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Author Topic: Low cost homebrew red LED light source for cyan tracks Warren Smyth Expert Film Handler Posts: 158 From: Auckland ,New Zealand Registered: Aug 2003 posted 08-19-2003 11:32 PM                     Thanks for the advice Jeff and Steve. You comments certainly seem to rule out the option of red filters. Sam, I take your point (no pun intended) about the difficulty of achieving uniformity of light across the slit with a single point source.If we still had variable density recordings it wouldn't be such a problem, but let's not go down that track (no pun again). Diffusing the beam, while not very efficient, may be worth a try. A mini anamorphic lens is a thought, but there would still be fall off of light at the sides. However the commercial solution, an array of leds seems less than perfect for that matter, having a row of "hot spots" at the source. Anyway, I will be grateful for availability details on the Red Star led Jeff and will contact you. A $10 investment is worth it just for something to experiment with. I don't anticipate running cyan tracks for a while and really just want someting I can use in case I get caught. By that time, there may be a more economical option.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 08-20-2003 10:18 AM                       Warren makes a good Point about an array having a series of hot spots and is indeed correct. They do get diffused and amount to perhaps 1 dB bumps but they are there. You can see them when aligning Dolby Digital readers. These bumps are of no consequence in digital but are less than perfect for analog. Jack Cashin developed a bar LED to combat this problem. This LED emits a shaped beam of light with no bumps. However, the yield on the production run was something like 40% and essentially the part was deemed not manufacturable and the effort was discontinued.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 08-20-2003 12:52 PM                     Century played with an anamorphic reader in the imfamous AnaFet Kinoton makes a read reader assembly that uses a quartz lamp(similar to a slide projector lamp) and a dichroic filter as a replacement to some of there machines  |  IP: Logged Phil Radosevich Film Handler Posts: 22 From: Centerville, IA, USA Registered: Jun 2000 posted 08-22-2003 04:58 AM                     I runa jaxlight have for 4 years mostl for trailers,run in dts for the feature,thanks to mark and a fire, I noticed last night that jaxlight has some sort of preamp if that's what it is does anyone know? Jeff what have you done about that?  |  IP: Logged Jeff Stricker Master Film Handler Posts: 481 From: Calumet, Mi USA Registered: Nov 1999 posted 08-22-2003 07:14 AM                     Phil, from the tests I've run using standard silver tracks additional amplification wasn't needed. I'm still looking for a piece of cyan track film to see if the amplifier gain is adequate.  |  IP: Logged Ray Derrick Master Film Handler Posts: 310 From: Sydney, Australia Registered: Sep 2001 posted 09-16-2003 10:15 PM                     Sam said in relation to the Dolby red LEDs "They do get diffused and amount to perhaps 1 dB bumps but they are there." The amount of diffusion is determined by the distance of the LED array from the point of focus (ie the film). As the LED is moved further back from the film there is a point (about 1mm from the film) where optimum diffusion is achieved. Any closer and the elements start to come into focus. Any further away and the array becomes so magnified that once again the diffusion effectively diminishes. The spacing between the individual LED elements in the 2 x 12 element array is equal, both horizontally and vertically. If a LED array is correctly positioned 1mm back from the film, adjusting it's vertical position in relation to the lens produces a broad peak in output from the cell, but no discernable dip attributable to the gap between the upper and lower row of elements (at least not that we have been able to measure). It stands to reason that if there are no "bumps" vertically there should be no bumps laterally either. So how did you measure these 1dB bumps in the analog light source Sam? Of the 12 elements in the Dolby array only about half of them actually fit behind the soundtrack, and are therefore visible to the solar cell, so the remainder are pretty much wasted (except to provide some margin for lateral alignment error).  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 09-17-2003 01:05 AM                       I've never actually measured them in analog domain, but like everyone else have seen them in setting up the Dolby Digital readers. I'm estimating that the ripples convert to about 1dB in analog. Lonny used to advise rotating the LED holder to minimize the ripples although the SR.D could care less.  |  IP: Logged Ray Derrick Master Film Handler Posts: 310 From: Sydney, Australia Registered: Sep 2001 posted 09-17-2003 05:56 AM                     Hmmm. So 1dB translates to an 11% variation in light level, yes? Considering there are approx 6 LED elements across the whole stereo soundtrack, that's approx 3 LEDs per channel, so we would see about three such "bumps" across the width of each track. Wouldn't such an 11% variation produce rather horrendous distortion? It would be interesting to test for the actual intensity of any such "bumps" with some accurate measurements. Dolby's illumination uniformity film is not at all suitable, but a weave test film immediately comes to mind. I think there was one such test film available in the past, anyone remember what it was called?  |  IP: Logged Stephen Furley Film God Posts: 3059 From: Coulsdon, Croydon, England Registered: May 2002 posted 09-17-2003 06:51 AM                     Phil, Jaxlight uses the existing cell with a new red light source, this produces a reduced output compared to a silver track with tungsten source, hence the need for the extra pre-amp.  |  IP: Logged Ray Derrick Master Film Handler Posts: 310 From: Sydney, Australia Registered: Sep 2001 posted 09-17-2003 06:30 PM                     I said "weave test film" but "snake track" was the term I was looking for.  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 09-17-2003 11:31 PM                       Ray states there 6 elements across the whole sound track. I believe there are 12 x 2, right? Am I missing something? On the subject of the magnitude of the ripples, I probably mis spoke but I do remember some figure being mentioned many years ago when Dolby was developing an analog optical scanner where you could see the little ripples. Some engineer extrapolated the significance of the ripples and it stuck in my head but I don't remember the figure and if it was worst case or best case.  |  IP: Logged Steve Kraus Film God Posts: 4094 From: Chicago, IL, USA Registered: May 2000 posted 09-17-2003 11:47 PM                    Sam, Ray said that only 6 are actually illuminating the active area of the track: quote: Of the 12 elements in the Dolby array only about half of them actually fit behind the soundtrack, and are therefore visible to the solar cell, so the remainder are pretty much wasted (except to provide some margin for lateral alignment error)  |  IP: Logged Sam D. Chavez Film God Posts: 2153 From: Martinez, CA USA Registered: Aug 2003 posted 09-18-2003 12:43 AM                       Steve, I thought of that immediately after the post. Still, I don't believe it's quite that extreme. Track is .084. LED array is .144. Don't forget the diffusion or spread works inward as well as outward so the LEDs on he edges contribute as well.  |  IP: Logged Ray Derrick Master Film Handler Posts: 310 From: Sydney, Australia Registered: Sep 2001 posted 09-18-2003 04:29 AM                     Sam, I've not set out to be argumentative here so please don't take it as such. However I did say "approx" six LEDs not "exactly" and it is based on actual observation of the image that is "seen" by the solar cell though a reverse scan lens with the LED positioned both right up against the film, and moved to it's correct position away from the film, not on postulation from theoretical dimensions. You can also get a reasonable idea by simply laying a piece of buzz track centered directly over a Dolby LED and you will clearly see three complete LEDs on either side laying completely OUTSIDE the inner margin of the buzz track image. By my math that leaves 6 complete LEDs laying within the soundtrack area. However this second method does not take into account the magnification of the LED array when properly positioned back from the film and observed through a lens which is focused onto the film. As the LED array is moved away from the film, the image of the LED array beomes magnified as it becomes diffused. So the number of elements visible to the solar cell becomes less, even though the image becomes diffused. Now any "bumps" across the light source, as "seen" by the solar cell, will still be due only to those elements visible to the solar cell, not due to any diffused light eminating from elements outside the view of the cell. BTW, Obviously I am only referring to one of the two rows of elements for the purposes of this discussion. Now come think of it, it would have made more sense to offset the upper and lower rows such that the lower elements sat half way between the upper elements to minimise any non-linearity. Now why didn't Dolby think of that?  |  IP: Logged David Buckley Jedi Master Film Handler Posts: 525 From: Oxford, N. Canterbury, New Zealand Registered: Aug 2004 posted 08-05-2004 01:08 AM                        Jeff quote: Jeff Stricker I need to get another red Luxeon star to test my other soundhead that has a stereo solar cell quote: Jeff Stricker Also, does someone have a small amount of film with a cyan track to test this contraption? Did you ever get around to performing these two tests? If so, how did it all go?  |  IP: Logged

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