Featured Image open rel="lightbox"

How and what do I change to get the images to open in a lightbox and not the image’s url page?
Thanks!

<div class="row">

                    	<!-- START PORTFOLIO LOOP -->
                    	<?php $loop = new WP_Query( array( 'post_type' => 'our_work', 'orderby' => 'date', 'order' => 'DESC','posts_per_page'=>12 ) ); ?>

                         <?php while( $loop->have_posts() ) : $loop->the_post(); ?>

                        <div class="col-sm-6 col-md-6 col-lg-4">
                            <div class="portfolio-item img-responsive">

           						<a href="<?php
$feat_image = wp_get_attachment_url( get_post_thumbnail_id($post->ID) );
echo $feat_image;
?>" rel="lightbox" target="_blank">
                             <?php if ( has_post_thumbnail() ) {
									the_post_thumbnail( 'full', array('class' => 'img-responsive') );
									}
								?> </a>

                            </div>
                        </div> <!-- end col -->

						<?php endwhile; ?> <!-- END PORTFOLIO LOOP -->

                        </div>
                        <!-- /.row (nested) -->

                    </div>
                </div>
                <!-- /.col-lg-10 -->
            </div>
            <!-- /.row -->