Does LED clor make a difference in the brightness of edge lit signs?

[knight_toolworks]

I am playing with a new design and the sample sign I made with the same leds but red seem much brighter then using while leds in another sign with the same logo. I don't have any more leds right now to check things out.

[steve_g]

Steve

Not all wavelengths of light are equally visible, or equally effective at stimulating human vision, due to the spectral sensitivity of the human eye. The overall luminous efficacy of a source is the product of how well it converts energy to electromagnetic radiation, and how well the emitted radiation is detected by the human eye.

http://en.wikipedia.org/wiki/Luminous_efficacy

SG

[knight_toolworks]

I am trying to work out if the surface treatment is causing problems or just the color of the light. edge lighting is different so I don't know much.

[joe]

Yes, that's right, red is considerably brighter than white. I say that but there are some new white LED's that are much brighter. If you call your supplier they can fill you in.

Joe

[paul_z]

The most likey cause of the brightness difference is that the white LEDs need about 2.7 volts per led to start emitting. The red LEDs start emitting at a much lower voltage. If you used the same current limiting resistor for both white and red, the red will appear to be much brighter (even though the human eye precieves red "poorly" http://www.ndt-ed.org/EducationResources/CommunityCollege/PenetrantTest/Introduction/lightresponse.htm)

If you have a dc milliamp meter (usually on all mulitmeters) measure the current going through the leds and adjust the current limiting resistor so that the leds are running at half their rated current.

Paul Z

[knight_toolworks]

Ok this gives me the info I need. I just wanted to know if I was on the right track before I started spending more money. I plan on using higher wattage LEDS.
what about blue and green and yellow?

[paul_z]

What a coincidence … I just stumbled across some test results I did on relatively high efficiency LEDs.
For each LED color I measured the voltage when the current through the LED was 10 ma.
Red 1.8V
Yel 1.9V
Green 2.9V
Blue 2.8V
White 2.8V
A couple of notes
1. If I remember correctly, there is no truly green LED. Instead, an LED of shorter wavelength (higher energy photons such as blue) shines on a phosphor in the LED which then glows as green. That is why the green voltage is higher than one would expect.
2. The white LED is similar except multiple phosphors are used to make an approximation of white (broad band) light.

Paul Z

[knight_toolworks]

yes this works I had to tear apart my last sign but the red is much better. going to high output leds should take care of it. I was worried my face treatment would be a problem. well it cam be inf the cuts are very deep.
http://i154.photobucket.com/albums/s266/knighttoolworks/posting/136dcb92.jpg
http://i154.photobucket.com/albums/s266/knighttoolworks/posting/d4db3a12.jpg