Deep Red vs High Efficiency Red in this datasheet? - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: December 19, 2018, 06:33:11 pm
content: http://www.avagotech.com/docs/AV02-0699ENDeep Red says on page 6 that its Luminous intensity is typically 770 ucd, but High Efficiency Red says 270 ucd.Does this make sense? Shouldn't the "high efficiency" be brighter for a similar current?In the peak pixel current on 5 it separates the deep red from the others, but it is 18.7mA vs 17.1mA, so current is very similar.My question is, which one would use less current for a similar level of brightness?Why two reds? Is one a different LED type that is more efficient?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2018, 07:21:47 pm
content: Haha, their "high efficiency red" isn't.Yeah, I don't see any reason to doubt that their currents are similar, or that their emissions are different from what they say. That means the "deep red" is REALLY bright, because it's on the far side of the luminosity spectrum. Go figure!Incidentally, that also means their green is shit, in radiative terms -- it's merely comparable by luminosity, but it's at (nearly) the peak. Old fashioned GaP green I imagine -- not very good efficiency in general (and part of the reason why green laser pointers aren't just a laser diode and that's it, they're an IR-pumped doubler instead!). There are phosphor+GaInN greens today (you can tell because they drop 3V just like blue LEDs, and have similar quantum efficiency but correspondingly worse power efficiency which is actually worse than white LED phosphors, which is why they aren't favored for illumination purposes), but they probably never updated the product line to add them (or can't because of the voltage drop).Tim

author: alank2
author_rank: Super Contributor
timestamp: December 19, 2018, 07:26:16 pm
content: It is odd. I do have all but the "deep red".I set them to a low current setting, around 10% of maximum:green "2963" is very niceher "2962" is closeorange "2964" is decentyellow "2961" is much dimmer than the restI wonder how the deep red would compare, but i don't have one.The goal is bright display with least current use...

author: james_s
author_rank: Super Contributor
timestamp: December 19, 2018, 07:33:11 pm
content: Quote from: T3sl4co1l on December 19, 2018, 07:21:47 pmOff topic here but there are direct injection green laser diodes now. They cost more than the DPSS green lasers (which became insanely cheap) but I'm sure somebody must sell pointers using them by now. I bought one of the diodes and built it into a module, the green color is a tad more saturated looking and slightly more pleasing to me than the yellowy DPSS green.

author: ajb
author_rank: Super Contributor
timestamp: December 19, 2018, 07:35:41 pm
content: Quote from: alank2 on December 19, 2018, 06:33:11 pmQuantifying the subjective experience of "brightness" is complicated. The Candela is a unit of luminous intensity and is equal to lumens per steradian--in other words, it's the amount of radiant energy (watts) corrected for human spectral sensitivity (watts->lumens) emitted in a given solid angle (cone). This is typically measured over a fairly small angle near the viewing axis of the emitter.This reveals two big confounding factors when comparing the brightness of LEDs via the the cd figure:- The two LEDs may have very different distributions of their radiated power in space. A wide angle LED will have a much lower cd rating than a narrow angle LED even if the two emit exactly the same number of photons. The narrow angle LED will be much more intense when viewed straight-on, but the wide angle LED will be more intense at wider viewing angles. It's up to you which is more important to you.- The perceived brightness of the LED depends on how sensitive the human eye is to its spectral content. This has a related confounding factor: Human vision is complicated. More on that later. The big thing here is to understand that the human eye has a spectral response that peaks somewhere around 500-550nm and tails off at wavelengths above and below that. When you get towards the ends of the spectrum, the relative change in brightness over a small change in wavelength can be huge. The spectral response is generally defined for two different vision conditions: the photopic regime, where the eye is adapted to bright conditions (better color perception and detailed vision in the center of the FOV, because the color-sensing rods, which are concentrated in the fovea at the center of the retina are dominant), and the scotopic regime, where the eye is adapted to dark conditions (much higher sensitivity at the expense of poor color perception and peak acuity, because the color-insensitive rods, which are more spread out over the retina and less concentrated in the fovea, are dominant). Luminosity functions have been defined for both of these conditions (actually, several curves have been defined empirically), and by multiplying the power spectrum of a source by the correct luminosity function you get a luminosity spectrum. If you integrate the power spectrum you get watts, and if you integrate the luminosity spectrum you get lumens. Wikipedia has a nice graph showing a couple of the standard curves here:https://en.wikipedia.org/wiki/Candela#/media/File:Luminosity.pngThat all said, assuming that the two displays have similar angular emission distribution, and assuming that they have been quantified under a suitable luminosity function, the one that is subjectively brighter SHOULD have a higher candela rating. If the radiated power of the two devices is the same, then the source at 635nm should be about 1.57x times brighter than the source at 645nm (using the CIE 1978 photopic curve, they have weights of 0.21702 and 0.13812, respectively).However, eye sensitivity at the very edges of the visual spectrum is poorly quantified and highly sensitive to viewing conditions--far more sensitive than can be understood via a fixed luminosity curve. If you look at some other LED datasheets, like the Luxeon Rebel Color line, you'll find that they bin most of the colors by luminous flux (lumens), except for the deep red and blue wavelengths, which are binned by power. So I suspect that in this case the candela rating of your displays is simply not a good reflection of the subjective brightness of the display due to the area of the spectrum involved.As a general rule, I would never solely rely on datasheet figures for LED brightness. Datasheets are fine for a basic idea, but the exact measurement methods and conditions are pretty much never specified, and the appearance of an indicator is highly subjective and context-dependent anyway. It's always going to be best to get some samples in and look at them subjectively in different conditions and with different drive currents and see what works best. This is especially important if you have lightpipes or lenses involved!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2018, 07:37:54 pm
content: Right, that's the last factor, emission angle. DR may be narrower? Who knows.Tim

author: ajb
author_rank: Super Contributor
timestamp: December 19, 2018, 07:46:21 pm
content: Quote from: james_s on December 19, 2018, 07:33:11 pmYes, the DI greens are 520nm, so a bit less yellow than the 532nm DPSS. They are more expensive, but come with the advantage of much improved optical stability, albeit with the same shitty beam quality exhibited by all direct injection diode lasers. That said, building agoodDPSS module that modulates well at reasonable power levels is a lot more expensive than the plethora of cheap shitty DPSS modules out there might lead one to believe, because they are inherently much more complicated and once you get above a watt or two it gets very difficult to make them reliable and stable. At higher power levels where you don't need superb beam quality or coherency, DI green tends to be way more economical than DPSS green.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 19, 2018, 08:24:21 pm
content: One thing to remember in LEDs is: They can be absolutely anything. Assume nothing.Reputable LEDs "for illumination" can easily be incandescent-bulb-level inefficient; cheap Ebay finds may be order of magnitude better than reputable products; an LED which says "efficient" can be order of magnitude less efficient than one that doesn't say this.Also, it's very typical that when a company such as OSRAM manufactures and sells an LED, they state front-page efficacy of something like 45% [power converted to light], or 120 lm/W [for a white LED], but when you read further, beingverycareful about your exact part number, combine all the numbers using their minimum values (having to ignore most of the data, since it's not related to the actual part being sold), and do the math again with these numbers, they only guarantee like one fifth of that! The gotcha: the front page spec only applies to an imaginary bin you cannot buy from any distributor on the planet, and even then using "typical" numbers. And this isn't limited to the front page specs; most of the datasheet is useless if there is a binning table at the end of the DS that tells you that the product you can buy performs down to 3x worse than what you just read. This is called "bait&switch". It's also a case of underspecifying their product to hell so that if they ever produce a bad batch, they can just sell it - just like those Aliexpress vendors do as well! -, but given poor competition, customers will still buy the OSRAM and do their own evaluation, finding out their "typical" values, and hoping they'll get similar batches in the future, trusting in a name brand's manufacturing quality control - but without any guarantee. (This is, at least, what I have to do.)With LEDs, assumptions only hold by luck. Be careful, understand all the numbers, and avoid traps (such as datasheet values and distributor parametric search values based on unobtainable bins).It's worth understanding that "high efficiency red" is the name of acolor- it has absolutely nothing to do with physical efficiency, except the fact that if two LEDs output the same number of photons per energy, the "high efficiency" red looks much brighter (and a bit orange, actually) because our eyes can see this wavelength (such as 620 nm) better (instead of 660nm). But no onenecessarilysays anything about two LEDs outputting the same number of photons per energy, or even close! It may be, or it may not. If you pick thousand randomly sampledmodernLEDs, you'll easily see 1000x varience in efficiency, the best are converting electric energy to light at 50% efficiency, the worst at <0.1%, and yes, the worst can be Avago (now Broadcom), Cree, Osram, any good brand. Any of this isn't always clear in the datasheets, especially since many LEDs areonlyrated using spot measurement units, not total light output, and conversion is non-trivial.Often, it's just easiest to buy and test. This way has an added benefit of taking all optical beam shape irregularitities, psychovisual effects, etc. into account automatically. Do side-by-side tests.

author: alank2
author_rank: Super Contributor
timestamp: December 19, 2018, 09:17:12 pm
content: Thanks everyone; a ton of knowledge here!!!!

author: David Hess
author_rank: Super Contributor
timestamp: December 20, 2018, 01:22:24 am
content: Very old "high efficiency" red LEDs (GaP?) only had high efficiency at low currents but this made them suitable for low power circuits.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 20, 2018, 03:42:32 am
content: Quote from: David Hess on December 20, 2018, 01:22:24 amModern green and blue LEDs also need surprisingly little current to get a visible glow. I modded a clock to have the backlight be always on at a very low level - something like a 100k resistor across the transistor that switches the blue LED backlight. The 1000mAh battery easily lasts months between charges. At some point, I stuck in a piece of fluorescent orange paper to act as a phosphor, to make the glow less harsh at night.

author: Zero999
author_rank: Super Contributor
timestamp: December 20, 2018, 11:08:45 am
content: Quote from: alank2 on December 19, 2018, 07:26:16 pmLEDs are most efficient, in terms of power in vs radiant power output, in the red-orange and blue-violet ends of the spectrum. Green may seem brighter, but that's because it's near the peak response of the human eye. Yellow is very poor, so much so, a deep blue/violet/near-UV LED with a phosphor can give higher efficiency, than a yellow die, but the forward voltage is higher and it's much more expensive. Improving the efficiency of LEDs in the middle of the visible spectrum is still work in progress.Deep red will be dimmer than orange-red, mainly because the eye is less sensitive to it, although the radiant efficiency of LEDs also drops at longer wavelengths.Another reason why blue LEDs light so well at low currents, is because the human eye's peak response shifts to shorter wavelengths, at lower illumination levels.