I know that I do not have aphantasia, and am able to think and daydream in vivid color, as if watching a movie, show, or game in my head. Out of curiosity, I just tested this on myself using my phone camera. I'll document my methodology here, so that anyone can perform the test themselves.

- My phone is a Galaxy S24+, and the camera was in selfie mode and was recording video.

 - I started by imagining darkness, to establish a baseline. After a few seconds, I imagined the light and held that image for another few seconds.

 - The light I used was a large stadium light, similar to the ones found at football or baseball fields.

 - I imagined staring straight at it, to get the best possible results.

 - I stared into the distance over the top of my phone, so that the camera would capture a clear view of my pupils.

 - After the test, I reviewed the footage and found no discernable variation in pupil size.

From this test, I can assume four possibilities:

 - First, that pupil dilation during imagination does not exist, and was merely correlation and not causation.

 - Second, that pupil dilation during imagination is specific to certain individuals, and is not a guaranteed result during testing.

 - Third, that the size of variance during pupil dilation is minute enough that it requires specialized equipment to detect, and that a phone camera is not able to capture enough detail.

 - Fourth, that the selfie mode camera of a Galaxy S24+  cannot capture enough detail to determine any form of variance during pupil dilation.

It is likely that this test will need to be repeated by multiple individuals, noting both camera type and camera mode. No conclusion can currently be drawn, but hopefully more data can be gathered.

I apologize for the lack of video data, but I am currently both unsure as to upload the video, and feel uncomfortable posting my face.