I have order increment id like 100000096 and i want to get its invoice id, please help me to achieve this in magento, i have tried with

```
                 $incrementId = '100000096';
               $order = Mage::getModel('sales/order')->load($incrementId);
                   $invIncrementIDs = array();
                     foreach ($order->getInvoiceCollection() as $inv) {
                         $invIncrementIDs[] = $inv->getIncrementId();
              }

```

But it returns an empty array.