LED based point source enlarger

Lachlan Young

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mshchem said:
Is there a big difference between point source and ordinary condenser light sources?
Click to expand...

Yes - in regular condenser it's a fairly diffuse source (big frosted bulb) going through condensers that collimate the light somewhat; in a point-source it's a tiny undiffuse source (literally a point-source) contained in a non-reflective/ flare-free environment, with the condensers being of specific focal lengths (from recall, upper condenser FL = distance from lamp filament, lower condenser FL = distance to lens) - and you need to be able to adjust the relationships within this to get optimal performance - it's also where APO/ high MTF lenses can come into their own in terms of wide-open performance. The big advantage an LED has for a point-source system is that it can be operated under PWM control, unlike the older systems which adjusted exposure via a pretty standard dimmer (with consequent light colour changes).
 

distributed

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Thank you for the link to the article. It's a good read even today.

I recently got a Polish Krokus 3 Color enlarger (up to 6x9 cm) that is a "typical condenser enlarger" according to the article, i.e. to be used with an opal lamp. As a quick experiment I taped an 3x3 RGB LED (https://www.mikroshop.ch/LED_LCD.html?gruppe=7&artikel=681) into the opal lamp socket. Running the LED at half power[1] produced a very bright image on the baseboard. I have made the same experience as you: negative grain looks very sharp through a grain focuser, but dust on the condenser features prominently, too. When I move the LED around I see patterns moving on the baseboard. I believe these were caused by the 3 LED dies per color.

I proceeded to put a piece of tracing paper in the filter drawer a centimeter or two above the negative. Now the grain was hard to see in the grain focuser and the image on the baseboard is dim. If i moved the tracing paper to some place in the light path between the LEDs and the filter drawer, I would get behavior in between.

My experience touches on questions 1 and 4 of yours. While I was able to get good light output from one 3x3 composite LED I don't think the resulting system is desirable without some diffusion introduced. Of course with diffusion your light output and sharpness will take a hit. If you had a system to move LEDs into place, you could maybe avoid this tradeoff, but I am not sure. I would like to encourage you to experiment a bit with different LEDs and light paths with quick and dirty experimental setups before you commit much time and material into a fancy solution.

[1] 150 mA current. I use the same LEDs at the same current in a Durst M305 (35 mm) diffusion head and I get a not terribly bright, but adequate, illumination.
 

AgX

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In contrast to panel-light, mixing chamber or classic opalized-bulb/condenser illuminations, a true point-light illumination needs a condenser(system) that projects an image of the light source into the lens to certain point and has to be adjusted in position for any enlarger-lens extension.

The better designed the condenser, the more light gets onto the board, and the better the eveness of board illumination is. The same time the play one got at positioning the condenser becomes smaller.
 
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mshchem

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OK, I'm still a little confused. I have Beseler 4x5 enlargers, typically I use dichro (diffusion) light sources. I also can use the condensers, which are adjustable for film size. I rack the condensers (as a pair) up and down, the distance between the film changes, but the distance between the lamp stays fixed. I even have the adapter that allows the use of the dichro source with the condensers. I have found with the color head and the condensers that this setup added about 1 grade of contrast with VC black and white papers.

I know Beseler also offered a Point-source lamp, that could be substituted for the ordinary lamp house, still using the standard condensers, for "extremely critical work like electron microscopy"
I'm interested in how this topic develops.
 

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Marco Buonocore

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Hello John,

Point light sources can only be used in an enlarger with the lens wide open, otherwise you see the shadow of the lens iris as you noted. Traditionally, a point light source has been used in tandem with a variac. Durst's was called a Varipoint, Beseler had a Resistrol, Omega had their DM power supply. These units allowed you to dim or brighten the light as needed, as an alternative to using the aperture of a lens. One of the drawbacks of these variac units is that as you dim the light, the colour temperature of a tungsten bulb lower significantly. This can be a problem with variable contrast paper, as it will generally lower the contrast. With an LED bulb, this wouldn't be an issue.

Because you are using the lens wide open, to achieve the sharpness you're looking for, a good lens and a well aligned enlarger will go a long way.

To try and answer some of your questions:

1.) A tradtional tungsten point light source is uncoated, and quite bright - I would say comparable to a PH 213 Opal bulb. I use a 20V 100W bulb as my point light source. A single LED, focused correctly, would be quite bright I'm sure, so your exposure times aren't too surprising. If it's too bright, a variac or dimmer is your answer.​

3.) the condenser set up is absolutely critical for a point light source. You need the correct condensers for the magnification you are enlarging, and for the lens you are using. To give an idea of this, you can look at the Durst 138 manual pages 30 and 31. You see the different combinations of condensers used for different scenarios. There is some math behind this, but for because I use a Durst 138, I stick to their chart. For your Durst 805, you'd have to figure things out.​

I can say this much - if you use the correct condensers, and focus the lamp (which is different to focusing the negative) properly, the light is almost perfectly even in illumination. I use one of Darkroom Automation's enlarging meters, and you can move it to any corner of the easel, and it's pretty much bang on - there is very little vignette at all.​

4.)The positioning of the lamp is quite critical. With a tungsten bulb, you are actually using the filament of the bulb to focus the light, which makes it easy enough. On my Durst 138, you can move the bulb on three axis: up & down, side to side and front to back. This makes the task easy. On some of the Beseler 45's I've seen, you could move the up and down a decent amount, which was good enough.​

You focus the bulb separate from focusing the negative - it's an extra step. In terms of sharpness, this is where you get it. The difference between a print made from a properly focused lamp and an improperly focused lamp is night and day. As I said, with a tungsten bulb, you use the projected image of the filament on your baseboard as a visual aid to doing this. How you would do it with an LED lamp I don't know. I've thought about it, but never came up with a good answer, so I'll be interested to see what you find out. I'm sure there is a mathematical calculation for how far the point light must be from the condensers, but applying that in practice sounds easier said than done. Having the visual reference of the filament makes this extra step sort of painless.​

I treat the point light source as another tool in the drawer. The sharpness is plain to see in the grain focuser and in the print. It's pairs wonderfully with lith printing. The extra contrast it gives is a blessing when printing with old graded papers, that may have lost some of their contrast over the years. Plenty of drawbacks as well. Dust and scratches are much more pronounced, so you have to spend time and effort keeping things clean and tidy. Dodging and burning, in practice, are more difficult - the shadows cast by your hands and tools are much harder edged. The way a bulb behaves in a variac is a pain in the neck as well - on lower settings, there is a significant bulb turn on or ramp up time. This changes the way you dodge and burn for sure, and with how you make test strips, if you want predictable results.

Good luck,
 

distributed

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michael_r said:
I would use ND filters on the lens for light attenuation rather than messing around with varying the output of the light source.
Click to expand...

Why is that? Do you expect output control to interfere with something else in the printing process? My gut feeling is that it would be easy to change the LEDs output with simple current control with virtually no effect on the optical characteristics of the LED.
 

Marco Buonocore

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Just another quick bit of advice, that michael_r's post got me thinking about. An ND isn't a bad suggestion, but you want to be wary of anything in the light path. A glass carrier, for example, is a must because you are using the lens wide open - but anti newton glass is a dead end. You can clearly see the pattern of the AN glass projected on the image. If you can source optical quality anti-reflection glass for the your negative carrier, that is the way to go. I would worry that an ND would introduce flare.

Durst sold special coated condensers for the 138 and 184 enlargers. I have seen similarly coated condensers for Omega D series enlargers. These are pretty rare, but in my testing they made a considerable difference. Durst even sold a special coated mirror that their condenser system uses, although I've never seen or used it. These peripheral items were made for a reason - flare is a real issue with a point light source.

It's a fussy system, no doubt, but the end product is unique enough to justify it.
 

Paul Howell

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The DYI enlarger I build out of a Federal came with a glass negative carrier, not sure what glass it has but no issues with patterns, it was build in the late 40s or early 50s the condensers are uncoated, wondering if that why it has even greater contrast than I expected. The negative carrier is a 6X9 but came with a mask for 35mm and 6X6, even with a very bright bulb it was rather slow with 35mm. My friend still prints with it, it is her second or third enlarger but she quite likes the look with 6X7 Tmax 100 negatives. She thought about adding a fan to keep the temperature down, it uses a 250 watt halogen bulb. Likely not worth the cost of fiddling with the housing, would have to be machined in some way to allow a air hose to be attached, not sure that would cause additional dust. But if a LED light source works might be worth her while to convert from the halogen, but not sure if a LED bulb could raised and lowered in the Federal lamp housing.

BTW, point source enlargers were made to make prints from microfilm, very sharp and high contrast.
 

distributed

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@michael_r: Thanks for your response. I also don't have a good overview of what changes with different operating conditions, thought that you maybe had some insight because I'm interested in the topic and so I asked

I am only familiar with one effect: temperature dependence of light output. Given a current through an LED, light output is lower with higher junction temperatures. Whether you go for current control or PWM or current control has some influence on this. With PWM you turn the LED full on for some time, then off for some time. The junction is constantly heating and cooling off at the PWM frequency. I suspect this effect is small, but I don't have a thermal model to tell. With current control, the junction temperature rises until it reaches stability and then stays there.

Obviously the exposure is also different for the photographic paper: With PWM you get short bursts of high luminance at PWM frequency (100 Hz - 10 kHz would be a typical range) but with current control you get constant, lower luminance. If I was trying to achieve a "medium brightness" by using a high powered LED at short duty cycles, I would expect high intensity reprocity failure at some point.

However I haven't read about either issues. I would like to look into them with my LED head, eventually.
 

John Tindle

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Thank you all for the many, many helpful suggestions. And also a whole bunch of background facts. I am learning fast.

Herewith a few things that I have drawn from the conversations and one or two things I can contribute.

The issue of the iris in the projection lens is a point well taken from several folks. It means of course that I need to get a lens that works well at maximum aperture .....

I had also come to the conclusion that I shall need to move the source back and forward in the lamp housing, Fortunately there is plenty of room to do that in the M805 head. So I have ordered a worm driven stepper motor to do the job. When that arrives it will be a matter of trial and error as to optimum positioning .... unless I can figure out the optical path. The old 1/f=1/v+1/u equation from my early physics lessons strings to mind, assuming of course that the condenser behaves accordingly. Marco, thanks in particular for the link to the Durst manual, it makes really interesting reading. I am assuming that the numbers shown next to the condenser elements are the focal length of each lens. The Bimacon 75 is symmetric I think, but no idea what the focal length of the elements are. When I have worked that out it will be time to trawl ebay etc for a small collection of condensers!

Something I know a little more about are LEDs. If I were to use a "white" LED, different currents can change the colour slightly ..... I have some high CRI white LED where the manufacturer states that running the LED above 60% power will result in a loss of the high CRI rating. White LED use a mix of phosphors to downshift the blue/UV light. My assumption is that some of those phosphors reach saturation at lowish power ......

But that is not the case here, because I am using a near monochromatic royal blue LED. At higher temperatures (generated by higher power) they do show a marginal shift, but so small as to have no effect on VC papers. Ditto the green that has yet to be installed.

Dimming LEDs is not usually done by varying voltage ....the voltage difference between off and blown is quite small. The two normal methods are Pulse Width Modulation or constant current. I have chosen the latter, partly because it is possible to calculate to a high degree of accuracy the light output for a given current input. Because this is possible over around a 5/6 stop range, the need for ND filters is significantly reduced. And of course I get decent exposure control.

Again, many thanks to you all
 

John Tindle

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Paul Howell said:
But if a LED light source works might be worth her while to convert from the halogen, but not sure if a LED bulb could raised and lowered in the Federal lamp housing. .
Click to expand...

Paul,

I would not try a white LED bulb (cf LED chip) for three reasons.
1) Most LED bulbs put all their light out in one direction. If your lamp housing happens to built in a particular way, you will get lots of light. For me (with a M805) this would be a disaster as the light is taken from the side of the bulb.
2) Lots of LED bulbs are heavy on the blue and very light on the green
3) many cannot be dimmed and those that do may well suffer colour shift.

Conversely, it is a very low cost experiment, so why not try it!
 

John Tindle

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Marco Buonocore said:
Hello John,

Dodging and burning, in practice, are more difficult - the shadows cast by your hands and tools are much harder edged.

Good luck,
Click to expand...

Marco
Good point, had not thought of that. From my VERY limited experience so far, there doesnt seem a good zone (ie poor focus).
I am hoping / expecting not to suffer the variac problem because LED start up is instantaneous and doesnt suffer the horrendous colour shift at startup
 

Paul Howell

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I gave mine DYI away, I'll let my friend know in case she wants to experiment. I was really excited, thought it would be great for 35mm 5X7, but just too sharp, gain was very noticeable, and even at 5X7 dust and small negative imperfections showed. But my negatives from the 60s and 70s were developed for contrast, needed for newspaper printing process at the time. Maybe with a good low contrast 6X9 negative would have like it better.
 

koraks

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There was a thread about this in the B&W section of the forum a few months ago. No problems seemed to be associated with PWM dimming. In my own experience, I never noticed any problems with it either. PWM works fine, also for high-end/high-power LEDs, and I see no reason not to go with this option if dimming is desired.

One caveat with trying to use LEDs as a point source is that while you may get away with a single blue LED, for green, you might need more than one, because of the inherent sensitivity of the paper and the efficiency of green LEDs.

I can't offer much advise in terms of trying to make LEDs into a point source. For my own work (which requires color, so also red in addition to green and blue), it's a dead end street since there simply are no single-element LEDs that are powerful enough, so I stick to LED panels, which means a diffuse light source, but I do keep the condensors in my 138 in place, so it's a diffusor/condensor setup effectively.
 

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Learned in the UV enlarger thread: for point source light you have to first converge the light to straight path and then colliminate it to the enlarging lens.

I converted my Fujimoto G70 to leds by just taping WS2812 strip inside the light chamber so that the leds become diffused light source. That is basically how it works with the original halogen lamp. I'm using condenser lens and it works fine. Grain is sharp.
 

koraks

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vedostuu said:
converge the light to straight path and then colliminate
Click to expand...
Collimating means you bend the rays so they become parallel. So here it seems you're basically saying the same thing twice: converge to straight path sounds pretty much the same as collimation to me. Perhaps a drawing would help? Do you mean collimate first, and then converge them into a single node that aligns with the upper condensor?
 

AgX

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What vedostuu desrcibes would mean installing an additional lens element between film and enlarging lens. A no go.
 

radiant

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Sorry, lost in translation. First the rays are converted to parallel rays from point of source and then concentrated/focused to the lens. Two lenses then yes.

Douwe is explaining it in his interview, see here: https://www.photrio.com/forum/threads/making-an-uv-enlarger.92824/page-2#post-2428398 - about at 22 minutes mark.
 

AgX

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Likely he just misunderstood the design of a condernser. To reduce abberations (which not only are an issue for imaging, but lighting rays) at the classic condenser a single element convex lens was substituted by two plano-convex lens element with inbetween them paralled rays. Thus this paralel part is not a necessity for illuminating the fil image, but to optomise the the condenser as such.
 

distributed

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vedostuu said:
I converted my Fujimoto G70 to leds by just taping WS2812 strip inside the light chamber so that the leds become diffused light source. Grain is sharp.
Click to expand...

That is another experiment I will have to try. I still have an old RGB LED strip lying around where the back is already separating from the clear flexible plastic cover after about 5 years. Definitely not quality material, but should work
 

radiant

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distributed said:
That is another experiment I will have to try. I still have an old RGB LED strip lying around where the back is already separating from the clear flexible plastic cover after about 5 years. Definitely not quality material, but should work
Click to expand...

It is worth testing for sure. I might be still the only who has done it I have a friend who is going to do the same in autumn.

I tested a blue COB- LED kindly sent to my by @distributed just by shooting the light down towards the condenser lens and it worked fine. I cannot remember did I play around the the aperture but maybe I would have got the same issue described here.

The reason I used LED strip was to avoid any problems with direct light hitting the condenser lens and to "emulate" how the enlarger originally works with halogen light.

ps. distributed: I haven't forgotten the print I promised.. I will some day send it to you
 

koraks

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vedostuu said:
I tested a blue COB- LED kindly sent to my by @distributed just by shooting the light down towards the condenser lens and it worked fine.
Click to expand...
When I did the same in my very first experiments in converting my 138 to LED using a 100W RGB COB led, I got very distinct differentiation between the individual led elements / separate 'color zones' when projecting an image on the baseboard, unless I used a diffuser.
 

Marco Buonocore

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My knowledge of LEDs as enlarger bulbs is pretty much nothing, but I'd like to add that this sort of effect can occur using a tungsten point light source as well. If you are using the wrong condenser combination for a particular lens, or have your bulb significantly misplaced, you will see all sorts of coloured light on the baseboard. Sort of like you get with a prism, but not as pronounced. Is it possible that this was what you were seeing?
 
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