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Humans looking at colored lights each at the same luminescence perceive yellow as the "brightest" color. Green is somewhat dimmer, and red is very much dimmer. Using a common formula for calculating mean luminance, 30% is red, 11% is blue, and 59% is green (R=645.16nm, G=526.32nm, B=444.44nm).

Theoretically, I should choose luminous intensities using ratios of Blue=90x, Red=33x, Green=17x, and I'm guessing, Yellow=15x.

Testing random LEDs from my junk box, a lux/fc meter with expired calibration, and two volunteers, Blue=152x, Red=143x, Green=23x, Yellow=15x.

The application is for indicators easily distinguished under office lighting. All will have the same package (T1,or T1.75), similar viewing angles (about 60 degrees), with the same lens (all clear/tinted, or all defused). They will be driven based on the data sheet's test current and luminosity curve.

I am assuming the all the other zillion factors like efficiency, encapsulation, geometry, orientation, etc. are specified constant (viewing angle, lens), or they are incorporated into the millicandelas (mcd) rating provided by the manufacturer.

1. Anyone care to comment on these ratios, based on real life experience?

My experimental red number calls for 3 times more luminosity than the theoretical one.

2. What mcd target values/colors would your experience suggest for LEDs viewed under office lighting?

My testing conditions can't give results in mcd, so my test data doesn't help. Once I get a suggested mcd for a color, I'll use the ratios to get the other mcd values. And of course, you can't buy exactly what the calculator says... so "rules of thumb" rule!

Update... ---> Think of a traffic light. What if drivers complained that the yellow lamp was too dim that the weren't sure when it was lit...?

I don't want my users saying they can always tell if the color1 or color2 idiot light is on, but they are never sure about color3 without getting their eye next to the device.

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  • \$\begingroup\$ What I have discovered is that for red LED's, a little current goes a long way. A red LED with 1 mA subjectively seems just as bright as a green LED with 3 mA or so. Blue LED's also seem brighter than green ones with less current. So, basically, if you tune a red, green and blue LED to have the "same brightness," I believe you will find that green needs the highest current, blue will be second, and red needs the least current. My assumption is that this effect is the result of poor luminous efficacy in green LED's. \$\endgroup\$ – mkeith Dec 30 '15 at 6:21
  • \$\begingroup\$ In real life, the colors that you're not getting with RGBA (amber) have an effect on the perceived color temperature. I'd almost hesitate to use blue LEDs at all if you're aiming for a color temp below 4000K. I'd suggest reading more on luminescence, as last I checked, green was the color we were most sensitive to. \$\endgroup\$ – Dave Dec 30 '15 at 8:02
  • \$\begingroup\$ There is an excellent article on colour ratios that gives good background for achieving equal perceived intensities at ledsmagazine.com/articles/print/volume-10/issue-6/features/… \$\endgroup\$ – Peter Smith Dec 30 '15 at 10:36
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    \$\begingroup\$ Related, but from the opposite perspective : electronics.stackexchange.com/questions/203264/… \$\endgroup\$ – Brian Drummond Dec 30 '15 at 11:15
  • \$\begingroup\$ @Peter Smith. Read the article, it was excellent but not relevant because I'm not trying to color match/balance. \$\endgroup\$ – HiTechHiTouch Dec 30 '15 at 21:21
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I finally got my hands on some known diodes (as opposed to grab bag stuff) and adjusted them to equal perceived brightness (3 human observers) at a comfortable intensity for viewing from 15 feet (4.5m) away under good office lighting with the desk task light on.

Led part    (Vcc=5.15v)          nm   mCd  Vf@mA    Ohms   mA   Vdrop  E-mCd
WP3A10ID    HE Red (GaAsP/GaP)   627   30  2.0 20    470  1.0    1.84     3
WP132XGD    Green (GaP)          565   30  2.2 20    470  1.0    1.84     3
WP132XYD    Yellow (GaAsP/GaP)   590   30  2.1 20    360  4.3    1.95    13
VAOL-3LSBY1 Blue (InGaN/Saphire) 470  700  3.5 20    33K  0.2    2.47     7

nm, mCd, and Vf@mA are from the data sheet.

The last columns are the resistor I used, the calculated current, the voltage drop across the diode, and the calculated effective mCd at my reduced current. The supply voltage is 5.15V, and all diodes are T1 format with diffused 60 degree viewing angle lenses.

The blue diode was the lowest intensity (mCd) which I could find, and you can see I had to scale its current way back. All LEDs are well under-powered from data sheet values.

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  • \$\begingroup\$ How are you measuring the luminous intensity of these LEDs? It seems strange that the blue one has a much greater luminous intensity than the others when if they are the same perceived brightness then they should all be the same. Also, red is not 527nm; that's green. It should be 627nm in the table. \$\endgroup\$ – Oleksandr R. Feb 12 '16 at 11:55
  • \$\begingroup\$ @oleksandr The mCd figure is from the data sheet, and of course is only a relative number for comparing devices running full strength. The equal perceived brightness was determined by 3 human judges, and is empirical. Part of my confusion leading to the original question appears to confuse you too. {frown} \$\endgroup\$ – HiTechHiTouch Feb 13 '16 at 14:01
  • \$\begingroup\$ Oh, I see. You should probably delete this column from the table, then, as it certainly doesn't apply to your conditions. Considering that the datasheets do state how luminosity varies with current (etc.), you could perhaps use the datasheet to estimate what fraction of the full-brightness luminosity is actually being produced under your conditions. By the way, cd is lm/sr, so your measurements with the lux (=lm/m^2) meter may have been thrown off by different angular distributions or measuring distances. It should give the same number for all LEDs otherwise. Or maybe it's just a bad meter. \$\endgroup\$ – Oleksandr R. Feb 13 '16 at 15:40
  • \$\begingroup\$ @Oleksandr, I checked the data sheets, and the relative intensity vs current graphs are straight lines. Red, Green, and Yellow Relative intensities are 100% at 10 mA making the formula mA/10. Blue is 100% at 20mA, making its formula mA/20. This yields the Data Sheet RIs you are interested in as R=10%, G=10%, Y=43%, B=1%. These numbers are how much under the manufacture's ratings my operating points are, and have no correlation to the intensities perceived by my human test observers. (FWIW, multiplying by the mCd ratings give calculated mCd outputs of R=3, G=3, Y=13, B=7.) \$\endgroup\$ – HiTechHiTouch Feb 14 '16 at 14:42
  • \$\begingroup\$ It's somewhat interesting that the RGY datasheets use 20mA as the reference value in the specs (as for Vf) but use 10mA=100% in their graphs. One might wonder if this is what they intended??? \$\endgroup\$ – HiTechHiTouch Feb 14 '16 at 14:47
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The problem here is using the mcd value of the LED is wrong. Candela is Photometic value. The mcd is therefore already compensating for the sensitive of the human eye.

You would have to convert the mcd to Radiometric milli-Watts using the CIE Photopic Luminous Efficacy Table for each wavelength.

It would be better to adjust the current to the correct radiometric value while measuring the LED flux with a spectra-radiometer.

Rough translation factor for common LED wavelengths
Luminous Efficiency

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