curves and the spectra of optical, X-rays and $\gamma$-rays from these two pulsars (Zhang and Cheng, 2001b). Although the mean free path of the primary photons from the outer gaps of these two pulsars is much shorter than the light cylinder and hence the radiation from secondary pairs still dominate in the observed spectrum, the thickness of the outer gap in these two pulsars are 0.25 and 0.3 respectively. Again the emission regions cannot be approximated by a single surface. In Figure 8, we have compared the model spectra and the observed spectra of PSR B0540-69 from optical to gamma-rays. The more detailed comparison between model results and the observed data of PSR B1509-58 and PSR B0540-69 can be found in Zhang and Cheng (2001b)

[Fig. 8. The comparison of predicted phase-averaged spectrum with the observed data for PSR B0540-69. Observed data at optical waveband are taken from Middleditch et al. (1987), Hill et al. (1997). The data at ROSAT energy range, BeppoSAX energy range, COMPTEL and EGRET are taken from Finley et al. (1993), Mineo et al. (1999), Hermsen et al. (1994) and Thompson et al. (1994) respectively. The solid curve represents phase-averaged spectrum. The dot-dashed, long-dashed and short-dashed 