[OperationContract]

public List<LightPh> GetI()
    {

        ... here i read a database and populate a list of LigthPH
    }

public class LightPh
    {


        public int IdImagen { get; set; }
        public string nombre { get; set; }
        public int IdProd { get; set; }
        public byte[] imag { get; set; }
        public bool isp { get; set; }


    }

<asp:ScriptManager ID="ScriptManager1" runat="server">
    <Services>
        <asp:ServiceReference Path="Serv/JavaAjax.svc" />
    </Services>
    </asp:ScriptManager>

<script type ="text/javascript">
    var i = 0;
    var imid = 1;
    var l = 0;
    var re;

    function GetValueFromServer() {

        JavaAjax.GetI(onSuccess, onFailure);

    }

    function onSuccess(result) {
        l = result.length;
        re = result;
        go();
    }
    function go() {
        if (l > 0) {
            document.getElementById('TextBox1').value = re[0].nombre;