This segment was at once fun and frustrating. I have my LED paper flasher hooked up and some basic software to make it run. This was pretty straightforward. The interesting part is whether it will work or not and how to know the light levels are right. I started out with a 3 LED battery powered light meant to be stuck in dark closets or cupboards. I connected it to a PWM output on the Arduino so I could control its intensity. This gives me a range of 0-255 which is 8 stops of brightness. I also fitted a piece of translucent perspex in front of it to diffuse the light. The LEDS are very bright.
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| LED light source when off |
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| LED light source when on |
I now needed to see if the I could get the light levels right to flash paper. My first attempts were without a filter to help establish baselines. I set the enlarger at a 30cm height (based on its scale). I used Ilford MGIV RC glossy paper as I have a lot of it and it is relatively cheap. My first exposure was with the brightness set to 128 or half the range. All timings are 1 second increments as timed by my Arduino with me moving black card over the test strip as prompted by the Arduino. The first results are below.
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| 3 LED 128 Brightness No Filter |
If you look carefully on the right there is a light grey strip. The is 1 second of exposure. Way too bright. So next I programmed the brightness way down to 8 (4 stops) with the following result.
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| 3 LED 8 Brightness No Filter |
Better but I don't like flashing paper at only 1 second. (The white/black vertical lines are marks I made with a Sharpie which tends to dissolve during developing.) Something more radical is needed. So I cut the leads on two of the LEDs. I started to make exposures from the lowest setting (1) 3 stops from the previous.
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| 1 LED 1 Brightness No Filter |
This is basically blank across the whole strip. So turned it up to 2.
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| 1 LED 2 Brightness No Filter |
Again nothing. So now 3 and then 4.
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| 1 LED 3 Brightness No Filter |
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| 1 LED 4 Brightness No Filter |
Now we are seeing some gradation so this is getting dialed in now.
Next to try some filters. These I bluetacked on for now. The filters are important because I don't know the actual spectrum of the white LEDs. They are often a mix of discrete colors like florescent lights. I start with filter #0.
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| 1 LED 8 Brightness #0 Filter |
Brightness 8 doesn't give much tone.
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| 1 LED 16 Brightness #0 Filter |
Brightness 16 is about right.
Next is filter #5.
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| 1 LED 16 Brightness #5 Filter |
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| 1 LED 32 Brightness #5 Filter |
Brightness 16 works well here. I note here that the same intensity of light gives very similar exposure on the #5 filter as the #0 filter. With incandescent light the #5 filter is rated one stop dimmer than the #0. I suspect the LED emits more light at the #5 end of the spectrum.
So this settles levels. I changed back to the #0 filter for the next test. I need to see if the diffusion is even across the whole easel. I dragged out my 16x20 easel which is my largest. Rather than chuck a whole sheet of paper in I stuck small squares of paper around the edges with 2 in the center. One in each corner and four at 12, 3, 6, and 9 o'clock. (I missed picking one up at the 3 o'clock position so the following scan has a gap.) I set the intensity to 16 and set the timer to 8 seconds to give a visible light grey. The following is the result.
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| Flash test strips from around the 16x20 easel |
Disappointing. There is a distinctly darker center and looking carefully the corners are brighter than the sides. Comparing to to test strips the difference from light to dark is a little over one second of exposure. This is about .3 stop difference visually. It looks quite visible here but I am unsure how uniform other techniques are.
I analyzed the scanned images in Photoshop and measured a histogram for each square in the center then computed some log values to confirm the visual inspection. It shows worse.
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LED Histogram |
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| Log2 ratio |
Log2 |
|
Position |
| 0.00 |
7.0 |
128 |
center |
| 0.50 |
7.5 |
181 |
lower left |
| 0.43 |
7.4 |
172.6 |
left |
| 0.50 |
7.5 |
181.5 |
upper left |
| 0.25 |
7.2 |
152 |
top |
| 0.30 |
7.3 |
158 |
upper right |
| 0.37 |
7.4 |
165 |
lower right |
| 0.23 |
7.2 |
150 |
bottom |
|
|
|
|
|
7.0 |
|
Min |
| 0.50 |
7.5 |
|
Max |
This analysis confirms more like a half stop difference across the baseboard. Not good.
A New Approach
So I needed to make a few changes. First the LED has an intrinsic lens which tries to focus the light in a beam which is opposite of what I want. The diffusing perspex cannot combat this. I decide to replace with a small incandescent lamp from a Maglite flashlight. They are very small and can run off the same current as the LEDs. I also re-purposed a diffusion dome from from a lost Sekonic light meter. I placed this inside same holder with the new lamp.
I ran some test strips and the best was setting the brightness to 128 as see below. This would give a 3 second flash time to be just under the toe.
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| Incandescent Brightness 128 test strip 1 seconds intervals |
I then ran the same 16x20 coverage from the same 30cm enlarger height.
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| Incandescent 16x20 coverage |
I made the same Photoshop measurements and got better results. This time less than 1/3 of a stop.
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Incandescent
Histogram |
| Position |
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Log2 ratio |
Log2 |
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| center |
|
0.00 |
7.6 |
191.42 |
|
| lower left |
|
0.23 |
7.8 |
225.14 |
|
| left |
|
0.14 |
7.7 |
211.56 |
|
| upper left |
|
0.19 |
7.8 |
218.45 |
|
| top |
|
0.08 |
7.7 |
201.9 |
|
| upper right |
|
0.24 |
7.8 |
225.46 |
|
| lower right |
|
0.27 |
7.9 |
231.22 |
|
| bottom |
|
0.13 |
7.7 |
209.13 |
|
|
|
|
|
|
|
| Min |
|
|
7.6 |
|
|
| Max |
|
0.27 |
7.9 |
|
|
I haven't learned what a sufficiently good number is here. My light meter indicates my enlarger gets about .2 Ev uniformity across this same baseboard. Many postings indicate that 1/3 Ev across a baseboard is typical so I will leave this here for now.
(To be continued)
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