Flat Field Correction in PhotoLine

[shijan]

Due nature of diffused light combined with my rather wide angle 39 mm macro lens, seems it is near impossible to get perfectly uniform backlight for film scans. I tried many different custom-made light sources and light designs, but in best case scenario light is ok, but not perfect. So i decide to test well known Flat Field Correction method.
Some test results:
it works really perfect in Linear gamma and in 32 bit. It makes field 100% flat and also removes all tiny color casts.
Flat Field layer should be in Linear gamma, so if put Levels inside, set Auto Levels clip to 0% and adjust only white clipping point - final corrected image brightness will remain unchanged (Levels applied in Linear Gamma is same as applied Exposure).
When applied to real photos, you need to add tiny amount of Blur to Flat Field layer to avoid high frequency noise.
In Gamma Corrected space in 32 bit it is possible to notice some tiny uncorrected areas.
In Gamma Corrected space in 16 bit it is possible to notice some visible uncorrected areas.

Compare examples visualized with high resolution Historgam in PhotoLine:

[PhilM]

When I need to scan old 35mm film, I use my smartphone to light it.
I display a white image on the phone. And place the phone about 1cm away from the surface of the film.
This setup guaranties a pretty uniform lighting.

As far as the lens, I use my Olympus 60mm macro.
With a crop factor of 2, this is equivalent to 120mm full frame.

Phil

[shijan]

He he. Things are not so simple. I spend about a year to design and test film scanning device. Sort of hobby project that will be manufactured in 2022 in very limited amount of copies. Real uniform light is a real challenge, especially when attempt to fit light source inside limited space.



I tested earlier few smartphones and light panels results where bad. LGP-based panels may have less vignette than direct diffused light, but they have a lot of other problems. LGP patterns always produce multiple non uniform color and brightness spots. LEDs line array usually located only at one edge and produce additional non uniform gradient across frame. On top of that IPS Panel filters light and outputs thin R G B peaks. LEDs CRI on smartphones is usually rather low for quality film scanning tasks. Probably some newer and high end smartphones may have better CRI. Pixels are visible even from far distance.
OLED screens may provide more uniform brightness, but same as IPS panels they output separate R G B peaks and so depending of luck a lot of colors between those peaks may be distorted. OLED CRI is rather low. This is just limitation of OLED technology yet.

Flat Field Correction may help to improve with smartphone light source as well. This is sort of essential correction. It is widely used in Photography to fix vignette wide lenses. With same method it is also possible to remove dust spots from sensors.

PhilM, can you share RAW photo of empty frame of your smartphone backlight for tests?

Here is my example from Smartphone. 24x36mm area in the middle of 5" screen. Focused about 8 mm from screen surface:

[PhilM]

I do not have time and space available at this time to setup my scan rig.
Sorry.
Interesting subject ...