Well, photo-red is Cree's name for 650-670nm peak deep red LEDs that are supposed to help in photosynthesis /used in horticultural application along with royal blue LEDs. Apart from that, the true red color of 660nm is aesthetically more pleasing than the orange-red 630nm to some people (including me).
I would have to agree with this, except in those instances where I specifically want a shallower red for design reasons! (
http://www.candlepowerforums.com/vb/images/smilies/thinking.gif http://www.candlepowerforums.com/vb/images/smilies/confused.gif ) Most of the time I find red shallower than 640nm to be orangey and to not posess that rich, satisfying red goodness that has been comparatively so easy to get from incandescent lighting. Similarly, red in the range of 640~645 is just barely beginning to get there, and it's not until it gets longer than 650nm that my red-snob eyes start being totally satisfied. (Curiously, this fact plays itself out in my Nichia 219 Hi-CRI tint-snobbery as well.)
I will literally mod the standard-red LEDs to deep red or create my own lights including deep red, not for plant growth or photographic paper safety, but because I often find it creates an aesthetically superior effect in the lighted scene. Yes, a scene lit for people to view, not for some scientific/biological/photographic purpose. I'm talking theatrical and Christmas displays here. I was buying Lumex 660s before the Chinese or Rebel ones even became available.
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Sure, I find it then takes at least twice as much power in the red but don't care. Couldn't care less in fact that it takes twice as much power.
http://www.candlepowerforums.com/vb/images/smilies/eek.gif Also note that I am not one of those 40% of women who can apparently see more shades of red than the rest of us. Actually I'm male. But don't worry, I do still use "normal" shallow red LEDs for when that is the best color.
I think YMMV applies here, in my opinion more than LED manufacturers seem to realize.
I think I once heard our green cones stop responding at approximately 650nm so that's why light shorter than this wavelength starts looking orange. Supposedly, go longer than 650nm (peak) and you can't tell red differences anymore because the green cones aren't responding. (although the light will look increasingly dimmer for the same power and efficiency). Perhaps this is where that 40% of women excel? I never could find exactly which red wavelength band(s) the researchers had in mind, if any.
Similarly what we call violet is actually the truest form of blue, where the green cones aren't responding anymore. Wavelengths longer than about 430nm start looking increasingly greenish, so that a normal 465nm blue LED is already activating the green cones quite a bit, not just the blue ones, and not just because of the greener portion of the blue LED's spectrum. I can still tell subjective differences in color between 405, 420 and 430nm LEDs so I don't know if this is due to the tail end of green cone response still trailing off into the longer wavelength part of the violet LED's spectrum, and/or some other mechanism. (?)
