A Better Light Source For Scanning Color Negative Film

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A Better Light Source For Scanning Color Negative Film

Originally published 2024-08-05, updated 2024-08-11

tl;dr

Narrowband trichromatic (RGB) light sources are superior to broadband (white) light for scanning color negative film; there are very good reasons why all commercial film scanners use RGB. (film scanners were using blue LEDs at least a decade before they would be widely used in consumer products!) Scanning film with an RGB light source produces high quality results with minimal post-processing, without the need for specialized software.

the results

Both scans captured using Fujifilm X-T30. White light scan captured using 95+ CRI 5000K light source. RGB scan captured using custom 450nm+525nm+665nm light source.

responses to frequent comments and questions

but i can just edit the white light scan to look like the RGB scan

You can’t get the exact same colors as the RGB scan by adjusting the white balance or individual color channels of a white light scan; the comparison image above has been updated to emphasize this. In fact, this is physically impossible because the light interacted with the film in a fundamentally different way in the white light scan vs. the RGB scan before it was reduced to three bins of wavelengths approximately representing red, green, and blue by the camera sensor.

but the colors in the sample scan(s) still don’t look good to me

Getting the desired colors from C-41 film, whether by darkroom printing or scanning and editing, has always involved some level of manual adjustment. Photography is art, and as such it has always involved making subjective decisions about how images should look; all photos are interpretations of raw data and it’s up to the photographer to decide what interpretation looks best. The best starting point for editing is a scan that represents the image data stored in the dye layers of the film with minimal crosstalk.

but the way you inverted the negatives isn’t technically correct

I think it’s safe to say that most photographers shooting film in 2024 are looking for aesthetically pleasing colors, not technically perfect colors. If you have a more technically accurate way to process your scans, then by all means use it (also, write an article about it or build open-source software to automate the process!); but using a narrowband light source instead of a white light source will still get you closer to the results from a professional scanner.

but won’t this make all film look the same?

Scanning film with RGB light does not neutralize the differences in the ‘look’ of different films; actually, it should produce results that are much closer to the intended look of the film when printed on RA-4 paper. The differences in look come from differences in the film’s sensitivity to light during exposure and chemical interactions that affect how the dyes are formed during development, not the absorption characteristics of the dyes themselves. It is also worth mentioning that many people have preconceived ideas of what film should look like that are based on highly edited or low-quality scans; old, faded prints; or even software emulations.

the simplified explanation

Color negative (C-41) film stores an image using cyan, magenta, and yellow dyes. Dyes appear a certain color because they absorb some wavelengths of light; for example, yellow dye mostly absorbs light in the 400-550nm range (which we perceive as violet through green), while allowing other wavelengths (yellow through red) to pass through the film. These dyes are not intended to produce a human-viewable image, but rather to attenuate certain wavelengths of light for making prints on photosensitive paper.

White light sources emit light over a broad spectrum of wavelengths. When white light passes through C-41 film, some of these wavelengths are only slightly absorbed by two or all three of the dyes, which means that these wavelengths aren’t useful for recovering the information stored in the individual dye layers - for example, the 580-620nm band (yellow through red-orange light) is mostly transmitted through the film, with a small amount being absorbed by both the magenta and cyan dye layers.

With darkroom printing, this is not an issue, as RA-4 color print paper (and motion picture print film) are, by design, most sensitive to specific bands of wavelengths that avoid these ‘overlaps’ between the dye layers in negative film.

What about scanning film with a digital camera? Digital camera sensors are designed to capture light in a way that allows for a faithful reproduction of the colors that humans would perceive (this is a gross oversimplification.) C-41 film was not designed to be directly viewed by humans.

Not only is the camera sensitive to wavelengths of light that are effectively ignored by RA-4 paper, there is significant overlap between the color channel sensitivities of digital camera sensors: light in the ~580-600nm range is sensed by both the red and green channels, and light in the ~480-500nm range is sensed by both the green and blue channels.

Because the overall light transmission of C-41 film is heavy biased towards yellow and orange wavelengths due to the orange mask, this means that a lot of light gets through that interferes with both the red and green channels in the camera sensor. The red channel of the scan will contain not just information from the red-blocking cyan dye, but also the green-blocking magenta dye. The green channel will contain information from all three dye layers.

note: the orange color of C-41 film is often misunderstood to be part of the film base, or an orange tint evenly applied over the whole image. It is actually a corrective mask made up of yellow and magenta positive images.

So if you scan color negative film with white light, you can’t just invert the colors and expect a good result. You need to do some math to try to recover the individual dye layers of the film from ambiguous input data. This can’t be done in standard image editing software, which is why dedicated software for negative conversion exists. This process will require extensive tuning for every combination of camera and light source used, and the color depth of the resulting positive image will be relatively low because much of the information recorded by the camera is effectively useless yellow-orange light that has passed through the film without being significantly attenuated by any of the dyes.

To avoid these issues, (almost?) all commercial negative film scanners use a trichromatic narrowband light source with red, green, and blue LEDs to minimize overlap between the dye layers in the film. When doing it this way, no software trickery is needed to recover an image and it’s really as simple as inverting the color channels and setting the color balance and black/white points.

so what LEDs should I use?

When considering the limitations of digital camera sensors, the ideal wavelengths are >650nm for red, 520-550nm for green, and <450nm for blue, as shown in the diagram above. However, to the best of my knowledge, there aren’t any commercially available light sources combining LEDs of these wavelengths, so the only way (for now) to get an ideal light source is to build one yourself, which is discussed in the next section of this article.

The next best alternative is to use general purpose RGB LEDs, which typically use 620-630nm for red and 460-480nm for blue. The results will not be the absolute best possible, but will still be far better than with white light. The Fuji Frontier SP3000, a popular professional scanner, uses 630nm for red and 465nm for blue (source), but it does not have to contend with the color channel sensitivity of a typical digital camera - it has a monochrome image sensor and combines three exposures taken with red, green, and blue light.

If going this route, I would recommend placing an array of RGB LED strips or an off-the-shelf RGB video light panel behind a diffuser made from some combination of diffusing film and white or matte clear acrylic sheets to get the most even illumination possible.

what about other light sources?

OLED displays do contain separate red, green, and blue emitters, but they typically have broader emission spectra than inorganic LEDs. They will produce results that are significantly better than a white light source, but not as good as RGB LEDs.

LCD displays emit white light that has been filtered through red, green, and blue filters and recombined. The results will depend on the emission spectrum of the exact display used and should be better than a white light source and worse than RGB LEDs. Newer high-gamut LCDs use special backlight LEDs designed to emit in narrower bands than typical white LEDs, so they will work significantly better than older ones.

White light is still ideal for scanning positive (slide) film, which is intended to be projected with white light. However, a narrowband light source may still be helpful for recovering colors from faded slide film.

scanning film with RGB

When scanning film with a narrowband light source, it’s easy to get good results without using any specialized software. Using software designed for processing white light scans to process RGB scans may give suboptimal results.

my scanning and processing workflow

1. scan

Scan all frames as RAW using fixed white balance and exposure. Exposure should be set such that none of the color channels are clipping.

2. import

Import scans into image editing software. Use a linear RAW profile. Apply lens corrections if necessary. Optionally, use Lens Cast Calibration in Capture One or Flat-Field Correction in Lightroom to compensate for any unevenness in the light source.

3. initial inversion

Invert the black and white points for all scans using the Levels or Curves tool and apply an appropriate curve to set the brightness of the output image.

4. white balance correction

Manually correct the white balance on one scan, and if necessary, adjust the red, green, and blue channel min/max levels to remove color casts in the highlights or shadows. Apply these adjustments to all scans from that roll of film, and check other scans and make adjustments as necessary.

5. fine tuning

Fine-tune the black and white points and curve for each scan using the Levels or Curves tool.

part 2: the hardware

To get the best possible results, I designed my own custom RGB light source. All design files can be downloaded from the GitHub repository.

the electronics

The light source uses six each of deep red (665nm), green (525nm), and royal blue (450nm) 2835 package LEDs, with adjustable brightness for each color channel. The LEDs are driven by Diodes Incorporated AL8860 constant current buck drivers.

Schematic (PDF)

Interactive BOM

build notes

The PCB should be fabricated with black soldermask to prevent reflections off the soldermask or fluorescence of the substrate material from affecting the emitted light.

Either a 19-24V barrel jack AC adapter or a 20V-capable USB PD supply can be used as a power source; if using an AC adapter, J6, U2, R6, R7, R8, and C4 do not need to be installed. At the time of writing, all components can be sourced from DigiKey except for the IP2721 USB Power Delivery IC.

Be aware that not all 2835 LEDs, even ones from the same manufacturer, have the same polarity. If using other LEDs than the ones specified in the BOM, be sure to check the datasheet before installing.

the diffuser

To diffuse the light, I designed a 3D printed enclosure to hold a stack of optical films similar to what is found in the backlight of an LCD display. I’m sure it’s possible to do better, but I’m not an optical engineer. A diagram of the diffuser assembly is shown below.

build notes

The diffuser film and microprism brightness enhancing film may be difficult to source. At the time of writing, they are available at a reasonable price from a variety of sellers in China. Brightness enhancing film is also being sold by CineStill under the name CS-LITEBRITE. It is also possible to obtain these materials by disassembling an old LCD display.

Alternatively, the brightness enhancing film can be omitted and some combination of diffuser film and matte or white acrylic sheets can be used instead. This hasn’t been tested, but it should provide satisfactory results.

All 3D printed parts should be printed in ABS, ASA, PETG, or a similarly heat-resistant material in a neutral color. Using white material or painting the inside of the diffuser housing white may improve optical performance. 4x M2x4mm and 4x M2x10mm socket head cap screws are required for assembly.

putting it all together

The light source is suitable to use with a variety of existing film carriers, but was designed with future expansion in mind. More open source 3D printable film scanning gear is coming soon :3

license

The PCB schematic, layout, and Gerber files and the 3D CAD files for this project are released under the CERN Open Hardware Licence Version 2 - Weakly Reciprocal (CERN-OHL-W V2). The text and illustrations in this article may not be reproduced without permission.

A Better Light Source For Scanning Color Negative Film