Why are the TV screen dark, blackish bands?

[vivitern]

I was trying to capture the scanning process of a CRT TV screen in a photo. It gave me these dark, broad bands, shown below: Fig. 1 a,b,c

I'm trying to understand the cause for this banding. However, I couldn't understand it clearly in very similar questions here, here, or here. All web sources I visited very briefly explained it as a difference between the scan-rate of the TV and the camera, and none provided a diagram.

Fig. 2 a,b,c

I assume what is going on is in Fig. 2. The blue lines indicate the scan lines where cathode rays hit most recently (phosphorescence occurring), and black lines indicates where the cathode ray has not been in the image, so phosphorescence is all that is left and a dark band appears.

I assumed this, because out of 10 photographs I took, I found only the above 3 types of photos, and none of the type shown in Fig. 3 or Fig. 4:

Fig. 3. an Fig. 4.

I also assumed that the camera is much faster than the TV scan rate, so it could take picture in an instant.

My question is, Have I assumed correctly? or is something else going on?

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Additional information:

TV:     Videocon 120 W colour CRT, Electricity power supply 50 Hz. 
Camera: Nikon Coolpix L24 digital camera.

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(please also explain other possible situations, such as this one in Fig.5. I didn't experience it myself. It is similar to Fig. 4.

Fig 5. From this question

I'll be grateful if any explanations use diagrams. What if it is a LCD?

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I've obtained several fig3 conditions, and though they are less frequent, they're not very rare.

File properties show exposure time for this still-photo is 1/125 second.

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I've also took some videos (in PAL and NTSC mode... both gave same result), file properties show frame rate is 30 frame/sec; and run them on slowest motion. I found similar, but much brighter and quite unclear bandings, and it seemed from slow-motion video that the alternating bright and dark band rising on the upward direction. From that videos I screenshoot some frames as-successive-as-possible. . Red small bars added to indicate the margin of each banding

 

[frgood]

If I understand the explanations you have collected, would not the camera have to take the picture at the exact same time as the scan completes a full pass?   I believe that is the syncing process between the camera and the TV. Both devices are scanning. So the camera must start scanning the moment the TV completes a scan.

[touji]

If I'm understanding this correctly, this is an issue with syncing the shutter speed of the camera and the TV. What screens do is that they are refreshing parts of the screen at a certain frequency, and the types of bands you receive are from the discrepancy of the refresh rate of the TV and the shutter speed of the camera. If your shutter speed is too fast, you will be capturing the screen in a single frame where there are certain parts of the TV that are refreshing.

 

If your camera has the capability to manually adjust shutter speed, try a shutter speed of say 1/10 as this should give you 3 full frames to capture (assuming your CRTV is 30hz).

 

Let me know how it works out!

 

[Nik Habicht]

What Touji said, you need a slower shutter speed on the camera to capture that.  1/30th or 1/15th should work.....

 

If you're worried about holding it steady, try to shoot 4-6 pictures in continuous, the center images should be sharper because you're not pushing the shutter button down or releasing it....

 

Bracing your elbows on a table, or other solid service can also help you hold the camera steady....