How to the make the intenisty of RGB LEDs as same?

[Narmu]

Hey guys, can anyone help me with in resolving this problem?

I'm using three LEDs namely Red, Green and Blue for my project. I have to drive these LEDs at a particular constant current where these three LEDs emit light at the same intensity. I tried characterizing these LEDs with a LUX meter. My idea of determining its current was the point where all the three LEDs intensity meet. But, when implemented, it dint happen. Green's intensity was too low. Intensity is very important in my project. Kindly, someone help me in solving this.

 

[kam1787]

**broken link removed**

 

[Narmu]

so, what is it?? im not able to understand it... can we make the intensity of the red, green and blue led as the same? if so, how? and if not, why? please explain me in simple words bcoz im not that great in understanding physics...

 

[Audioguru]

Can't you see that the graph shows that the sensitivity of your eyes is different for different colors?

White light contains all colors. You can make white by mixing red, green and blue lights in different amounts.

 

[Narmu]

Yeah, i can understand that. But my question is, I'm driving all these three LEDs with a single current source of 20 mA and switching is done to sequentially pass the light into the sample and the transmitted light will be detected at the other end. But with 20 mA current source, each LED produces a different intensity when measured using LUX meter. I have to make the intensity of the LEDs same when driven by a single constant current source. For that the current has to be decided based on where the intensity of all these three LEDs matches. Now, when I characterized all the LEDs by varying currents, these three plots that i have got doesn't match at a single point where the current needs to be fixed. Is there any other way to make the intensity of these three LEDs same?

 

[FvM]

In simple words, you need to adjust the individual LED currents to get same luminuous intensity (according to eye sensitivity curve). LED datasheets are usually showing the eye corrected intensity (luminuous intensity in cd), so you can calculate a rough current ratio. But RGB LEDs have also intensity variations between production lots, e.g. 1:2 ratio.

In a first order, the intensity of all three LEDs will linearly vary versus current, so the intensity most likely won't met for any current. There will be however a slightly different curve for each LED colour.

 

[Narmu]

Is it something like, I require three different currents to drive these three LEDs in order to make the intensity same? I'm measuring its intensity using LUX meter. Is it accurate?

 

[FvM]

Yoa are asking for the Lux-meter specification which we can't know. You should check the subjective uniformity.

 

[kam1787]

Is it something like, I require three different currents to drive these three LEDs in order to make the intensity same?

yes. you have to correct for the different efficiency of the LEDs and adjust for the perceived intensity,