Light intensity and spectra for large scale enlargements (building LED light source)

[ChrisBCS]

Hi all, through my job and some other hobbies, I have made, and understand how to make, high intensity LED fixtures with various characteristics. The more I think about making big (36"+) enlargements, the more I think about the enlarger itself and not necessarily going commercial.

For that print size, what lighting intensities are generally required to give reasonable exposure times? Are most commercially available papers truly panchromatic with respect to wavelength sensitivity of the enlarger source?

 

[MattKing]

Are most commercially available papers truly panchromatic with respect to wavelength sensitivity of the enlarger source?

If you are speaking of black and white papers, with one exception, none of the papers have panchromatic sensitivity.
The paper manufacturers usually supply spectral sensitivity information. Here is a representative example from Ilford/Harman: http://www.ilfordphoto.com/Webfiles/2013116121925810.pdf
EDIT: your review of that data will also give you some information about the sensitivity of the paper, and what light sources it is designed for.

 

[ChrisBCS]

Excellent! Thank you!

Also, wrt to the other large print thread I started, I do plan on acquiring a more standard, "normal size" enlarger and working on the technical side of things, per your suggestion. I like the notion of being able to practice and understand better what does and doesn't work at a fundamental level while I plan for the future of larger scale.

many thanks.

 

[MattKing]

You might find this information (Modern Enlarger's LED light sources for Omega enlargers) interesting: http://www.modernenlargerlamps.com/Modern_Enlarger_Lamps/Model_3.html

 

[Leigh B]

Graded papers are sensitive to blue light.

Variable-contrast papers have two emulsions with different charactristics.
One is sensitive to blue light, the other to green light.

None if those emulsons is sensitive to red light, which is why safelights work.

Papers have sensitivity based on exposure to an incandescent bulb of about 100 watts.

These characteristics go back many decades.
Modern papers may improve on them, but not change them significantly.

- Leigh

 

[tedr1]

Here is some information that may help arrive in the ball-park for light intensity.

I set up the enlarger to produce a 16x20 print from a 6x7 negative. With no negative in the carrier I measured the brightness of light reflected from white paper on the baseboard with the lens aperture set for f5.6. It measured EV2 with grade 2 filtration and about EV3 with no filtration. Using Ilford paper (Multigrade Fiberbase) and grade 2 filtration the exposure time would be approximately one minute. The enlarger light source is a 250W halogen bulb used in a diffusion arrangement. It may be possible to work backwards from the EV values to arrive at an intensity value however my maths and physics does not go that far

 

[Michael Guzzi]

Isn't EV tied to an ISO value?

 

[tedr1]

No it is brightness value derived from a combination of shutter speed and aperture based on EV0 = 1sec and f1, and EV1 = one stop brighter, etc.

When the EV value is used to calculate an exposure setting for aperture and shutter speed then an ISO value is required to perform the calculation.

In this situation I am attempting to provide an approximate indication of the brightness of the paper surface, my light meter is calibrated in EVs and I gave the values above. The same brightness converted to an exposure setting is as follows: EV2 = 1sec at f2 and 100ISO. EV3 = 0.5sec at f2 and 100ISO.

 

[ic-racer]

I am attempting to provide an approximate indication of the brightness of the paper surface,

I'm sure your findings are what they are. But, of course, one does not expose photographic paper by bouncing light off a white paper surface. An incident reading of the light falling on the baseboard is also useful.

 

[tedr1]

My meter is reflecting spot type, by all means provide an example using an incident meter to assist the OP.