Electronics > Beginners
Match LED Brightness
wraper:
--- Quote from: sleemanj on November 04, 2019, 05:03:23 am ---The best way to match an led for brightness, with a resistance decade box and your eyeball.
Decide on a resistor for one led that produces the brightness you want, then hook your decade box up for the second led and adjust until you find something that makes them match.
--- End quote ---
Not everyone has resistance decade box. Using potentiometer instead of fixed resistor to adjust the brightness and then measuring it's resistance would do the job just as well.
BravoV:
--- Quote from: StillTrying on November 04, 2019, 03:06:07 am ---I don't know about SMD, but close up I can see a Super bright 5mm blue lit at less than 1uA, they're very bright at 1 or 2mA.
--- End quote ---
In my limited experiences, looking straight directly at the LED light path is very poor way to perceive it's brightness level at 1 or 2 mA (@20 mA rated LED) as they're still blinding, try shine it to a wall or bright object, or use two identical LEDs with different current to see the "optimal" brightness vs current.
Whales:
Side question: Do you need the LEDs to appear similar at close range or very long range (eg crowds)?
Keep in mind that different LEDs have different output angles. At close range things can match, but if you care about 50-100m away then make sure to tune them at that distance. Different output cones == different rates of decay with distance, ignoring the problems of off angles.
tooki:
Blue LEDs are all made with “modern” ultra high efficiency compositions. Red, yellow, and green (especially yellow) are often older, less efficient chemistries. (Modern emerald green LEDs can be insanely efficient. As power indicators, I usually run them at about 0.5mA.) The result is that at a given current, the blue LED can be massively brighter.
You can get a sort of broad idea as to the chemistry by the forward voltage: for a given color, a higher voltage tends to be more efficient. So for example a traditional green LED might be 2.2V, while a modern high efficiency one is 3.2V.
In any case, I totally agree with the recommendation to do some testing and basically adjust it by eye using potentiometers until you’ve found what values work. Between LED chemistry, human eye characteristics, and optical characteristics of different LEDs, there’s just no real way to rely on calculations alone.
Psi:
--- Quote from: Kasper on November 04, 2019, 01:16:41 am ---Short version:
Pick resistors for each LED that cause them to have the same current. For example if you want 5mA in each LED:
R = (Vsupply - Vf) / I
R = (Vsupply - Vf) / 0.005A
If they have similar current they should have somewhat similar brightness but you could go further...
--- End quote ---
Doesn't really work.
At the same current, or even the same wattage there can be a huge difference in brightness.
The efficiency of LEDs varies a lot by color and by the chemistry used.
Even for one color, like yellow, there are a few different chemistry that can be used and they can have wildly different efficiency.
Best thing to do is look at the mcd rating and only compare leds that have the same viewing angle.
That will match them up pretty well.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version