# Radiant versus Luminous Flux



## gilaLED (Jul 19, 2009)

I am new in LED, when I read the datasheet, May I know why some parts are measure using radiant flux while other are using luminous flux. May I know I know what is the different and why not standardise ?


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## 2xTrinity (Jul 20, 2009)

gilaLED said:


> I am new in LED, when I read the datasheet, May I know why some parts are measure using radiant flux while other are using luminous flux. May I know I know what is the different and why not standardise ?



Radiant flux = output power in watts. 
Luminous flux = also output power in watts, weighted by the human eye sensitivity function. This is defined as lumens. 

May I ask which LEDs you are referring to?

Usually LEDs in the visible-light spectrum, such as white, red, green or blue will quote luminous flux. LEDs that are invisible or minimally visible, such as deep blue, UV, infrared etc. will quote radiant flux -- because eye sensitivity is very low to those colors.

Usuaully radiant flux and luminous flux are useful for different reasons. In some case it may be desirable to have both. A white light source will almost always be used for human beings to see things with, so luminous flux (based on human eye sensitivity to the light) will be imporant. Things like UV or deep blue LEDs are often used to cure epoxy, or excite fluorescent dyes. In that case, how visible the light source is is not as important as how much power (hence, ability to perform those tasks I mentioned) it emits.


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## gilaLED (Jul 27, 2009)

Hi 2xTrinity,

Thank you for the explanation. The LED I am talking about is Royal Blue. Now I understand better.


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## thepaan (Jul 29, 2009)

This is an efficacy table for conversion.

http://hyperphysics.phy-astr.gsu.edu/Hbase/vision/efficacy.html

Royal blue is usually around 450nm so multiply the radiometric flux by .038 to get your approximate lumens.


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## LukeA (Jul 29, 2009)

thepaan said:


> This is an efficacy table for conversion.
> 
> http://hyperphysics.phy-astr.gsu.edu/Hbase/vision/efficacy.html
> 
> Royal blue is usually around 450nm so multiply the radiometric flux by .038 to get your approximate lumens.



Bookmarked!


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