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I am supposed to use an light having the following specs. as described in the below statement.

"A red light, equivalent in its luminescence to a 21 W light bulb is to be used."

On an average how much light does an 21 W light bulb emit (in luminescence)??

I am planning to go with a array of high power LED's as it is more energy efficient.

If its LED what should be the power rating of led to give enough luminescence as described in the above statement.

If not LED is there an better option provided it is economical and energy efficient.

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    \$\begingroup\$ That's kind of a loose specification. Is it supposed to be visually perceived as similar brightness? Does the 21W light bulb have a red filter in this comparison? LEDs tend to be directional - is there a reflector on the 21W bulb? If it's visual perception you can use lumen numbers for comparison. \$\endgroup\$ – Spehro Pefhany Oct 26 '15 at 15:35
  • \$\begingroup\$ Incandescent bulbs produce around 20 lumens per watt. \$\endgroup\$ – HighInBC Oct 26 '15 at 15:41
  • \$\begingroup\$ so it it around 400 lumens? @HighInBC \$\endgroup\$ – Raaghul_ Oct 26 '15 at 15:46
  • \$\begingroup\$ If I'm using 1W LED's how many should i use? \$\endgroup\$ – Raaghul_ Oct 26 '15 at 15:47
  • \$\begingroup\$ You could find the photon conversion efficiency for the LED. Or find an LED that lists its light output in lumens also and there is your answer. \$\endgroup\$ – MadHatter Oct 26 '15 at 15:52
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To find the luminescence (in lumens) of a 21W bulb, you can multiply the power in Watts by the luminous efficacy (lm/W). The efficacy for a typical incandescent bulb will be about 15 lm/W, so you're looking at about 315 lumens.

Then it seems you are trying to find the amount of LEDs that will provide the same luminescence. If the LED's datasheet has the luminous efficacy, you can use that. If it lists the luminous intesity in candelas (cd, or LED's are often also measured in mcd), you can use the relation Lumens = 4Pi * Candelas. The value 4Pi assumes the emission of light is uniform in all directions.

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