For Thyeth,
I think it would be very hard to accomplish with a cellphone, but I know a way you can do it.
Since these types of images end the photo with the flash, you will need at minimum 2 people, 3 would be better.
1) Make sure it's very dark (of course).
2) Put the cellphone on a tri-pod or something very sturdy (If you have a remote to control the camera on the phone, it would help. For example: Samsung Notes can use their stylus).
3) Start the picture with no flash in time-lapse mode.
4) The target or someone else moves the lightsaber.
5) Put your hand over the lens to block all light from coming in.
6) Flood the target with light.
7) The target freezes in position with the lightsaber (VERY IMPORTANT).

Quickly move your hand away from the lens and then back over the lens.
9) Shut off the light.
10) End the photograph.
Greetings Schaeffer,
Great images, full of action even though they are static pictures. I always wanted to try this type of Long Exposure photography but didn't have a camera/phone capable of doing it so I had to resort to photo editing (I just got a new smartphone so I'll have to check if it has Time Lapse on it).
Thanks for sharing, I hope your son's saber serves him long and well (so you can post more photoshoots - we like photos here

) and enjoy the forum.
point incoming.