ARRI Forum

Shooting on an Alexa XT Plus, I noticed what looks like a dead pixel on just one single frame. I've seen this before on a Red Epic, but never on an Alexa. It should be a simple fix in post, but I'd like to know what causes this, and if it can be prevented.

Hello there,

looking at the image, zooming in... I tend to say it is not a dead pixel:

• - only on one frame
  - it is white
  - it is 2x2 pixels in size
  - when you're shooting 16:9 HD on an ALEXA, we use an sensor area of 2880x1620 as source which is being downscaled internally to 1920x1080. So a dead pixel would never be that prominent and again as big and usually not white.

Therefore I lean towards the occurrence of "digital dust". That's high energy particles hitting the sensor (its source is natural or artificial radiation - depending on your location). Usually those look like white streaks since they tend to hit the sensor in a non-perpendicular manner; but its shape may vary: dots, lines, we even saw strange "curves". The location on the sensor/image is random and it occurs on one frame only.

Cheers,
Jan

Yes I think you're right. We were shooting 2K, and after looking at a 100% crop in LogC, I see it's actually bigger than 2x2. Here is a blown up view from Photoshop. So do all camera sensors develop this as they age, and are exposed to more radiation?

It is not a thing of age; can happen to all cameras at any time.
But it is more likely to happen on top of a mountain than in a shielded concrete cellar.

Most important: No harm is done besides the white spot in the frame.

Hi, yesterday while shooting and checking the footage I found what looks like the same issue. We are shooting with an Alexa Studio XR sensor mode 4:3 ARRIRAW.





Some years ago, when I was doing an internship at a camera rental house, happened the same during the shooting of a feature and the explanation given at the time was related to the sun storms and the gamma radiation... ¿?

This explanation has something to do with the "digital dust" given in this thread? It´s the same thing?

Hey Saavedrarolan,

a yes on your question. It's save to say that all white (aka high energy) traces that last for one frame and leave no damage on the sensor fall into this category.

Best,
Jan

Very Interesting, I'm currently experiencing this on one of my shoots. See below.

I just finished shooting a series of yoga workouts on a black background with the Alexa Mini (2 Camera Shoot). About a month after shooting, the editor noticed a number of white specks throughout the footage (both A and B camera). The white specks would randomly appear in a one frame and disappear in the next (no consistency to area of frame). After speaking with Arri, they believe that this is an environmental issue caused by cosmic radiation. We've currently found about 15 specks within 3 episodes of shooting (which were shot on 3 different days). See below for a couple of samples with the specks. Let me know if you've seen anything like this before. We're going to be shooting the next round in a week and are trying to prevent this issue.

(Update) I have recently tested (2) different Alexa Minis in a separate location and have had the same issues.

Best,

Chad Mahadevan

Dear Chad,

if you put the cameras in a lead shielded case in the very same location, you probamly won't notice anything.
(Well you might, if the cameras have been exposed to radiation before).

This is a effect you can notice on all digital cameras. Depending on the physical architecture of the photosites, the effect will show in diffrent shapes or colors.

Best,
Jan

Hey Jan,

Thanks for the response! Just to fill you in on our current situation. We have tested 6 Alexa minis, 1 Amira and 1 Alexa XT within 3 separate locations and have had the same digital dust issue. We have also tested a Sony f55 and a Sony FS7 and didn’t experience any digital dust. Is it possible that the Alexa sensor is more susceptible to experiencing white specks than other cameras?

Thanks again!

Chad Mahadevan

Hey Chad,

we have not conducted a broad comparison series with other cameras, but from our experience it is a random issue that has a geographic factor in it.

I cannot speak for other camera manufacturers.

Best,
Jan

Wow this happened to me one time a few months ago while out shooting in the wilderness at night.

I was scared my trusty ole Alexa Classic was starting to fail.

It's never happened since.

I feel better now knowing what it was I was seeing!

How often should something like this happen? I noticed it a while ago on two jobs back to back and was worried but found this thread and seemed to be exactly what I was experiencing so felt better. But I feel like I keep noticing it, sometimes several times on the same job. It seems like it's almost every time I use the camera I notice one. At what point should I be concerned about a sensor issue vs it just being this?

Dear Matt,
experiencing multiple digital dust events during a shoot should raise a call to the union due to possible radiation hazards on site. In this case we'd be talking about images similar to this: https://www.youtube.com/watch?v=t2GNvHRjcz8.

If they occur from time to time, one incident at a time, its is considered normal/background radiation. High energy particles are a natural fact. They tend to be more present in high altitudes than is concrete parking structures ob sublevel 2.

For a more detailed analysis of your footage, feel free to get in touch via digitalworkflow@arri.de.

Cheers,
Jan

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