Luminous intensity

[Dr.EM]

Hi. I see this used as a description for how "bright" an LED is. But I am wondering, is it the only thing that constitutes to its brightness? I ask because I see 5mm LEDs at 10000mcd, rated at 25ma, but then there are these;

**broken link removed**

some are up to 20000mcd, but can be driven at 350ma. Surely these are more than twice as bright? I know they have a wider viewing angle too, but I wouldn't think that counts toward the luminous intensity, whatever it may be. [/url]

 

[zevon8]

You're correct in thinking that it is more complicated than a simple number. Many things have to be considered, including how sensitive the human eye is to a particular colour. Human eyes are most sensitive to variations in brightness of the colour green, this is why "night-vision" uses the colour green, since we can discern more sublte differences in green than any other colour.

Anyways, what you need to refer to are the photometric charts for a given light source. These charts will give a graph of the light output over a range of off-axis alignments. Narrow beam LED's can be very bright when viewed "on-center" but output can drop very dramatically off-axis. LED's that give a large output over a wider cone ( viewing angle) will generally require a larger drive current. A 4000 mcd LED can be almost painfull to view if it has a very narrow beam, but spread it out, and it can seem like a regular 5 cent device.

here are some links to hopefully help:

https://www.theledlight.com/technical1.html

https://www.highend.com/support/training/lightingfaq.asp

 

[audioguru]

A 4000 mcd LED can be almost painfull to view if it has a very narrow beam, but spread it out, and it can seem like a regular 5 cent device.

Hi Zevon,
Thanks for the links.
If a narrow beamwidth LED has 4000mcd (on axis) , then its mcd will be much less if it has a wider angle.

Hi Dr. EM,
I don't like spec's that say "up to". I don't like to go through a bucketfull of parts to find some that work "up to" their ratings. I like to see "at least" because then it has a guarantee on its minimum performance.

Sony has a very expensive Li-Ion battery for their portable DVD player that is rated to play for "up to 5 hrs" which is much longer than most. On their website there is an asterisk following the rating. I clicked on the asterisk and it said "with the LCD off". How can you see a movie if you can't even see it?
Maybe their marketing dept got it all mixed up and it charges in 5 hrs if the DVD player is off. It is rated to charge for only 2.6 hrs which is probably its play time, in line with others.

Doubling the luminous intensity of an LED doesn't make it appear twice as bright. Like your hearing, your vision's response to brightness is logarithmic, so 10 times the intensity appears twice as bright (until your vision is overloaded and it hurts). Additionally, your iris forms an automatic level control that reduces increases in brightness if the increases are fairly slow.

I have LEDs with mcd ratings of 600 and 5000 with the same beamwidth. They both look the same and are blinding if you look at them close, but the brightest one lights-up the house across the street a little and the dim one doesn't. If their ratings were 2500 and 5000, or 5000 and 10000 then they would light-up the house about the same. :lol:

 

[awright]

As always, your best starting move is to google "luminous intensity."

There, you can find the definition:

"Luminous intensity: candela (cd)

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian."

It turns out that, in air, the frequency 540 x 10^12 Hz is about 555 nanometers - pretty much in the middle of the range of wavelengths seen by the human eye. Luminous intensity is a subjective descriptor taking into consideration the spectral (Photometric) response of the average human eye. {The comparable objective descriptor NOT taking into consideration the spectral response of the human eye is, "Radiant Intensity," in units of mw. per steradian (with no frequency/wavelength data involved), which is useful for IR or UV radiation, as well as visible radiation.}

The steradian is the unit of solid angle and is the angle subtended by an area r^2 on the inner surface of a sphere of radius r.

But your source does not have to illuminate the entire unit steradian to be measured. As a given amount of radiation is concentrated into a smaller and smaller solid angle with optics, the luminous intensity within that smaller solid angle increases. Thus, optics can increase the luminous intensity of an LED by concentrating the light emitted into a smaller solid angle, even though the total radiated optical power does not change. The result is increased brightness on-axis, but reduced brightness off-axis (as you see with the Mag-Lite). The Luminous Intensity of any sized beam of light can be determined by measuring the radiated power falling on a receiver with an arbitrary sensitive area and normalizing the result for the actual area (and distance from the source) of the receptor.

Unless you factor in the angle over which the LED spreads its radiation (reported as either the whole angle between half intensity points or the half angle from the central axis out to the half intensity point), you can't make a direct comparison of the total amount of optical radiation of two different LEDs, although you can compare the on-axis brightness to the eye of the two sources.

That doesn't make the luminous intensity useless. It is still a good indicator of how subjectively bright the source will look to the human eye. That is very useful if you know where the eye will be located relative to the axis of the LED as in, for example, a cockpit or dashboard display. It just won't be very useful (alone) in telling you how much light you can project onto an extended surface, as in the case of a "flashlight."

In fact, think of the on-axis brightness of a "Mag-Lite," as you change the focus by rotating the head. The luminous intensity can be greatly varied by changing the focus, but the total light emitted by the bulb and batteries does not change. Depends on what you want to do with the light.

awright

 

[eblc1388]

I have LEDs with mcd ratings of 600 and 5000 with the same beamwidth. They both look the same and are blinding if you look at them close, but the brightest one lights-up the house across the street a little and the dim one doesn't. If their ratings were 2500 and 5000, or 5000 and 10000 then they would light-up the house about the same. :lol:

I use LED as indicator on my projects and I found the high brightness water clear LEDs completely useless to my purposes. Look directly they are too bright and the effect is bad when viewed off axis.

Luckily I found some diffused blue LEDs(manufacturer recommends 20mA to give 1000mcd) which suit my purposes well. They are bright and efficient and give very pleasing illumination effect with less than 1mA of current but best of all, unlike all other blue LEDs it has only 2.6V voltage drop so I can light them up using a 3V multimeter. They give a uniform blue light instead of the near purple of that of a normal blue LED.

This is unlike all my other clear blue LEDs which all requires more than 3V to light up. Do you think this is a new type of LEDs?

 

[Dr.EM]

Ok, thanks people, thats a lot of info . So its the case that the 20000mcd LED requires so much more current because it has 120degree viewing angle as opposed to just 20deg like my 5mm ones.

I knew about green being the most apparently bright colour to us (I think it is the colour to which Cds is most sensitive too). I didn't know about the logarithmic vision scale though, thats an interesting fact.

If I was trying to light up a room, a 120deg viewing angle LED would be far more effective then? Also, it would be better if it was in green, which for some reason, in the case of those X-power LEDs, has a much lower luminous intensity (and costs a lot more). I'll try getting some of these LEDs, to see how they are for real.

 

[audioguru]

Hi Dr. EM,
Boy are you ever going to have a glaring light in the room.
I can't stand having a halogen bulb's glaring light on me, like the clear incandescent ones in a chandelier. For bare bulbs I always use "soft white" because they are diffused and don't glare.
You'll probably end-up covering the ultra-bright LED with a diffuser.
Green? Ugly colour.

All traffic lights in my city are LEDs now. They are so bright that I slow down for a red one a few blocks too soon. When I pullup to a red light I put my visor down, even in daylight.

There is a site on the web where a guy shows how he uses sandpaper on a clear LED to make it diffused! :lol:

If your vision didn't have a log response then you wouldn't see anything in moonlight and sunshine would over-expose everything.
Just like hearing leaves on trees rustle in the wind and hearing a rock concert.

 

[awright]

Dr. EM:

Your statement, "So its the case that the 20000mcd LED requires so much more current because it has 120degree viewing angle as opposed to just 20deg like my 5mm ones, " is probably true. Just be cautious about inferring too much information from statements like, "viewing angle," unless there is a statement of what it means.

"Viewing angle" is probably a phrase allowing them to report a larger angle than the, "half angle," or "beam angle," which will generally be accompanied with a statement that the angle is measured out to the half intensity point (FWHM), not just some arbitrary "visibility" limit, which would be subjective. The former requires a somewhat sophisticated photometric measurement setup. The latter involves a guy sitting at his desk saying, "Yeah, I can still see some light at this angle."

FWHM is the acronym for "Full Width - Half Maximum," the accepted method of reporting beam width. It is measured out to the angle at which you would measure half the radiant intensity (milliwatts per steradian) of the on-axis peak. There is a lot of information available on the topic by googling, "FWHM," but I have not followed my own advice.

Have fun

awright

 

[audioguru]

The high power LED doesn't look like it has a lens in front of it like most LEDs, so I think that it really does have a wide FWHM. It has a pretty good heatsink so it can handle the very high current to be extra bright over most of its wide "viewing" angle.
Viewing? Do people sit there just looking at it? :?: :?:

 

[awright]

Well, audioguru, "viewing" doesn't necessarily mean sitting in your Lazy-Boy with a beer in one hand and a clicker in the other, turning the LED on and off.

It should mean you get whatever function you are using the LED for satisfactorily fulfilled at the specified angle. An example being trailer tail lights, which I was trying to implement before getting distracted to another project. Buying high-brightness, red LEDs for tail lights won't do you much good of the high mCd numbers are accompanied with a very narrow beam angle, which some of them are. For safety, you want (and vehicle codes generally require) visibility within some fairly large viewing angle at some specified distance. Having blinding intensity straight out to the rear won't cut it, and if you've achieved high luminous intensity by concentrating the beam with an integral lens, you may have to invest in many extra LEDs and their installation time to cover the required angle.

awright