Why the wide pitch on RGB LED displays?

[EricB]

I've been following RGB unitary LED technology for years, and you see some displays here and there. What I niotice is that the pitch (space between rows and columns of LED's) is very large: 4mm or more, large enough to fit another row or column between. Other signs, the LED's are all touching one another.
This has limited the RGB displays to large signs, and even then, most go with sRGB (separate colors). I haven't yet seen a small text sign, in RGB. They all still use RyG.

So does anyone know if there is any reason RBB's have to be so far apart?

[PhotonWrangler]

Two reasons that come to mind are expense and thermal management. Full RGB displays are generally used in pricey commercial installations where the screen is going to be large and mounted far away from the viewers. In cases like this a 6mm dot pitch blends into a continuous display from normal viewing distances, so as higher density is a waste of money. Smaller, closer displays are better handled by plasma, LCD and other technologies.

Thermal management is also important, from the standpoint of preserving the life of the LEDs as well as maintaining color matching from pixel to pixel. Hot spots would cause changes in LED conductivity and thus affect the brightness of individual diodes, messing with the overall color uniformity of the display.

[jtr1962]

Preventing color bleed may also have something to do with it. If the LEDs are practically touching some light may transfer between two adjacent LEDs, affecting color rendition. This occurs because even narrow beam LEDs have a little light spilling out the sides. You can prevent this by surrounding the LEDs with something opaque, but this increases cost, and may impact thermal dissipation.

[EricB]

Thanks.

I wondered if it might be some sort of "thermal" type issue, but wasn't even sure if LEDs generated enough heat for thqat to be a problem.
Still, with both this and the color spill issue, why don't the tri-color RyG's have this problem, and can be touching each other?
I do see where the RyG's often have different brightnesses in spots (often making the yellows either reddish or greenish in places). But these are small text signs, and it doesn't matter that much. But I don't see such small signs with RGB at all. It appears that they cannot be made with the finer pitch at all, restricting them to the large signs only; rather than the large pitch being used because of the largeness of the signs.

So the thermal issue seems to be the best explanation. Might a greater thermal condition on RGBs be because of the three dies as opposed to two or one; or does it have more to do with the GaN? (I just saw some apparent two-die RmB only LED displays, and they still had the wide pitch).