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Author Topic: Light source for large format film copying  (Read 1097 times)

jwittes

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Light source for large format film copying
« on: February 11, 2020, 10:18:38 am »

I'm looking for recommendations for a light source (light table/box) for copying 8x10 negatives/transparencies using a copy stand. Evenness of light across the entire panel is extremely important as well as color temperature around 5000ºk.  Any advise or recommendations regarding this setup is greatly appreciated.  Thanks...
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Paul_Roark

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Re: Light source for large format film copying
« Reply #1 on: February 11, 2020, 11:32:54 am »

I once built an 8 x 10 head for my Beseler 4x5 enlarger by copying the design of a smaller enlarger head.  I used cardboard that I coated with titanium white paint to fabricate the head and used a 2 2/4 color head as the light source, so that I had the color filtration needed for variable contrast B&W paper.  It worked very well and cost almost nothing.  The 8 x 10 negative was simply put on a plain glass sheet that acted as the negative holder.  If found an Apo Rodagon copying lens that would cover 8 x 10 and was way cheaper than their 8 x 10 enlarger head.

FWIW

Paul
www.PaulRoark.com
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JaapD

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Re: Light source for large format film copying
« Reply #2 on: February 12, 2020, 06:27:10 am »

Out of the box thinking: what about using your (calibrated) monitor screen as light source for your 8 x 10 slides? Create a white window on your screen. Of course you’ll need to hang/mount the slide with a sufficient distance from the slide in order to de-focus the monitor screen.

Regards,
Jaap.
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digitaldog

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Re: Light source for large format film copying
« Reply #3 on: February 12, 2020, 12:48:56 pm »

5000K is a large range of possible colors of white so it’s kind of meaningless:
http://digitaldog.net/files/22Thecolorofwhite.pdf
Most important is the spectrum and ideally, one that’s smooth and devoid of spikes like seen in Fluorescent lighting.
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kers

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Re: Light source for large format film copying
« Reply #4 on: February 12, 2020, 12:53:00 pm »

I am also thinking about that - for smaller negatives- Best thing would be a continu lightsource ( light bulb like Solux) - combined with a milk perpex/glass (at some distance from the film) that does not filter visible colour out...

I don't know if a led lightsource spectrum could be a big problem.
For evenness the ledlight source has to be as large as the film format and at some distance from the perspex.
you could measure the evenness with a lightmeter (- I use it nowadays only for this purpose)
« Last Edit: February 12, 2020, 12:59:54 pm by kers »
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Dale Villeponteaux

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Re: Light source for large format film copying
« Reply #5 on: February 12, 2020, 03:42:01 pm »

Just a random thought without any experience to back it up, but could you use a Fresnel lens to collimate and
enlarge the transmitted light beam? The plastic ones might work and they're cheap.

Regards,
Dale
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Chris_Brown

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Re: Light source for large format film copying
« Reply #6 on: February 12, 2020, 05:11:01 pm »

I used a clean 2'x2' piece of milk white plexiglass and mounted it horizontally (like a table top, making sure its flat) and placed clean, white foamboard under at about a 45˚ angle. I placed two strobe heads (Speedotron) with parabolic reflectors about 4–6' away from the foamboard, and flagged off any stray light. I created an 8x10 "window" on the plexi table with gaffer tape and flagged off everything outside that window. I spot metered the light, adjusting lights and foamboard until an even exposure was read throughout the window area. This setup allows for hassle-free copying once the camera is positioned and focused (but always check focus on every shot). I never had problems, but I've used/blended multiple exposures to help shadows or highlights if needed.

I used this system as an assistant in studios in Detroit and Los Angeles with good results. Best results come from a PMT drum scanner, but a good digital capture is second best, IMO.
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TechTalk

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Re: Light source for large format film copying
« Reply #7 on: June 20, 2020, 01:57:50 pm »

This new product is currently under development. https://www.negative.supply/light

If you wish to build your own, 99 CRI LED strips are available here...  https://www.waveformlighting.com/absolute-series

For negative conversions, there is a free trial version of Negative Lab Pro available.  https://www.negativelabpro.com
« Last Edit: June 20, 2020, 02:59:34 pm by TechTalk »
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digitaldog

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Re: Light source for large format film copying
« Reply #8 on: June 20, 2020, 03:55:22 pm »

CRI is nearly a useless metric to report the qualities of a light source. Ignore in it. CRI, is a kludge to make a light source appear to be closer to daylight for marketing and light manufacturers. CRI was developed in large part to aid in the sales of Fluorescent tubes. There are BCRA tiles used to compare under a reference light source but only eight. That's too small a set of tiles. That make it easy to create a spectrum that will render the 8-14 tiles and doesn't tell us that the light source is full spectrum. It doesn't tell us how the other colors will render. My understanding is there are two reference sources; Tungsten for warm bulbs and D50 for cool ones: Above and below 4000K. That means that a normal tungsten bulb and perfect daylight both have a CRI of 100! As such, a high CRI is a decent gauge of how well a light will preform in your home but not such a great indicator of how well it will work for photography and proofing. Both a Solux 48 and a "full spectrum" tube from home depot may have a CRI of 97. I can assure you the Home Depot bulb has a giant mercury spike and some spectral dead spots. 

A better metric is called CQS (15 very colorful patches). That doesn't tell us about the spectrum which is even a better way to evaluate the illuminant. 
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TechTalk

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Re: Light source for large format film copying
« Reply #9 on: June 20, 2020, 08:57:58 pm »

I agree that CRI and all of the current color index ratings are severely limited in offering a complete evaluation of light quality and are subject to gaming of the numbers by LED light manufacturers. Testing for the intended application is always neccessary to insure satisfactory results.

I linked the LED manufacturer Waveform because they at least seem to take color seriously. They include quite a bit of detailed information on their website including downloadable photometric test data with CRI (Re and Ra), CQS, TLCI, and spectral power distribution data. https://www.waveformlighting.com/photometrics/TR_7001.50.pdf

They provide much more information than I'm used to seeing from other manufacturers. Their spec sheet also includes the TM-30-15 rating and minimum as well as typical specs.  https://www.waveformlighting.com/datasheets/CS_7101.pdf

If someone were interested in a build your own LED light, their site isn't a bad place to start. They have detailed photometric test reports available for a number of their products.  https://www.waveformlighting.com/photometrics/  As well as additional information that may be useful.  https://www.waveformlighting.com/high-cri-led   https://www.waveformlighting.com/tech/

LED lights are less than ideal compared to continuous spectrum sources, but they're not going away and are getting better. CRI is also less than ideal, but it's better than nothing as a starting point for comparing lights. It can at least weed out from consideration the really bad options.

I don't think that I've ever replied to a post of yours. Let me take the opportunity to say that I always enjoy reading what you've had to say. Also, many years back I've had the opportunity to listen to you speak at an event or two and chat with you a bit in person. You (and the greatly missed Bruce Fraser) helped put me on the right path to a better understanding in my early days of approaching digital color. Not to worry though, you are hereby absolved of any blame when I get my foot entangled in my mouth.

Best Regards and thanks for your long record of contribution to the community.
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BobShaw

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Re: Light source for large format film copying
« Reply #10 on: June 20, 2020, 09:41:37 pm »

A Boxlite would probably be ideal.
Before you have a heart attack over the price I bought one for a few hundred dollars on eBay.
You  can do screen calibration for more even light if you use Phocus.


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