Luminous flux of 0603 and 1206 LEDs with the same color and current rating

I am looking at SMD leds, and it seems that same leds of same color in 0603 and 1206 cases, having same rated current and same angle of irradiation, show different light flux, 1206 being several times brighter.

What is the explanation for this? My task is to get maximal brightness out of bunch of blue SMD leds. 0603 are smaller and cheaper, so for the same money I can place 3x times more of them, while 1206 have higher rated mcd flux.... Hi-power 1-3W leds which are rated in lumens does not work for me, as I would need them on flexible PCB.

I am puzzled... Any info on that?

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Can you please provide a few datasheet links to LEDs which are the same in 1206 and 0603 but have differnt flux. Also on what do you base the assessment of "same" - same part number, same binning, same ...? –  Russell McMahon Dec 18 '11 at 11:45
Could it be that they are measured per pre-defined unit of area? So an LED a third the size will provide a third the light, but is the same rating as it covers a third of the area? –  Majenko Dec 18 '11 at 16:30

I think the problem is a lack of understanding of what one means by "brightness". Brightness is a very ambiguous term. First of all, I'll make the assumption that you are speaking in terms of a human's perception of brightness. This limits our discussion to photometry. You have to now differentiate between two measures of brightness, i.e. Luminous intensity and Luminous flux (or Luminous Power). Luminous flux is the light power emitted in all directions and has SI units of "Lumens". Luminous Intensity is the light power emitted in one solid angle which has SI units of "candela". Think of a solid angle as a cone with it's vertex at the led emitter. You mentioned mcd as units, these are millicandela, so mcd is a measure of Luminous Intensity. You can't directly compare luminous Intensity (candela)) with Luminous Flux (lumens). I did find this tool to convert between the two units, but, it must have to make the assumption that the light is omnidirectional and is only accurate for such lights.

You'll have to get your units straight before you can make sense out of this.

In additon, you have to decide what your objective is. Are you trying to make a light with high luminous Flux or one with high Luminous Intensity? I would imagine that you are going for a light with high luminous Flux or you probably would be speaking of laser diodes instead of LEDs.

So choose whatever package that maximizes your Luminous Flux, i.e. Lumens. You did not provide enough information to give a better recommendation than this. That is, you did not specify any additional design constraints.

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It seems the issue was lack of such understanding by Chinese manufacturer. I've checked datasheets of reputable manufacturers, and flux is about the same for all chip leds. –  BarsMonster Dec 19 '11 at 8:54