Yes, that is the original idea; to expose film in enough controlled way that I can make comparisons between my own tests.
The deviations in light output are hardly measurable, so small that you can use the system as a sensitometer. You can make step wedges for comparisons.
Except for all the electronics and reliance on a calibrated scale that can fade. I have an electronic densitometer that is working but I am never sure when something will go wrong with it. As an alternative, I like ( Marshall Studios ) a visual densitometer takes advantage of the law of inverse squares.…It's in a way not too different from the first visual densitometers, where one compared the relative brightness of two beams of light, one passing through the sample…
A purely optical visual densitometer is as accurate as the operator's ability to discriminate fine changes in density. The density tablets are usually silver-gelatin or lampblack-filled gelatin, so it shouldn't fade in normal storage and usage.
The Physikalisch-Technische Bundesanstalt in Germany uses (or used up to the 2000s) a reference densitometer based on the inverse square law. There's a detailed paper on its construction and calibration but it's sadly behind a paywall.
Another (luxury) option would be a closed-loop system where the LED current is set by a servo loop to achieve the desired current in a photodiode operating in zero-bias mode. The photodiode becomes the critical component, and, IIRC, it is linear over a range exceeding the 10^3 needed in a sensitometer. No more any worry about intermittency effects in the film under test, transient effects in the LEDs, or thermal effects in the LEDs. Did I write luxury? But the assurance of being rid of spurious effects is priceless.Your time-based approach will work within reciprocity failure limits, as others have pointed out. Controlling the intensity with PWM might cause intermittency effects on the exposure, though IIRC those effects disappear at higher pulse frequencies. Controlling each led with a constant current is fine, but then the current-intensity relationship deviates from linearity and the design is complicated by having to program 3-4 decades of current sources.
For a sensitometer, I think it would be impractical to build an instrument on that basis. LEDs are a poor point source until you are quite far away.
Your time-based approach will work within reciprocity failure limits, as others have pointed out. Controlling the intensity with PWM might cause intermittency effects on the exposure, though IIRC those effects disappear at higher pulse frequencies. Controlling each led with a constant current is fine, but then the current-intensity relationship deviates from linearity and the design is complicated by having to program 3-4 decades of current sources.
Fully agree.assurance of being rid of spurious effects is priceless.
You know you can “drop some strings” in the channel to block some light physically. I don’t know what you call it when you “print” a line in space, but if the 3D material is opaque you can physically block some of the light in the path near the LED
At the lowest exposures dimming with a physical “screen” may give you better consistency. Especially when you make an exposure test for 1000 speed film. You can run the bulb at a slightly longer time (e.g., 1/1,000 second instead of 1/10,000) if you block some of the light going through the tube.Dimming is not a problem and doesn't need 3D printing at all. All FDM materials are more or less opaque. My device has been printed maybe 1.6mm wall thickness of black PLA and the dimmed light is still visible in total darkness - I was suprised about this.
Haaa. Thinking any test will settle any question is the mistake. But don’t worry you’ll get used to itI must have made a mistake here ..
The principle of a visual densitometer is simple.@radiant, I believe the PTB densitometer used a very advanced version of the original "grease-spot photometer" - by comparing a source of unknown intensity with a calibrated one and varying the distance until they are equal. If the emitter approximates closely a point light source, the relationship between distance and intensity is ideally an inverse square law.
"Box" would need grade 2-3 filter and overexposed grade 1-2 filter.
My conclusion: by overdeveloping HP5 by double time you need one grade lower filter. If this is true, then we can stop all discussion on these 10-20% increased development.
Your 6 minute development time is near 0.5 contrast and your 12 minute development time is near 0.66 contrast.Anyone? This bugs me, it cant be this way ..
Your 6 minute development time is near 0.5 contrast and your 12 minute development time is near 0.66 contrast.
I don’t think there’s anything wrong with your results. I could wish for more densitometer readings under 0.10
That’s why I suggested a barrier screen (or more thick diffusers) on the low end.
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