Magento : How to add multiselect to magento system configuration?

============local\Namespace\Module\etc\system.xml=======

<select_orderField translate="label">
    <label>Order Export CSV Field</label>
    <frontend_type>multiselect</frontend_type>

    <source_model>Namespace_Module_Model_system_config_source_view</source_model>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>0</show_in_store>
</select_orderField>

========== local\Namespace\Module\Model\System\Config\Source\View.php==========
<?php
class Namespace_Module_Model_System_Config_Source_View
{
    public function toOptionArray()
    {
        return array(
            array('value' => 'Order Number', 'label' => Mage::helper('adminhtml')->__('Order Number')),
            array('value' => 'Order Date & Time', 'label' => Mage::helper('adminhtml')->__('Order Date')),
            array('value' => 'Order Status', 'label' => Mage::helper('adminhtml')->__('Order Status')),
        );
    }
}
?>