PIC 18F4550
Use phase control at the required brightness level in normal circumstances and use a PIR motion detector to detect a movement and trigger full phase angle for full brightness for a predetermined time. Write a program with the said sequence an dprogram the PIC. But its quite easy to do this even without any microcontroller. Just use a standared fan dimmer and connect bias resistors for controllling brightness, through a relay in conjunction with the PIR...Cheers