The first and second order spectrum of IRAS 4B at  ~100 μm. The second order light at 100 μm (blue), calibrated from the  ~200 μm spectrum, matches the standard first and second order spectra (red and green). The lines in the blue spectrum are artificially weak, by a factor equivalent to the ratio of first to second order light, because the lines and continuum are treated separately. The dotted vertical line shows the location of the CO 13–12 line at 200.27 μm.