I have noticed that in urban areas at night, blue lights have
a tendency to stand out. One application of this is the blue lights used
for police emergency phones in some college campuses and the like.
Until now I believed the noticeability of blue lights was a result of "blue impact" or stimulation of the blue-sensing light sensors in human eyes. I even made up a unit of "blue impact" that I called the "blumen" for "blue lumen". Back when the lumen was defined in terms of the original 1924 official photopic function (same as the 1931 CIE "Y-bar" function), "blumens" were equal to lumens times the ratio of z to y chromaticity coordinates. (The 1988 redefinition of the photopic function messes this up, generaly just a little.)
So I notice that a major college campus where I did delivery work at night has LED blue lights for police emergency phones. Most have the usual 470 or so nm blue LEDs. I notice that current bright blue LEDs have peak wavelength mainly around 466-470 nm. But the blue peak of human vision is a little below 450 nm. And I wonder about the lack of blue LEDs with output concentrated towards this wavelength.
Since the days when 466-470 nm narrowband blue LEDs superseded 450 nm broadband ones, I wondered about why all the easily available blue LEDs were 470 nm ones when 450 nm ones could yield more "blumens". I suspected that 470 nm ones were favored merely because they produce more lumens and more candela - which are specifications that people would recognize and which are useful to put on a spec sheet but do not relate to the less-known "blue color impact" or my "blumens" or blue-relevant derivable units such as "blue candela" (candela times ratio of total Z-bar to photopic total of all wavelengths produced.)
Back to 450 nm blue for the usefulness of 450 nm blue...
I have noticed that Cree makes deep blue LED chips. I managed to get a few LED lamps with 450 and 460 nm (dominant wavelength) versions.
My thanks to Opto Technology for providing me with samples of OTL450A-1-1-46-D-2 (450 nm version) and OTL460A-1-1-46-D-2 (460 nm version) back in 2001!
Both of these are reviewed in one of Craig Johnson's blue LED pages, specifically this one on older GaN and InGaN blue models.
So I get a 470 nm LED lamp and tape black electrical tape around it to make its output distribution as similar as possible to that of the deeper blue 460 and 450 ones that I got. Then I measure optical output of each of these with an unusually broadband solar cell (black single crystal silicon basically satellite grade). I adjust/change the dropping resistors for each LED lamp to get solar cell current outputs (to a milliammeter) proportional to wavelength - ideally equalizing optical power output, and assumed not deviating much from that since all lamps in question are narrower band LEDs in the same basic color of the visible spectrum.)
Results of 450 vs. 460 vs. 470:
I put the three LEDs on a board and took it outdoors in an urban area at night with a friend to see how they stack up. We went to different areas of a large apartment complex parking lot:
Dark, dimly lit with high pressure sodium, brightly lit with high pressure sodium, lit with metal halide, and lit with incandescent.
I illuminated a piece of paper with them so that I had three "light sources" of equal size, directional characteristics, and equal optical power.
Results: The 460 and the 470 mostly came up as equally visible and noticeable, with the 450 looking dimmer - about 3/4 the brightness of the other two.
It appears that "blumens" count a little, and not nearly as much as lumens. "Blumens" seem to have enough impact as to make the 460 nm LED about as effective as the 470 nm LED. The 470 has more lumens and less "blumens". The 450 has only slightly more "blumen" output than the 460, while having less lumen output than the 460. I am still trying to figure out how much "weight" a "blumen" has in terms of making blue lights noticeable. For now I have somewhat of a guess that the sum of lumens plus approx. 3 to 5 percent of "blumens" is an indicator of how easily noticed a blue light is outdoors at night in urban areas.
NOTE - in RGB LED displays such as large TV screens using R, G, and B LEDs, "blumens" seriously count for the blue elements and there is also desirability of best-matching the blue phosphor of the usual TV's and monitors. I think 460 is best for this. 450 has a little more blue impact than 460 but my recent tests show the advantage of this to be small and 450 has apparent color varying with lighting conditions (sometimes looks violetish) in a way that 460 avoids. So I think 460 is better than 450 or 470 for 3-color LED TV screens. But in other applications 470 seems better than 450 or 460, to my surprise.
I am still working on this occaisionally from time to time as of 2006.
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