LED displays: why two red LEDs but only one green/blue for full colour?

hatrickpatrick

Figured this was he best forum to ask in: was at a nightclub on Saturday night (Krystle in Dublin) and one wall is covered by a gigantic LED video display, on which they can show words, clips from music videos, random colour shows, etc.

Was sitting next to this screen and noticed something odd. Up close, although you get a perfectly full coloured display when you look at it from far away, when you actually look closely each of the pixels is odd: instead of being made up of three LEDs each as I would have suspected, each one has four - 2 reds, top and bottom, a blue on one side and a green on the other.

Anyone know why this might be? Are red LEDs dimmer and so need extra support in order to have full colour? Do our eyes perceive red less than green and blue? Why this imbalance? Had I been asked to build something like this without any prior instruction I would have made 3 bulb pixels with one of each colour - clearly I'm missing some facts.

Anyone know how this works?

mawk

hatrickpatrick wrote: »

Figured this was he best forum to ask in: was at a nightclub on Saturday night (Krystle in Dublin) and one wall is covered by a gigantic LED video display, on which they can show words, clips from music videos, random colour shows, etc.

Was sitting next to this screen and noticed something odd. Up close, although you get a perfectly full coloured display when you look at it from far away, when you actually look closely each of the pixels is odd: instead of being made up of three LEDs each as I would have suspected, each one has four - 2 reds, top and bottom, a blue on one side and a green on the other.

Anyone know why this might be? Are red LEDs dimmer and so need extra support in order to have full colour? Do our eyes perceive red less than green and blue? Why this imbalance? Had I been asked to build something like this without any prior instruction I would have made 3 bulb pixels with one of each colour - clearly I'm missing some facts.

Anyone know how this works?

Well green is dead center of our visible spectrum so we are more sensitive to it. And blue and especially UV diodes are more efficient generally. Maybe that is why?
But I would have thought to just drive the reds a little harder. Maybe having two reds is cheaper than a fancier control set up?

spideog7

Could it have been Amber rather than Red? LEDs are frequently RGBA, adding an Amber that allows for better yellow and white, and warmer less washed out colours.

Red is typically less perceptible and usually also has a lower forward voltage so consequently a lower power output at the same current, but driving it harder (ie greater current) make more sense than duplicating. Do more dull LEDs make an apparently brighter LED, I dunno?!