Electronic – Human perceived (relative) brightness of indicator LEDs

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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.

Best Answer

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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