considering the cost of an LED at this point. In many cases an LED will cost you more than its less efficient equivalent however not only will they consume less energy they should (as long as you buy a good LED) outlast your old incandescent or indeed fluorescent comfortably.
Okay so the last piece of this puzzle is our friend the Kelvin (as illustrated above). Now as a measurement of temperature you'd think the higher the number means the warmer the lamp right? No, it doesn't in fact the exact opposite is true (don't shoot the messenger it wasn't my idea).
Daylight 4600-6000K 
Cool White 3100-4000K
Warm White 2000-3000K
*K = Kelvins
Daylight is the brightest in appearance, many people find this somewhat harsh for day to day home usage but it is ideal for detailed tasks and work as it provides a very clean and clear light. It can have almost a blue hue to it.
Cool White is the next step and while still good for tasks requiring attention to detail it doesn't provide quite the level of clarity of the daylight lamp. This colour is closest to white in its appearance.
Warm White is often preferred for day to day lighting as it is the closest in appearance to the old style incandescent lamps many of us grew up with.
Final point here is that colour temperature (remember Kelvin) can have an affect on Lumens. Often the same wattage lamps from the same manufacturer will vary slightly with daylight offering more Lumens than cool white and in turn cool white offering more than warm white.
There are more variations in Kelvins and so many descriptions to go with them, true daylight, soft white and very warm white to name a few, but hopefully I have given you enough information here to find your way to a good light source for you (and not so much that you got bored and went to do something more interesting).
Anyway, I hope that has made things a little clearer (6500K gag there) but if it hasn't then ask away and I will try and help.
The thing to remember is this, buy a quality LED and you can trust the Lumens!
Wattage never denoted the brightness, it was always Lumens we just never used them day to day. 

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