Yttrium Aluminium Perouskite crystals are more efficient in light output than YAG (see graph). Also the emission spectrum peaks at about 378nm which corresponds closely to the maximum sensitivity of the S11 photo multiplier that is in general use in most electron microscopes. There is therefore a significant improvement in signal by using the YAP crystal rather than the YAG. The decay time of YAP crystals is 40ns compared with YAG of 80ns, so overall performance of YAP is superior to YAG.