Power spectrum of the line-mass fluctuations observed along the Pipe filament shown in Fig. 2 as a function of spatial frequency s (bottom x-axis) or angular frequency (top x-axis). In both panels, the black plus symbols show the observed power spectrum, P_obs(s). In panel a), the red dots show the power spectrum, P_obs(s)−P_N, obtained after subtracting a white noise power spectrum level ×10^-4M_⊙²/pc, marked by the horizontal red line and estimated from the median value of P_obs(s) in the 3.9–4.2 arcmin^-1 angular frequency range; the blue triangles show a similar power spectrum after subtracting the instrument noise power spectrum level 4.0 ×10^-5M_⊙²/pc, marked by the horizontal blue line and corresponding to the instrument noise level 1σ ~ 1 MJy/sr in our SPIRE 250 μm maps according to HSpot³. In panel b), the cyan dots show the noise-subtracted and beam-corrected power spectrum, P_true(s) = (P_obs(s)−P_N) /γ_beam. The vertical dotted line in both panels marks the FWHM of the beam power spectrum at 250 μm ( arcmin^-1; see Fig. A.1), which is also the highest frequency data point used to fit a power-law function. The vertical dot-dashed line is the Nyquist angular frequency (λ/ 2D) for SPIRE 250 μm data. The power-law fits to the power spectra P_obs(s) and P_true(s) have logarithmic slopes α_obs = −2.1 ± 0.2 and α_true = −1.6 ± 0.2, respectively. Considering only angular frequencies up to = 1.5 arcmin^-1, the best power-law fit to P_true(s) becomes α_true = −1.7 ± 0.3. (This figure is available in color in electronic form.)