linux/shell command to control screen brightness in android

I am working on a kiosk project using an android tablet.If there is no input power to the kiosk for a long time then, the tablet will eventually shutdown.In order to auto-reboot the tab when the power comes back I modified the code in my tabs battery animation file(ipod) using:

#!/system/bin/sh
sleep 300
/system/bin/reboot

However,during the sleep period(shown above),the tabs screen remains in the ON state (and hence takes longer retries for the tab to charge and bootup).I need to turn the screen OFF. What is the command I should use prior to the sleep command to turn my tabs screen OFF during the charging/bootup stage. Thanks !