Mark Lloyd wrote:

Every black body above absolute zero emits some visible light.

Would it always be in the limited frequency range this is visible to humans?

There would always be some visible light, but it's very faint at low temps :-)

20 PI=4*ATN(1)
30 LAMBDAU=7.8E-07'upper visible limit (meters)
40 LAMBDAL=3.8E-07'lower visible limit (meters)
50 FOR LT=2 TO 4'log F temp
60 TF=10^LT'temp (F)
70 TK=(TF+460)/1.8'temp (K)
80 GAM=.0143879/(LAMBDAU*TK)'Pivovonsky and Nagel (1961) polynomial term
90 CFRACU=0'initialize upper cumulative fraction
100 FOR M=1 TO 3
110 TERM=15/(PI^4)*(((M*GAM+3)*M*GAM+6)*M*GAM+6)
120 CFRACU=CFRACU+EXP(-M*GAM)/(M^4)*TERM
130 NEXT M
140 GAM=.0143879/(LAMBDAL*TK)
150 CFRACL=0
160 FOR M=1 TO 3
170 TERM=15/(PI^4)*(((M*GAM+3)*M*GAM+6)*M*GAM+6)
180 CFRACL=CFRACL+EXP(-M*GAM)/(M^4)*TERM
190 NEXT M
200 P=5.6697E-08*(TK^4)'total emissive power (W/m^2)
210 EFRAC=CFRACU-CFRACL'fraction of emissive power in visible spectrum
220 PRINT TF,EFRAC,P,EFRAC*P
230 NEXT

temp visible fraction total power visible power

100 F 6.010933E-22 531.1561 W/m^2 3.192744E-19 W/m^2
1000 2.753038E-07 24540.4 6.756065E-03
10000 .4668266 6.465423E+07 3.018231E+07

Nick