The camera was mounted on a tripod in totally darkened room.
The shutter was opened on "bulb" while subject wrote "1978" in the air with a burning sparkler.
To reduce connecting lines, I held my hand over the camera lens when the subject moved from one digit to the next.
Finally a hand-held flash was pointed at the subject and manually fired, then the shutter was closed.
The image had to be reversed (mirror image) to read correctly.