How quickley does a fast start energy saving light bulb come on?

Some Background to CFLs?

I am sure everyone  remembers the old energy saving light bulbs which seemed to take an eternity to come on. People were often reluctant to turn off their lights because they knew that they would invariability take several minutes to come on. Now the situation these days has significantly improved but I though that I would try to add some statistics around this issue.

The Visual Aspect

Light level is measured in LUX but it is the humans brains interpretation of this fact that is critical. For example with dimmable LED candle bulbs sometimes you can dim them down and think that  they are still quite bright. However when you utilise a LUX meter to see what percentage of the original brightness they are dimmed to you normally find that the level is much lower than the brain interprets. For example where people say that the lamp has dimmed down to 30% of its original brightness then by measuring the LUX values you will find that it has dimmed  down as low as 10%.

At the other end of the scale there is the fast start energy saving light bulb issue where it does take some time for a compact fluorescent lamp to reach its full brightness. However with modern ones then visually when you turn them on you think that they are bright immediately. However when you actually measure the  LUX levels you find that although the light level is good there is still a period of time to reach maximum brightness. This lag in time is not relevant though because to the human eye the brightness is adequate even though after a minute or so it is further enhanced.

In order to illustrate his fact I have taken some measurements you apply a level of science to the matter.

LUX levels of energy saving light bulbs during warm up

The first bulb analysed was a Philex 25w bulb purchased a few years ago. This bulb was removed as it took too long to warm up! The LUX levels  below taked during warm up  illustrate what this means. The figures are the percentage of brightness reached after 5 minutes.


  1. 16% after 10 seconds
  2. 20% after 30 second
  3. 45% after 1 minute
  4.  81 % after 2 minutes
  5. 89% after 3 minutes

The comparison has been performed against one of the SLB 20 watt spiral energy saving light bulbs.


  1. 61% after 10 seconds
  2. 78% after 30 seconds
  3. 91% after 1 minute
  4. 95% after 2 minutes
  5. 99 % after 3 minutes

 Interpretation of these results

The interesting thing for me was the fact that although the SLB spiral bulb appears to come on instantly then in reality then the LUX level is only 78% of the final value after 30 seconds but in reality this is fine.
The obvious comparison to  this is the older bulb which only reaches a figure of 20%.

So this is the price we pay for saving energy but this electricity saving is very significant, but what I will say is this debate will only be with is for a few years because the LED prices are dropping and these reach full brightness instantly.

2 thoughts on “How quickley does a fast start energy saving light bulb come on?”

  1. I find this all so frustrating. More than ten years ago I had two CFL bulbs in my bedroom that came to full brightness in no more than a second. I never managed to buy anything as quick again. Now we seem to be thinking that 61% after 10 seconds is GOOD. I went to my Homebase this week with the empty box of a bulb that started in a few seconds, but they don’t sell them anymore. Nothing on their shelves promised less than 120 seconds. And no, I don’t want to spend £11 on one LED bulb. Dave Garner

    1. Hi David

      YOU just need to wait a short while. We have just got some LED filament GLS bulbs in which are very cost effective.

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