LED Question: Actual luminance vs perceived luminance

Using Arduino LEDs and Multiplexing
1

Hi all,

I made an LED array with SMT LED's. The circuit is 9 sets of one each, red, green and blue, each set in series and running on 12 volts through a 150 ohm resistor. The V forward of the LED's are:

red: 2.1V
green: 3.2V
blue: 3.2V

The series current for each set is, therefore 23.3 milliamps. See attached image.

Since each LED is rated for the same output luminance (brightness), I expected the board to produce basically white light. Yet, the light has a blue-cyan cast to it (even looks blue compare to daylight).

It seems as though the blue and green LED's are brighter than the red ones (although the specs say otherwise).

Anyone know why this is? Is the light really white, but looks blue to my eyes due to differences in wavelength sensitivity of the eye? Is the red not as bright as the other two?

Any ideas will be appreciated. I ultimately need to be able to get a balanced white light output from this board. This one is a test, but I plan to build a much larger one when I get it all figured out.

Thanks!

(click pic for full res)
led_array.jpg

2

The spectral purity of the primaries; the red, the blue and the green.
Is the red truly red or is it shifted to orange, and so on.

3

The human eye's sensitivity is not the same for all colours. It peaks at about yellow which coincides, surprise supprise, with the peak output of the sun.
So if all the LEDs are putting out the same amount of light then it will not be perceived as daylight.

Also the different forward voltages for each LED and using the same resistor ensures each colour has a different current so even if the spec says the brightness is the same for some fixed current that is not the situation you have.

4

Grumpy_Mike:
The human eye's sensitivity is not the same for all colours. It peaks at about yellow which coincides, surprise supprise, with the peak output of the sun.
So if all the LEDs are putting out the same amount of light then it will not be perceived as daylight.

Also the different forward voltages for each LED and using the same resistor ensures each colour has a different current so even if the spec says the brightness is the same for some fixed current that is not the situation you have.

THAT was kinda what I was afraid of. I really don't want to use individual resistors for each LED, nor do I have the room on the board to make each "set" all green, all blue and all red, then give each set a custom resistor.

Darn... I'm going to have to re-think this idea. Maybe I can purposely buy red LED's brighter than the G and B ones and fudge it...... :frowning:

5

Assuming the RGB LED's will be controlled using an Arduino then why not write a function that corrects the gamma curve for your particular LED setup.

6

I really don't want to use individual resistors for each LED,

That is puzzling, have you not got individual resistors at the moment?
You said:-

The circuit is 9 sets of one each, red, green and blue, each set in series and running on 12 volts through a 150 ohm resistor.

So have you got a R, G & B LED in series?

7

(12 - (2.1 + 3.2 + 3.2)) / 150 = 0.02333

8

OK Mr. Spock... 23.33333333333333333333333333333

9

Grumpy_Mike:
That is puzzling, have you not got individual resistors at the moment?
You said:-So have you got a R, G & B LED in series?

Yes. I did this.... the colors skewed by 1 place each time:

GND---RED---GRN---BLU-----////----12V (r=150 ohm)
GND---GRN---BLU---RED-----////----12V (r=150 ohm)
GND---BLU---RED---GRN-----////----12V (r=150 ohm)

10

Krupski:
OK Mr. Spock... 23.33333333333333333333333333333

You've got to be kidding.
I was answering someone else's question by recapitulating your figuring in the OP and you take it as a shiv?
I'd better go set myself on fire in the backyard.

11

Riva:
Assuming the RGB LED's will be controlled using an Arduino then why not write a function that corrects the gamma curve for your particular LED setup.

Right now, a red, green and blue are in series with one resistor. Can't individually control them yet.

What I wanted was to have the three colors intermingled as close as possible so that I would get white light. If I drive each color separately, I'll have to use one "block" of red, one of green and one of blue. Not enough room for all the resistors otherwise.

I've been thinking of trying this:

GND---RED---GRN---RED---BLU----////----12V (r=TBD)

....to get more red while staying with the series string. I don't need adjustable color, I need a reasonably good white (and no, white LED's are not white - as in full spectrum).

12

Sorry. I thought it was another one of those "snide" posts that CERTAIN people here make.

Didn't realize anyone questioned the simple Ohms law.

13

How would it look if you replaced one blue for a red and reorganized the pattern like this?
...or replace a green for a red to get this

...or put the reds in the corners, 3 greens in the middle row, remaining is blue. Then adjust the green resistor value to get white.

...or put the reds in the corners, 3 blues in the middle row, remaining is green. Then adjust the blue resistor value to get white.
Somehow, I'm missing the reason not to use white LEDs .

14

dlloyd:
How would it look if you replaced one blue for a red and reorganized the pattern like this?
...or replace a green for a red to get this

...or put the reds in the corners, 3 greens in the middle row, remaining is blue. Then adjust the green resistor value to get white.

...or put the reds in the corners, 3 blues in the middle row, remaining is green. Then adjust the blue resistor value to get white.
Somehow, I'm missing the reason not to use white LEDs .

Thanks for the idea. I think it's similar or the same to what I thought to try next. Instead of R-G-B, I was going to try sticking in another red, but keep the pattern repeating so that it shifted one for each line... like this:

R-G-R-B-R
G-R-B-R-G
R-B-R-G-R
B-R-G-R-B

I think this gives me the same "balanced" distribution of colors, but with more red.

As to your question about white LED's, those are just blue or UV diodes with phosphor on top of the die. Although they "look" white, the spectrum leaves a LOT to be desired.........

I suppose it would help if I explained what it's for... I want to make a "telecine" machine to transfer film images to digital. I plan to use an Arduino to control it all... the sequence will go like this:

(1) Index projector to the next frame
(2) Open the camera shutter (BULB mode - open as long as the shutter signal is applied).
(3) "Flash" the white LED array for the proper length of time to expose the image onto the camera CCD
(4) Close the camera shutter
Lather-rinse-repeat.......

Of course, the projector is "projecting" directly into the camera - no lens on the camera and the LED array provides the light (as opposed to the megawatt power lamp in the projector).

I have hundreds of reels of 8mm, super-8 and 16mm movie film spanning from the mid 1950's to now, and I want to transfer all of it to digital... and $425 per hour to have a lab do it is a bit much. Plus, it will be a fun project. I SHOULD be able to just load a reel, start the program and let it rip (oops "rip" is not the right word for film...) maybe "let it run".

:slight_smile:

15

dlloyd:
How would it look if you replaced one blue for a red and reorganized the pattern like this?
.

Update: Tried your idea... the color temperature is almost perfect daylight! The camera can white-balance any minute difference. FYI, here's the LED array, version 2:

(click pic for full res)
led_array_v2.jpg