LED Brightnes.... - Page 1
Forum section: electronics / beginners

author: Christopher
author_rank: Frequent Contributor
timestamp: December 26, 2014, 03:07:48 pm
content: This is something I've not covered before, very simple, always just put 5mA current though an LED and expected it to light... If it's too bright then change it.Now I am choosing some LEDs for a project which has three RGB 1206 LEDs on together. The LEDs should all be of a similar brightness: not so bright they blow you eyes out when looking at 0->30 degrees.I have chosen the series of LED already :Red550mcdGreen18mcdBlue250mcdThe luminous intensity is a linear function of current, and in this case is measured at 20mA.So according to the datasheet, the Green LED running at 5mA will give me about 4.5mcd(+-15%).So my overall question is are the relative intensity values for Red, Green and Blue measured with any relation to each other??According to this the red LED will be super bright and the green LED will be really really dim...

author: leppie
author_rank: Frequent Contributor
timestamp: December 26, 2014, 03:36:53 pm
content: You need to consider the observer spectrum.

author: German_EE
author_rank: Super Contributor
timestamp: December 26, 2014, 04:57:24 pm
content: The human eye sees different colors at different efficiencies, with the peak at about yellow/green and just to add a further complication women see colors slightly different to men. The only thing I can suggest is that you take an LED of each color and a 1K potentiometer then visually align them behind a piece of frosted plastic until you see white, then note the resistor values.

author: edavid
author_rank: Super Contributor
timestamp: December 26, 2014, 05:56:26 pm
content: Quote from: Christopher on December 26, 2014, 03:07:48 pmOf course, that is the whole purpose of the candela unit.QuoteRight. You might want to pick different LEDs.Quote from: German_EE on December 26, 2014, 04:57:24 pmThe candela unit already incorporates the correction for this.

author: Falcon69
author_rank: Super Contributor
timestamp: December 26, 2014, 05:59:58 pm
content: The only way to do this, so each LED appears to be the same brightness to the human eye, is to get one of these RGB's and test it with different values of resistors.You can't go off the data sheet or the lumens output of these to determine that.But, as edavid already pointed out, RGB's should already be calibrated for this. You will have to get one and test it.

author: Christopher
author_rank: Frequent Contributor
timestamp: December 26, 2014, 06:05:45 pm
content: I will get some in. Kinda admitting defeat as I want to understand some physics behind it.They are separate LEDs for each colour .

author: Falcon69
author_rank: Super Contributor
timestamp: December 26, 2014, 06:18:12 pm
content: I can't help you there. But I know from experience, that when I get LED's in, I have to manually test them to make sure they seem to be even in brightness when I look at them by changing the resistor values.I remember reading somewhere that LED's are made with a phosphorus or something, and depending on how much is used, determines how bright it gets. That's why you see a 3mm RED LED that is 14,000 MCD and another 3mm RED LED that is only 4000 MCD.I wish there were some type of other figure on the data sheet that determines the brightness level of these LED's based off how the human eye perceives it. Would make it much easier to determine the resistor values, and also to determine which LED to get for us 'Beginners'.But, I have gotten in RGB LED's before. The ones that require just one resistor value to operate all 3 (green, blue, and red) colors inside it. And, it appears that the LED's of each color are the same in brightness level as I look at it when it changes color. I don't know if this is the same for ALL manufacturers of the chips, that is why you must get a sample and test for yourself. I would advise doing this BEFORE placing a huge order of them, otherwise, you may be disappointed.

author: edavid
author_rank: Super Contributor
timestamp: December 26, 2014, 06:38:44 pm
content: Quote from: Falcon69 on December 26, 2014, 06:18:12 pmYour wish is granted... this is what the cd/mcd spec on the datasheet is for!

author: Falcon69
author_rank: Super Contributor
timestamp: December 26, 2014, 06:43:21 pm
content: then my eyes must be screwed up. I have gotten a Red and Green LED 3mm that both say 14000 MCD (super bright) and the green is waaaaaaaaaay brighter.

author: edavid
author_rank: Super Contributor
timestamp: December 26, 2014, 06:49:06 pm
content: Quote from: Falcon69 on December 26, 2014, 06:43:21 pmOr one of the datasheets was wrong, or they had different viewing angle specs, or the red one was damaged

author: Zero999
author_rank: Super Contributor
timestamp: December 26, 2014, 07:48:41 pm
content: Quote from: Falcon69 on December 26, 2014, 06:18:12 pmOnly LEDs which emit a more than one wavelength at the same time use phosphors. An LED with a blue or shorter wavelength is coated with a phosphor which emits other wavelengths to give various shades of white, pink or purple. Normally red, orange, yellow, green, blue and violet LEDs have no phosphor. There is a variation in brightness because the human retina is most sensitive to green an LEDs are generally most efficient at emitting red and blue wavelengths but it depends on the type of semiconductor used for the LED.

author: Simon
author_rank: Unknown
timestamp: December 26, 2014, 09:18:00 pm
content: I don't know if I'm reading the definition of a candela correctly but it seems to account for the different sensitivity to different colours. The other thing to note is that the measurement relates to the direction of the light rather than the sheer light output so the shapes of the LEDs also need taking into account.

author: SL4P
author_rank: Super Contributor
timestamp: December 26, 2014, 11:58:31 pm
content: I think we've all reached the point where product, combinations, datasheets, specs and eyes are all variable in their accuracy.I know that if I want any degree of repeatably in LED output,- I note the manufacturer, brand & part number being used- then start tweaking the resistors until I get the output I want- those three variables all go into the BOM as a matched set for the product under development.Also helps in the future when someone wants to service the LED array- they can be sure of identifying the exact / best match replacements.