how to extract value of captcha image and check that value

Hi,
in my application , iam using captcha for security.
in my login example i want to check the captcha value.

here is my jquery code
$('#btnsave').click(function (e) {
e.preventDefault();
var txtval=$('#txtcapcha').val();
var captcha = $('#imgCaptcha').val();
if(txtval==captcha)
{
//go to another page

}

});

when i tried to display captcha value in alert box it is showing empty.
how to solve it.


Regards
Baiju