I've lowered the light source, added more diffusion and made a few prints. The results are promising, but there needs to be more 'tests'..... always MORE TESTS!
;-)
I've lowered the light source, added more diffusion and made a few prints. The results are promising, but there needs to be more 'tests'..... always MORE TESTS!
;-)
Looking good! Go make prints
Seriously though, one can test, test, test, and up to a certain point, that's useful. But personally in a project like this, at some point I just start making prints and then fix problems as I run into them. It's easy to overshoot the target.
That's what I'm doing..... making prints and then we'll see what happens! Everything is a test of one sort or another! Happy New Year!Originally Posted by koraks
Looking good! Go make prints
Seriously though, one can test, test, test, and up to a certain point, that's useful. But personally in a project like this, at some point I just start making prints and then fix problems as I run into them. It's easy to overshoot the target.
I've been using an enlarger LED head for a couple of years now. It works great with multigrade paper. I made it to fit inside a Durst Laborator 1000 when the condensers are removed. But thinking of those starting with analog photography, I made a second one in a way that it works like one of those old Graflarger, from Graflex, fitting on the back of a 4x5" camera. In case anybody is interested, maybe in using just one or another feature, the complete project is here: https://apenasimagens.com/en/enlarge...plete-project/
That's a very nicely documented project indeed! And I think I remember the writeup of your original LED light source back when I started out working on the same thing. Ultimately I decided to do a couple of things differently, and based on that experience, I have some concerns about your design, particularly the electronics. Firstly, the LM317 current limiter configuration is probably not thermally stable. For B&W work, this will *probably* not matter much, but for color work, the resulting color drift will be problematic. It took me a long time to figure this out in my design. Besides, especially on the blue and green channels, there's very little headroom for the LM317's to regulate the current, and I doubt that this is beneficial for linearity.
Secondly, there are some thermal issues. Mounting 3W LEDs in close proximity on regular FR4 board seems like a bad idea again in terms of thermal stability. These LEDs are intended to be mounted on a heat-conducting substrate. I see you did enlarge the center pads a bit as a makeshift heatsink, but the C/W value of this will be orders of magnitude lower than a proper thermal management solution such as a metal-core PCB (which you can also etch at home) with a proper heatsink. A more minor issue is the 7805 that is used without a heatsink, but since you only have one relay active at a time, you probably can get away with it. Note that with a 7V drop across it, even a modest current draw of 100mA from the 7805 will heat it up beyond the point where you can touch it without burning yourself. A buck converter in this place would achieve the same but with greater efficiency and no need for a heatsink. The current pass resistors (the 3.9R ones) used in conjunction with the LM317's appear to be 0.5W types, but you're setting them for a 320mA current, which makes them dissipate around 0.4W. This is far too much for a 0.5W resistor; 2W types should be used instead for good stability. Keep in mind that the resistors heating up will also affect the current, creating further non-linearities in the light output. You're also dumping a lot of heat into those LM317's; even on the blue channel it'll be close to 0.5W per device and it'll be far more on the red channel.
Thirdly, there's an odd omission of a base resistor in the TIP120, apparently relying on the GPIO impedance (which is in the range of 25 Ohms or so if memory serves). For a MOSFET this would be OK, but when using a BJT (even a Darlington pair) it's again a somewhat risky approach. I've seen lots of Arduino projects fail because of a lack of understanding of how a BJT works and that it's fundamentally different from a MOSFET. Indeed, I'd suggest for simplicity to replace the TIP120 with a logic-level MOSFET.
Another issue is the LEDs themselves; it's been tested by several people before and it appears that the blue on RGB LEDs often doesn't allow a true grade 5 to be achieved. Moreover, you noted that you haven't tested your design for color enlarging, but that it should work in theory. My experience is that in practice it won't work very well, because the red is also problematic. With these kinds of LEDs, it's difficult (or in fact, impossible) to get clean primaries and prints free of color crossover. For B&W this is of course not a concern, although the red may still fog B&W papers, so its use as a focus aid with the paper already on the easel must be approached with caution.
Is there any particular reason why you're choosing 252 (0xFC) as the maximum value for the green channel? Not that it matters much, but as you noted, you want lots of green and less blue, so why not max out the green channel for grade 0? Also, I'm a bit surprised you did lay out a PCB for the LEDs, but not for the rest of the electronics. It would have made the wiring and installation into the enclosure a lot cleaner. Technically and functionally, the result is the same of course, but the odds of making a mistake during assembly are in my experience lower when working with a PCB.
All considered, I think the approach ASA1000 has chosen is more robust, more efficient as well as much easier to implement for the average hobbyist.
With SO many LED 4X5 heads are either in production or DIY, we need a test or master full size 4X5 negative to compare
progress
https://www.stouffer.net/TransPage.htm does not have one
Perhaps we can make them....
How many would need color?
How many master negs?
What would be good enough?
Tin Can
YMMV. The thought you put forward is an interesting one, and it gets complicated really fast. The question is mostly to what extent it's necessary, which will be a personal decision.
For B&W, you'd have to at least standardize on a paper and developer to get consistent outcomes across different setups, in addition to which a Stouffer wedge would be a good starting point.
For color, at least the paper would have to be standardized and there would have to be 'master' negatives with good consistency. Perhaps the process control strips as used by e.g. Kodak would be a possible starting point, although these are very limited; AFAIK they only offer three greyscale patches.
Honestly, I personally wouldn't bother, since for me, what counts is if I can do what I want/need to do with a setup. That's also what my previous post mostly touched upon: does the system make accessible the entire contrast range a B&W paper has to offer, and is it capable of printing presentable colors onto RA4? If so, and it's also robust enough to survive years of regular use, it's good enough for me. It's an entirely subjective 'standard'...
I used miles of this FujiFilm, but not useful in our scenario
https://www.tekscan.com/products-sol...oaAkpjEALw_wcB
Tin Can
Koraks, Thanks for your considerations. As they arrived when I have already printed hundreds of pictures using that LED head, I can't be but happy that I was not aware of those points you made at the time I built it. I could have abandoned the whole thing or could be stuck until today thinking of better ways to do it.
Nevertheless, as I believe your remarks are worth to consider, I was trying to conciliate the theoretical grounds of your thoughts and the evidence I have that the project, as it is, works consistently and has done so for many years now.
Thermal processes are also a matter of time. Heat travels at its own speed. What is probably happening is that in normal use the whole thing is not “on” long enough for those thermal concerns to become a practical problem.
As for the Blue LED and its inability to yield contrast 5. I did not make any comparative test against MC filters. But I remember checking Ilford papers with the spectral response of MC papers and concluded that Royal Blue 440-450 nm would be suitable. Furthermore, I have a Leitz Focomat with MC filters, side by side with my LED Durst Laborator 1000 and I never felt I was hitting the ceiling with the LED head. If there is a difference I never felt it as a constrain for my printing practice. I actually avoid in my negatives the need of using contrast 5.
About the 252 instead of 255, I don't remember why I used it. Probably that was a result of a math function and I entered the figure as I got it.
I am sure that my project was not any final word in using LEDs for enlargers. It was not at the time I shared it and even less today. For me it serves me very well, but there are certainly better ways to implement that basic idea of LED enlargers.
Hehe, yeah, I see what you meanIn the end what matters is that it works for you. The reason I offered my concerns is mostly because you appeared to present this as a project that others might replicate. Perhaps I misunderstood that part!
Also, as I mentioned earlier, your projects are certainly an inspiration and as such your first LED head also helped me along in a way. I think I might even have gleaned the LM317 idea from you back then, but as you can see, I had to abandon it for my purposes.
Things are easier today than when you did your first LED head; it's much easier now to get off the shelf LED drivers with PWM capability built in at low prices.
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