tinyOS light (send packet if on else no)

Question

I am want to send packet if the light is on (on 500) not from (0) else not send. what I should to do ??

Please please help me on that

  event void Read.readDone(error_t result, uint16_t data) 
  {
    if (result != SUCCESS)
      {
    data = 0xffff;
    report_problem();
      }
    if (reading < NREADINGS) 
      local.readings[reading++] = data;
  }
}