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I'm considering building some crown moulding lights with LED strips. I'd like to get light equivalent to a 30W spiral CFL bulb (package says 1800 lumens) plus a bit more, so let's say 2500 lumens. This is for a home office.

I want high wattage and color adjustable. Found some 24W strips. Is there a way to determine the lumen output per ft? They are IP20 if that matters.

I found this table from another question here but I'm unclear on how to translate that into lumens/foot for an LED strip.

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    The only reliable information is going to be specific to the particular strips, as efficiency varies considerably across manufacturers. May 16 at 20:59
  • If you installed one strip and found it too dim, you could just keep adding strips until you were happy with the brightness. May 16 at 21:32
  • Not a great solution, that means cutting the strips up etc and I might not be able to return them in that state.
    – jcollum
    May 16 at 22:39
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    you may be on the wrong track with counting lumens ... the brightness of a room lit by an LED strip will look different from the brightness of the same room being lit by a CFL ... probably the easiest course of action is to determine the required number of strips experimentally
    – jsotola
    May 16 at 23:43
  • If I cut them up I don't think I can return them so surely there's some way to figure this out before I cut them down to size.
    – jcollum
    May 22 at 20:57

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Lumen-per-watt efficiency varies wildly by the quality of the LED. It also varies by the current through the LED - LEDs are more efficient at lower current, and less efficient when driven near their thermal maximums.

The data sheet for the LED strip will plainly state the lumens per segment at any given voltage. It will also state the current per segment at any given voltage. You can multiply current x voltage to get watts at that voltage, and then look up lumens at that voltage and compare.

These types of "LED strip" are rated by voltage (rather than current as a raw LED would be) because they are actually "segments" of 3-6 LEDs in series with a resistor. LEDs have a very steep current/voltage curve, so small changes in voltage has big changes in current, and thus power, and thus lumens. The resistor dampens this somewhat, so the changes are not so extreme. Generally, LED strips are sized so that on automotive battery float voltage with charging system operating (14 or 28 volts), the LED's power is limited to that which will be safe without an added heat-sink. LED strips are generally not used with heat sinks.

The cheapest LED strips may not have spec sheets, because they are opportunistically using whichever LEDs nobody else wants to buy, so the spec changes every batch. In which case you're on your own!

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  • huh well can I measure the lumens with a phone app maybe?
    – jcollum
    May 16 at 22:40

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