LED intensity/angle mcd?

It means 2x the 'view angle' theta e.g. 30 degrees, so it's talking about a cone of light with a sixty degree included angle ( for theta = 30 ).

For the same led 'chip' inside the led you got it.

No idea.

What do you actually want ? A narrow angle led or a wider one ? Whichever's more suitable for your application is better.

Graham

I suggest you view all the data sheets side by side and try to understand all the differences in detail.

The first one (670-1257) is a 10mm diameter LED with a wide viewing angle. The wide angle means that the light is spread over a wider area so it appears not to be as bright on-axis, but can be seen over a wider angle. Too far off-axis and the others may be difficult to tell whether they are on or off, depending on ambient lighting conditions. It also has a red *diffused* lens, which means that the light appears to come from the surface of the dome rather than through the 'water clear' lens that the others have. It's also tinted red, which affects how ambient light is reflected or absorbed. Chances are good it has the exact same type of 660nm color die in it as 670-1226, just a different molding.

The main difference between the second two is the color of the emitted light. They are both 5mm diameter LEDs. The 670-1224 emits 636nm wavelength light, which is more of an orange-red. The 670-1226 emits longer wavelength 660nm light. Both are water-clear epoxy lens. They are narrow angle (30°), which means they look much brighter when you look almost directly into them, as opposed to much off the axis. Most manufacturers provide a graph of brightness versus viewing angle. The different dies have slightly different forward voltages as well (higher typical forward voltage by 300mV for the 670-1224, which emits shorter wavelength light).

BTW, please get into the habit of referring to such parts by the manufacturer's (Lumex, in this case) part number rather than the distributor catalog number. You will find it infinitely easier to deal with the manufacturer's data sheets and when you change distributors or shop around you won't have to translate everything if you stay with the same manufacturer.

That said, why not buy all three, power them up at a sensible current (in series is good for side-by-side comparisons) and look at the differences for yourself?

Best regards, Spehro Pefhany

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cone

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The ratio of the intesities for a given led die with different theta's is the inverse of the ratio of the square of the tan of each theta. Since tan15=0.268 and tan30=0.577 we would expect the narrow angled device to be (0.577*0.577)/(0.268*0.268) =4.64 times 'brighter' than the wider angled device.

cone

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Which indeed seems to be the case here.

Graham

cone

more

It's diffused too, which should make it even worse.

Best regards, Spehro Pefhany

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Spehro,

Thanks for the reply. As I understand it, the (670-1257) part with the red diffused lens emits a closer pure red light than the other options, but also has less total power output due to the red colored lens which filters out some of the light?

The 670-1224 emits more of an orange-red color, but also appears brighter compared to the 670-1226 which is closer to pure red but rated at a lower mcd?

How does one compare mcd rating to color and apparent brightness? I'm guessing orange LEDs of the same power output will appear brighter than pure red LEDs of the same power output?

-Bill

It's the same color as 670-1226, just with a huge 10mm diameter lens that is also tinted and diffuse (like a regular frosted incandescent bulb as opposed to a clear glass candelabra bulb). It's less bright looking right on axis because the light goes all over the place, and is emitted from a large surface.

Yes, although the difference in brightness is not much.

The number is already corrected for eye response AFAIUI, so compare directly. But the eye has a log response so a 2:1 difference in brightness doesn't look huge.

Best regards, Spehro Pefhany

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snipped-for-privacy@att.net wrote: [re Radio Shack 670-1257 and other LED's]

All else being equal (eg illumination pattern and viewing angle, besides power output, LED efficiency, "ordinary" eye) that might be so, since as mentioned before and as shown in some of the following webpages, the eye's sensitivity is dropping off as wavelength increases from orange to red.

However, I don't recall that you've said yet what it is you are trying to do (other than having blue and red LED's with about the same apparent brightness). Will the LED's be panel indicators, or do they illuminate something? From how far are they being viewed? Under what lighting conditions? Proper answers depend on knowing your requirements.

I don't know what you are trying to do, and don't know your requirements, but will mention a few things anyway. First, based on a nonscientific look at some ebay prices, it's substantially easier to get red/green/white LED's that are bright, vs. yellow/orange, so much so that your concerns about eye sensitivity probably are moot. (See ebay examples below.) Second, most of these LED's are bright enough that they are dangerous to look at, making them unsuitable for use as panel indicators if driven at full power. Third, several of these will produce visible (ie, noticeable but not very bright) light at currents of a few hundred microamps, and could be used as indicators with currents of a milliamp or two.

Auction# $Each Item and pertinent specs from auction pages

7619669955 .23 100x 5mm Red LED 25ha 15000 mcd typ. at 20mA 7620754821 .32 50x 5mm Red LED 20pa 20000 mcd min at 20mA 7619480649 .60 20x 10mm Red Led 30va 80000 mcd claimed at 60mA 7620924300 .17 30x 3mm Yellow 20va 4000 mcd typ. 5000mcd claimed 7620916574 .12 100x 5mm Yellow 20va 4000 mcd typ. 5000mcd claimed 7619668700 .26 50x 10mm Green 13ta 65000 mcd typ. at 20mA 7619668531 .36 50X 10MM White 13ta 110000mcd typ. at 20mA 7619670585 .26 50x 10mm Orange 13ta 45000 mcd typ. at 20mA 7620929777 .24 50x 10mm Yellow 12va 60000 mcd typ. at 20mA $Each= (Price and shipping total dollars) divided by (number of LED's). The angle (in degrees) of the LED light is listed 4 different ways in the above auctions, which I've indicated by ha, pa, va, and ta, as follows: ha, half angle; va, viewing angle; pa, "50% Power Angle"; ta, "1/2 Viewing Angle". Last auction also has a "Radiation Diagram". Note, where typical mcd ratings were given I listed those rather than the max values, or if typ not given, min instead of max.

-jiw

Just to clear up any misunderstanding about photopic units:

An Xmcd led of any colour has *exactly* the same apparent 'rightness' as a led with the same mcd of any other colour. That is what a Candela (milli candela or whatever) is by definition. It is not a measure of the power emitted by a light source it is a measure of the human response to the radiated power. (An infra-red led obviously emits power (in the infra-red region) else my TV remote control would not work but it emits none in the visible region ad thus emits zero milli candela's.)

could even be the same 'brightness' - iff eye kood spel propurly