"Double Exposure" Effect on Candles/Lights

Here's a screenshot from tonight's airing of Dad's Army (click for bigger):


Notice predominantly the red tint around the flame of the candle which is kind like a "shadow" effect of where the candle was a momet previously. This is better demonstrated by the actual video in question:
http://bbc.in/1DQQHVA (scene starts from 10:25)

I did read somewhere about the cause for this "shadow/double exposure" effect on things like candles and lights and it was either due to the cameras used or the videotape used in studio recordings in the 1970s - it certainly wasn't unique to Dad's Army. And of course I can't find it.

I presume the effect could appear with any of the three primary colours and it just happened to be red in this case?

It was due to the tubed cameras, wasn't it?

Yes. Bright lights would "burn" into the tube, causing a similar effect to what happens if you look at something bright. CCD's were much better in this respect, but can end up smearing a bright light vertically over then entire picture.

The brightness of the signal took several frames to decay/clear from the tubes in the camera, and it faded away slower in the red tube, hence the red halo lasting longer. The overall affect was to give comet trails.. There were compenating circuits that could reduce the effect.Thank god for CCD cameras in the nineties.

I have actually seen the effect on some filmed stuff. I'm presuming they must be colour telerecordings of video material (it was on a music video shown on a 1980 TOTP that was on BBC4 last year, for example), as I don't think this effect would happen with telecined film, unless I'm wrong?

Yes the effect was prominent on old studio recordings of bands performing etc under bright studio lights, especially on the 70s recordings of TOTP.

No - you'd see it on something shot on tubed cameras and then telerecorded (kinescoped for US readers) to film. Film telecined to video wouldn't have had it by the time we got to colour telecines. TOTP often got US stuff shot on video, delivered on film.

Sticky/laggy tubes and comet tailing, along with image burn in, were a fact of life with tubed cameras. I remember being really disappointed when TOTP moved to Elstree, as it switched back to tubed cameras, afte havingr just switched to CCDs mainly at TV Centre. Although CCDs would white or red line (well early ones did) on highlights, giving you a sharp white or red line the full height of the picture on highlights, this was a lot less annoying, in some ways, than a tubed handlheld covered with spots left over from the lasers on a previous performance...

Yes, a combination of lack of sensitivity of the tubes and the ability of a tube to discharge the target plate. Generally but not exclusive to the red tube as it was the least sensitive and therefore always the first light out of the colour splitter prism in the optical path to get as much light as possible to the 'red' tube. A couple of methods to try and reduce it - ACT (anti comet tail) and HOP (highlight overload protection) - different patents I believe.

CCD/CMos sensors don't suffer from it, but they are not completely 'perfect'. Looking at the current pictures from Rio if you look at some of the camera shots with bright specular 'point' highlights you will notice a blue halo effect around the highlight. I pointed this defect out to Sony years ago, suggesting it was the blue sensor not liking single wavelength light from some sources (think fluorescent tubes) - they noted what I said but clearly wouldn't/couldn't do anything about it. I've noticed when the BBC do a scores/position info graphic using a static indoor arena shot as the background that the specular highlights have blue blobs around them!

Noggin, don't forget a few budget stations might have had tubed based colour telecine channels, particularly in the US as they could handle stills as well as film and were therefore considered good value. Southern's Dover station had one, but I couldn't tell you the make.

Interesting thread. Black and white TV also seemed to suffer badly from the effects of bright lights which sometimes appeared dark at the centre of the light, or developed dark halos like this...



I presume the cause is similar?

Yep - artefact of the Image Orthicon tubes used in B&W cameras and early US colour cameras. In colour it looks very odd indeed - you see it in early Johnny Carson show clips. European colour services were later and launched, in the main, with plumbicon colour cameras (which continued to be used until CCDs - with a few Saticons and other variations on a theme)

Have look at this link:

https://en.wikipedia.org/wiki/Video_camera_tube

... and look for the section on dark halo.

Yes - artificial detail correction added 'in tube'. Probably one of the reasons people loved IOs in the B&W days - as they gave such a sharp/crisp-looking picture. (They also suffered less from lag than Vidicons, which used to 'smear' a bit).

AIUI the EMI 2001 was originally designed to take Vidicons (when it would have been the 2000?), but the BBC were expected to reject it as plumbicons were so obviously better), so a re-engineered 2001 with plumbicons was quickly developed.

Incidentally AIUI CBS and Philips/Norelco in the US worked very hard to get Plumbicons to work as they realised the existing tubes were terrible AND RCA were effectively part of NBC (or vice versa) and CBS really didn't want to have to buy (or continue to buy) RCA colour cameras (effectively buying from their rivals)