[email protected] (Hal Murray) wrote in
I'm not sure that citing it as any guide to precision matters much. I just
tried that test I mentioned, I used a Weller WSD-81 'station' and an LR-21
iron, set at 450°C. I could see it before my eyes were fully dark-adapted,
but could not resolve colour. I could see it pretty much equally with
photopic or scotopic vision, done by arranging wide angle changes off-axis
from central vision, but I could only resolve any detail when it was at the
centre. Not much either, I couldn't see the tip shape very clearly but I
could see the silhouette of the heavy guard coil wire as it passed across it.
This appears to me that you were seeing the very-dimly-glowing tip and
maybe the likely-slightly-hotter heating-element-area mostly with scotopic
vision.
450 C is 723 K. The surface over the heating element was likely
somewhat hotter, likely mid to upper 700's K.
The 798 K "Draper point" appears to me to be the threshold of achieving
significant stimulation of photopic vision to the point of seeing color
more than gray.
What did surprise me was that a small near-IR sniffer I built could not see
it. It's not very sensitive but I thought it ought to if I could, given the
stuff I've used it for before, so eyes are obviously rather good at this.
What wavelength range is your "near-IR sniffer" good at, and what
radiant power density in that wavelength range is lowest it will sense?
If you report this, I can figure temperature necessary to achieve this.
Couldn't see a thing at 400°C though.
I am not surprised - that is 673 K.
I consider myself optimistic at thinking that I may be able with best
dark adaptation to dimly see with blurred outlines and no color,
incandescence at 700 K.
I calculate the following values of candela per square centimeter to
photopic vision corresponding to the following temperatures low enough
to achieve glow so dim as to be seen primarily by scotopic vision despite
spectral content at wavelengths long enough to favor photopic:
775 K: 3.25 E-7 (s/p ratio is .131 on a scale where 555 nm
monochromatic yellow-green = 1)
750 K: 1.22 E-7 (s/p ratio is .119 on the above scale)
725 K: 4.27 E-8 (s/p ratio is .107 on the above scale)
700 K: 1.40 E-8 (s/p ratio is .096 on the above scale)
===============
Temperature at which s/p ratio achieved by a blackbody is "unity" as in
same as that of 555 nm yellowish-green monochromatic narrowband light:
2093 K,
which I have determined that a USA-usual 120V 100W lightbulb of
"Big 3 brand" and rated to last-on-average 750 hours and to
produce-on-average 1670-1750 lumens at 120 volts, to achieve at around
53-54 volts, maybe give or take another volt.
I am aware that a few lower-current longer-life vaccum-containing
incandescents have a fair chance of having color temperature this low or
slightly lower. However, it appears to me that more-usual for tungsten
incandescent lamps with design current low enough to be served better by
vacuum than by argon-nitrogen gas fill, along with design life expectancy
around 2,000-3,000 hours, is for color temperature to be not far from
2360 K (which achieves s/p ratio of 1.16 on scale where 555 nm achieves
unity).
The USA-usual 120V 100W incandescent with rated average life expectancy
of 750 hours, rated to produce 1670-1750 lumens, and having CC-6 or CC-8
filament, and of "Big 3 brand", appears to me to typically have color
temp. of 2870 K (2865 K by a slightly older definition revised by a
redetermination of one of 2 constants in the "Blackbody Formula"), appears
to me to achieve s/p ratio of about 1.42.
I hope the above supports my impression that human photopic and scotopic
vision have nonlinearities that differ from each other. I sense that
there is dynamic range compression achieved by both, less for human
photopic vision than for human scotopic vision.
I sense that human scotopic vision achieving greater dynamic range
compression than human photopic vision does, is the explanation for seeing
barely/hardly/minimally/dimly - incandescently-hot objects with color
"degrading to gray" (my words) as temperature decreases from about 800 K
or so to mid-700's K or so.
- Don Klipstein (Jr) (
[email protected])
And what makes THAT interesting is that it implies a bluish
