stormpetrel
Group owner
This is my second attempt with making a proper light source for my Omega D5 condenser enlarger based on color LEDs. My first attempt consisted in a large network of green and blue leds (diffusing mode) combined with two highly powerful 15W green and blue leds used as punctual source. Both green and blue channels were current controlled independently. The head could be used as a diffusing light source with a piece of diffusing plastic instead of the 3x condenser lenses or as a condenser light source when the light source was replacing the standard light bulb.
This light source can be seen working on this video. Sorry this is in french but the interesting bits are at 1m30 and at 2m35.
At this time, I was not enough aware about the principle of a condenser enlarger. There were no problem with the diffusing setup but at the end I realized I would prefer to use my enlarger mainly as a condenser enlarger. The problem here is that I did not take into account the formation of an image of the light source on the film plan (slight offset between the green picture and the blue picture) , said differently the light source should be as uniform and punctual as possible which requires the use of a diffusing dome as the two main leds do not act as a perfect unique punctual source despite the short distance separating them). This is not a surprise for the traditional printer. but unfortunately there was not an easy way to integrate such diffuser on my board.
The second problem was the wavelength of the LEDs (navy blue and green). As we will see in the next chapter they are not the most suitable wavelengths, so I have done a bit of research optimizing the new light source.
I would like to share those information with the community as it might be useful for your future design.
* photographic paper sensitivity
I did an extensive review of the the light sensitivity of all the photographic papers available on the market nowadays. Unfortunately it was not possible to get the datasheets from all the manufacturers but still we have enough information to optimize the wavelength of our leds.
I have split the compilation into 3 groups: normal variable contrast papers, warmtone variable contrast papers and fixed grade paper.
The light sensitivity unit is an arbitrary unit. The interesting information here is at which wavelength each paper is the most sensitive and at which wavelength the control of the grade is done.
Normal variable contrast papers:
https://www.flickr.com/photos/stormpetrel_geek_mode/8573619183/in/photostream/lightbox/
As you can see on the graph, most of the papers are quite sensitive in the violet domain (400-425nm). Note the light sensitivity is a log scale and the sensitivity of many paper drops a bit in the blue. Regarding the control of the grade, things append between 500nm and 575nm but mainly between 520-525nm (yellow green).
For the warmtone picture, the Ilford paper behaves like the normal VC papers however this is not the case with the other papers. Grade control is performed between 520-530nm for the Foma paper and at above 545 for the Forte paper. In the low side of the spectrum, we see that the Ilford paper is more sensitive in the violet area compared to the 2 others papers which are more blue sensitive (I wonder if there is a mistake in the Forte paper datasheet).
Fixed grade papers are completely different beasts. There are very sensitive to the green light (475-500nm).
https://www.flickr.com/photos/stormpetrel_geek_mode/8573619107/in/photostream/lightbox/
Based on all those information , I have selected the following wavelengths for the LED of my new light source: 420-425nm violet and 520-525nm (yellow green). Those are the wavelength were I will have the best bangs for my bucks based on my use.
The new light source consist in 12x 3W 520-525nm green LEDs and 9x 420-425nm violet 3W LEDs spread on a 6cm of diameter PCB. There is a second board on which are place 4x far red 1W led (inactinide light source). This boards is mounted on the first board.
The diffusing dome comes from a Philips energy saving 18W bulb (OD=69mm)
Next post: illumination modeling.
This light source can be seen working on this video. Sorry this is in french but the interesting bits are at 1m30 and at 2m35.
At this time, I was not enough aware about the principle of a condenser enlarger. There were no problem with the diffusing setup but at the end I realized I would prefer to use my enlarger mainly as a condenser enlarger. The problem here is that I did not take into account the formation of an image of the light source on the film plan (slight offset between the green picture and the blue picture) , said differently the light source should be as uniform and punctual as possible which requires the use of a diffusing dome as the two main leds do not act as a perfect unique punctual source despite the short distance separating them). This is not a surprise for the traditional printer. but unfortunately there was not an easy way to integrate such diffuser on my board.
The second problem was the wavelength of the LEDs (navy blue and green). As we will see in the next chapter they are not the most suitable wavelengths, so I have done a bit of research optimizing the new light source.
I would like to share those information with the community as it might be useful for your future design.
* photographic paper sensitivity
I did an extensive review of the the light sensitivity of all the photographic papers available on the market nowadays. Unfortunately it was not possible to get the datasheets from all the manufacturers but still we have enough information to optimize the wavelength of our leds.
I have split the compilation into 3 groups: normal variable contrast papers, warmtone variable contrast papers and fixed grade paper.
The light sensitivity unit is an arbitrary unit. The interesting information here is at which wavelength each paper is the most sensitive and at which wavelength the control of the grade is done.
Normal variable contrast papers:
https://www.flickr.com/photos/stormpetrel_geek_mode/8573619183/in/photostream/lightbox/
As you can see on the graph, most of the papers are quite sensitive in the violet domain (400-425nm). Note the light sensitivity is a log scale and the sensitivity of many paper drops a bit in the blue. Regarding the control of the grade, things append between 500nm and 575nm but mainly between 520-525nm (yellow green).
For the warmtone picture, the Ilford paper behaves like the normal VC papers however this is not the case with the other papers. Grade control is performed between 520-530nm for the Foma paper and at above 545 for the Forte paper. In the low side of the spectrum, we see that the Ilford paper is more sensitive in the violet area compared to the 2 others papers which are more blue sensitive (I wonder if there is a mistake in the Forte paper datasheet).
Fixed grade papers are completely different beasts. There are very sensitive to the green light (475-500nm).
https://www.flickr.com/photos/stormpetrel_geek_mode/8573619107/in/photostream/lightbox/
Based on all those information , I have selected the following wavelengths for the LED of my new light source: 420-425nm violet and 520-525nm (yellow green). Those are the wavelength were I will have the best bangs for my bucks based on my use.
The new light source consist in 12x 3W 520-525nm green LEDs and 9x 420-425nm violet 3W LEDs spread on a 6cm of diameter PCB. There is a second board on which are place 4x far red 1W led (inactinide light source). This boards is mounted on the first board.
The diffusing dome comes from a Philips energy saving 18W bulb (OD=69mm)
Next post: illumination modeling.
