620nm versus 660nm. Deep red is a longer wavelength - farther from orange. There’s also 720nm “far red” (near-infrared).
So 660nm. deep red is better because it’s less orange?
There is no better. The deep red looks less orange next to a 625nm led, it is also about 4 times less visible (you need 4 times the optical power to achieve the same brightness).
Not too exact, but think like this:
typical 620-630nm red is just like something 80-ish CRI red - which are most common in bicycle tail lights
deep reds with 660nm is like getting red in 95 (or above) CRI
Without comparing typical red to deep red, the previous one will look still red, but when you see the deep red, then that will be ‘THE’ red, and the typical red looks rather like an orange-ish red.
But as djozz says, the human vision is less sensitive to deep red (but even for 730nm far red, most people’s eye gets triggered a bit, and for even 780nm near infra red, people can expect barely visible light - but I guess, staring into that could be harmful, as there is no good sense of ‘too much’ light entering, which would trigger the pupil’s diameter decrease).
For visual use, the ‘white’ LEDs usually don’t go much past 660nm, because then, it would be necessary to convert much light to reds, which we just barely pick up, then overall efficiency would suffer.
There’s also Luxeon CZ, Z & Rebel in 660nm
XP-E deep red 670nm.
XP-E2 red 620nm
I focus on LEDs for flashlights. If they are too small or if I do not find any DTP MCPCB I do not care.
Luxeon CZ is just a C with less power and Z has a weird footprint without the history of the Rebel.
Rebel is in the table.
Djozz is right, the deeper red that a lot of manufacturers have various names for like "photo red" or whatever oslon calls their different nm rated lights, are only really noticeable to a novice like me is when white wall shining two together. Put a 620nm xp-e2 next to a "photo red" 660nm xp-e, and the 620nm appears orange. But, after using the deeper reds, the tint is noticeably more pleasant than the 620nm from a tint-stob (pleasant to the eye) perspective.
But, there's also a good reason to use them since a lot of red lights are for hunting. I've never gone for the "far red" above 660nm because too much of it is out of the visible human spectrum. The higher nm you go, the less output you can perceive, so the photo red lights won't ever have as much visible output. Unless you're using digital night vision, which is another story.
Considering red led's generally have low visible output anyway, I don't notice much difference in functionality between a 620nm xp-e2 and a photo red xp-e. However, tint-snobbery aside, there is a basis for the distinction if you want to use the light for hunting. While a lot of that light is still in the human range, it is even more removed from the visible spectrum of a lot of animals that are hunted at night (feral hogs, predators, etc). Not really scientific, but here's a brief explanation on why I like the higher range reds in addition to the subjectively "nicer" tint:
In short, I have a C8 with a de-domed xp-e2 620nm normal red, and a xp-e photo red, both around 2A driven. I notice no difference in range or real usefulness other than the photo red looks red and the 620nm is orange side by side. If it makes a difference to the wildlife you're trying to spot, so much the better. I just know I like the appearance of the deeper red once I got used to it, and the added bonus is being less visible at night to animals. Just be careful not to shine light above 660nm near your's or someone else's face. With infrareds, just because it isn't visible doesn't mean it isn't damaging your retinas.
This is probably not what you’re looking for, but since it is interesting and available, it seemed worth mentioning:
Clemence has recently started selling Nichia E17A phosphor converted red emitters. While he used to offer a modified Jetbeam Jet-u with them, I believe now you can just order the emitters, optionally presoldered onto a compatible MCPCB.
Because it is phosphor converted, it has a wider spectrum than a typical red LED. These means more extension into the deep reds, but also into the red-orange. That could potentially aid object discrimination, but also could mean more likely to be perceived by animals.
Here’s a comparison to the XP-E2
| Red XP-E2* | Red E17A | |
| Peak | 625nm | 645nm |
| Spectral FWHM | ~25nm | ~100nm |
| Max Rated Current | 1000mA | 700mA |
| 350mA Output | 84 lumens | 31 lumens |
Obviously, single XP-E2’s are capable of significantly higher output. On the other hand, if throw isn’t needed, Clemence can mount four of them in a close-packed “quadtrix” format. I expect that would work decently well with a Convoy S2+ and one of the TIR optics that the Convoy Ali Express store has available. I don’t know how the beam shape would be if used in a smooth reflector like in a C8.
/* Edit - The photo red XP-E2 is closer to the E17A in peak wavelength, but Cree only gives the output for the photo red in mW of optical power, not lumens. If this table is accurate, then the 650nm photo red XP-E2 should produce 43 lumens at 350mA
Cutter au has a “NEW Product Release: Latest Version of the Cree Photo Red 660nm, now in XPG-3. This is a preorder, stock available late May 2019”
link to product page: https://www.cutter.com.au/product/xpgdpr-l1-0000-00g01/
According to the official spreadsheet the red XPG-3 can handle 1.5A max and the XP-E2 1.0A.
Djozz measured the XP-E2 can handle a bit more than ~2.5A I wonder how much the XPG-3 could get max and how it compares to the XP-E2.
According to the spreadsheet the Photo Red XP-G3 has a typical wavelength of 645nm and peak wavelength of 670nm. The XP-E2 only has a peak wavelength of 650nm listed. I’m confused again, which one is better in terms of tint? = more red.
Define: more red
The XP-G3 datasheet says:
Peak Wavelength Minimum: 650nm
Peak Wavelength Maximum: 670nm
Dominant Wavelength: 645nm
The range of peaks is listed, as I understand, because there can be variation between batches.
Peak is where wavelength with the highest intensity. I wasn’t aware dominant wavelength was a separate specification, but from a quick search, I guess the difference is peak is based strictly on spectral power density, but dominant sounds like it is in some manner a perceptual specification. I guess maybe it’s adjusted based on the human eye sensitivity to the wavelength that would appear identical from a true monochromatic source (color LED’s are close to monochromatic, but not quite).
The XP-E2 datasheet does not specify dominant wavelength. The peak wavelength minimum and maximum are the same as the XP-G3.
Both should appear basically the same.
Closer to the deep red wavelengths. The 620nm XP-E2 is indeed a bit orange-ish.
Thanks for the explanation. But XP-G3 has better parameters and should handle a bit more current. Maybe I’ll organise a small groupbuy (approx. 30 leds).
I see. In my perception (everyone differs a bit in how far red they see), at longer wavelenths than 660nm the colour of red does not change anymore and your eyes do get more and more insensitive for it. A good compromise I think is 640nm: looks very red and not orange, and you do not need bucketloads of output (as in: radiometric output) to be able to see something.
Hmm, so if the XP-G3 is slightly less into the far red wavelengths than the XP-E2 it’s actually good.
In my perception and opinion yes, but there is a sort of myth that the higher the wavelength, the better while in fact you are going to see less and less instead of a “redder” colour.
And there’s the hunters who like all sorts of colours for as many reasons as there are colours, but I do not know much about that.
Digi-Key lists 645nm for the XP-G3.
As soon as they will become available at mouser, roughly late july I’ll organise a small groupbuy 30-35 leds from them. Even with import taxes the price should be decent.
From Mouser for G bin of red XP-G3:
"End of Life: Scheduled for obsolescence and will be discontinued by the manufacturer."
Strange.
:+1: good idea, in for 5