A hard-light hologram was a hologram which was solid.
Strax's Fighting Drone is an example of a Hardlight hologram. In 1894 Strax tinkered with the drone coursing it to malfunctioned and begin roaming Paternoster Row. Most of the residents thought that they were being hunted by a Ghost and blamed Madame Vestra for it. (PROSE: The Spectre of Paternoster Row)