colour with $A_V=0$ as the optically thin extreme ($\lim_{A_V\to 0}(\lambda_1/\lambda_2)_\mathrm{cl}$). In other words, the FIR colour calculated with $A_V=0$ (in fact $A_V\to 0$) represents the case where the dust is illuminated by a radiation field without extinction. In the following, $A_V=0$ means that the dust is illuminated by a radiation field without extinction, not that there is no dust. ]{}

Data {#sec:data}
====

Analysis of the *AKARI* data {#subsec:analysis}
----------------------------

H08 analysed eight BCDs observed by FIS onboard *AKARI* with four photometric bands of *N60*, *WIDE-S*, *WIDE-L*, and *N160*, whose central wavelengths are 65, 90, 140, and 160 $\mu$m with effective band widths of $\Delta\lambda =21.7$, 37.9, 52.4, and 35.1 $\mu$m, respectively [@kawada07]. The measured FWHMs of the point spread function are $37\pm 1''$, $39\pm 1''$, $58\pm 3''$, and $61\pm 4''$, respectively, for the above four bands. We exclude Mrk 36 from the sample, since it is not clearly detected at *WIDE-L* (140 $\mu$m). The other seven sample BCDs in H08 are listed in the lower part of Table \[tab:flux\].

  -------------- ---------------------- ---------------------- ----------------------- -----------------------
  Name                   *N60*                 *WIDE-S*               *WIDE-L*                 *N160*
                  ($\lambda =65~\mu$m)   ($\lambda =90~\mu$m)   ($\lambda =140~\mu$m)   ($\lambda =160~\mu$m)
  UM 420           $0.46\pm 0.05$ Jy      $0.37\pm 0.04$ Jy       $0.14\pm 0.03$ Jy          $<0.36$ 