当angularjs用于表单验证时如何集成验证码

I use angularjs and php for my project and angularjs for my form validation and it's pretty good so far. But I haven't thought of captcha till now and how to add that. There is an opensource phpcaptcha but not sure whether I can use that(since I'm using angularjs for validation). What could be the best way to include captcha in my form?

Here's the part of my form with angularjs:

<tr>
    <td><label>Enter the college Name:</label></td> 

    <td><input type="text" size="50" minlength="2" maxlength="50" ng-model="$std.clg" name="clg" placeholder="Enter College Name Only In Alphanumeric Characters" ng-pattern ="/^[a-zA-Z\s]*$/" required onkeyup="numberCheckCollege()"> 
    </td>
    <td>
         <span style="color:#F47216" ng-show="student.clg.$dirty && student.clg.$error.required">
        College Name is required 
        </span>

    </td>

 </tr>
<button ng-disabled="student.$invalid" ng-click="register($std)">Submit</button>

Here just before the button I would like to add captcha. Since I'm using angularjs for validation, Am I being forced to use the same for captcha too?