Halation and its simulation in Dehancer

Halation

Halation is a physical effect visible on film as a red-orange halo near the contrasting boundaries of over-exposed areas, as well as a red flare in the middle tones. Usually Halation is produced around bright light sources.

Halation OFX
Westworld Season 3 (2020), Kodak Vision3 500T 5219


Enlarged detail

Where and how to find it?

Modern color films contain a special anti-halation layer, which significantly reduces visible Halation, but does not eliminate it completely. In order to see Halation clearly, you can remove the carbon-based backing layer first, then shoot the film and develop it as usual.

Today the CineStill company produces film without anti-halation layer. It’s buying Vision 3 motion picture films from Kodak in bulk, then uses its own technology to remove the backing layer, cuts film in short strips and packs them in standard 35 mm or type-120 film rolls, ready to use with conventional cameras.

Normally negative motion picture film is developed with original ECN-2 process. This process is similar to the usual color negative C-41 process but not the same. The principal difference is the additional step of removing the carbon layer as one of the stages of film processing.
Pre-removing of anti-halation layer before film exposure then allows simple development with C-41 process and chemicals that makes movie film usable to a wide range of photographers.
Developer formula, temperature and processing time are also slightly different. Therefore, color on the Cinestill films is slightly different from the original Kodak Vision 3 processed with original ECN-2, but in the study of the Halation effect it is not of fundamental importance.

Let’s compare the same film shot with and without anti-halation layer. Any motion picture film such as Kodak Vision 3 50D will do.

Halation

The original Kodak Vision 3 50D with anti-halation coating is shown on the left. On the right is the same film but with pre-removed backing carbon layer, known by photographers as the CineStill 50 Daylight.

Both scans show a clearly visible red halo near light sources. In the first case it is much smaller, but still present. Thus, Halation is a natural and essential feature of the real picture on film.

The nature of Halation effect on film

If there is no anti-halation layer, strong light passes through the film, reflects back from the rear surface of the film base, or from anything behind the film (such as the pressure plate or the other internal surfaces) and comes back to the film.

Color emulsion consists of many layers. Basically only 3 or in some cases 4 of them (as with Fujifilm Superia) are involved in the color separation process itself. They are sequenced specifically, and the order determined by the corresponding wavelengths and their distribution in the matter. The closest layer to the lens is responsible for registering the blue light component, then the ‘green’ layer follows, and finally the last and the deepest layer captures the red light.

Therefore, light reflected from the inner surfaces of the camera is usually filtered out from the high-frequency components (blue and green spectrum) and backlights mostly the ‘red’ emulsion layer, which is also the closest to the inner surface of the camera. This causes red halos to appear around strong light sources.

Sometimes reflected light is very bright and not fully filtered by emulsion layers. This allows it to reflect back and penetrate not only into the ‘red’ layer, but also into the ‘green’ layer too. In this case, the scattered light colour shifts to the yellow spectrum, and the Halation effect becomes orange.

You may notice that in different movie scenes shot on the same film, Halation tint may be different. Actually the effect is the same on film, but at the editing stage white balance for different scenes may be significantly adjusted, producing different hues.

Excessive and evident halos around light sources and near the contrast boundaries of over-exposed image areas are considered a flaw. However, small halos are completely organic and even desirable as an aesthetic feature for the image shot on film.

There is an anti-halation layer in all contemporary color negative films that eliminates and softens visible halos. In photographic and movie films this technology is implemented slightly differently.

Halation

This picture from Wikipedoa shows the anti-halation layer on modern movie film. It is the last (outer) layer of opaque carbon particles applied to the backing of the film stock. Black carbon absorbs light and prevents it from reflecting and passing all the way through the film back to the emulsion.

Why movie films are so special about this layer? There is no black painted or anodised film plate on movie cameras – this allows film travelling freely. So the light passes through the film and scatters inside the camera. The only way to stop the light from reflecting back and exposing the film is to apply the opaque anti-halation layer on the film base. On movie films this layer is made of black carbon particles. Removal of this layer is a special stage of film processing.

The anti-halation layer is not absolutely black. So it still reflects light, albeit much less than the inner parts of the camera. At the same time, the reflection occurs at close distances, which reduces the scattering radius and the size of visible effect.

Thus Halation is small but clearly visible. Sometimes it’s more obvious, sometimes almost indistinguishable but still noticeable. This depends on various factors, primarily the brightness and the light source type, as well as the exposure level and the degree of contrast on the boundaries where halo appears.

Generally, the brighter the light, the higher film exposure and the scene contrast, the stronger Halation effect is and the more its tint is shifted to orange color. In particular, incandescent light bulbs produce quite obvious Halation, but around LED lamps halos may not be visible at all, because the red component in their spectrum is usually insignificant.

Halation on film: examples

Before we move on to Halation simulation in digital processing, let’s look at a some examples of this effect in the movies shot on film.

Halation
Once Upon a Time… in Hollywood (2019), Kodak Vision3 500T 5219

Halation
Enlarged detail

Note that despite the high brightness of the light source sufficient for Halation to appear, the light source itself is not overexposed and does not produce clipping.

The conditions under which Halation occurs are not limited only to powerful light sources and night scenes. It can also be seen during daylight, especially near the high-contrast edges.

Halation
Once Upon a Time… in Hollywood (2019), Kodak Vision3 200T 5213


Enlarged detail

Halation is very typical around specular highlights on reflective surfaces.

Halation
Brat 2 (2000), Kodak Vision


Enlarged detail

It is observed that Halation usually goes hand in hand with the Bloom effect. This film effect is described in detail in a separate article Bloom: what it is and how it works.

Halation
Dirty Harry (1971), Eastman 100T 5254

Halation
Enlarged detail

The contribution of Bloom to the combined ‘Halation + Bloom’ effect may vary depending on the optics and film stock used. Sometimes Bloom is even more prominent than Halation. Sometimes Bloom is less visible. But anyway Bloom almost always comes with Halation (although these effects are of different physical nature). In this case, Bloom is often, though not always, tinted with a touch of Halation color. As a result, the cumulative effect usually looks like a large composite halo with varying color.

Visible size of Halation effect depends not only on the brightness of the source and the degree of Bloom contribution, but also on the format of the film (the frame size). The smaller is the frame, the larger the Halation and Bloom effects are in relation to it. For example, on 16 mm film, halos are more visible than on 35 mm film. On the ‘wide’ 65 mm negatives halos will be very small relative to the entire image.

Example of Halation on 16 mm film:

Halation
L’ONE | 7days (2018), Kodak Vision3 500T 7219

Halation
Enlarged detail

Example of Halation on 65 mm film:

Halation
The Hateful Eight (2015), Kodak Vision3 500T 5219

Halation
Enlarged detail

Halation effect with Dehancer OFX

We’ve invented a special tool to simulate a lifelike Halation effect in digital image processing. It’s based upon real physics on film and it is an integral part of Dehancer Pro and Dehancer Photo Edition OFX plugins. Effect controls are combined into a separate group called Halation.

Halation OFX plugin

Let’s see how it works. For this purpose we’ll use a synthetic image with all types of contrasting boundaries at different exposures.

Halation

The inner rectangle represents some sort of ‘light source’ with brightness varying from L=0 (no light or not exposed film) to L=100 (light is on and/or overexposed).

The horizontal gradient around the rectangle represents the background on which a light source is placed. In Lab coordinates absolutely black has L=0 (unexposed areas), and purest white corresponds to L=100 (large brightly lit and/or overexposed areas).

Hence, at the corners of the rectangle we’ve created the best conditions for Halation study. Two of them have the highest contrast:

1. The upper right corner corresponds to the brightest light source over the black background. This is, for example, light bulbs in the darkness:

Halation

2. Lower left corner represents unexposed (black) objects over white background. This could be a backlit human silhouette:

Halation

The remaining two corners on the test image correspond to the areas with complete lack of contrast. This is a bright light source on an equally bright background (top left corner) and a switched off or underexposed light source on an equally dark or underexposed background (bottom right). Under these conditions Halation doesn’t appear, but anyway we have to study the effect between the extremes.

So, let’s review each tool parameter in detail. When demonstrating the impact, other settings can be set to the excess values (for illustrative purposes).

Sensitivity
This option defines the exposure threshold at which halos generation starts.

Minimum value = 0 means that only very bright sources will produce Halation effect. The background can be either very dark or rather light. The lighter the background, the weaker the haloes appear on it.

Halation

Maximum value = 100 means that Halation is formed not only around very bright light sources, but also around muted ones. The background must be dark enough to make effect visible. The darker the light source, the less halos are produced around it.

Halation

Low Soft
High Soft

This pair of parameters determine how smooth is the threshold set by the Sensitivity parameter.

Halation

The higher the value, the smoother the threshold and the wider it is extended towards shadows (Low Soft) or highlights (High Soft).

Diffusion
Determines how far light penetrates into the ‘emulsion’ from the source edge. The higher the Diffusion value, the larger the geometric size (radius) of the halos.

Halation

Amplify
This is the overall Halation ‘power’. It is a virtual amplifier that emulates the sensitivity of red and, to a lesser extent, green ‘emulsion’ layers to the light scattered from the backing film layer.

Increasing the Amplify value makes Halation more prominent, in the meantime haloes become orange-yellowish because the green emulsion layer sensitivity grows as well.

Halation
Amplify = 0

Halation
Amplify = 100

Hue
Hue parameter sets the base Halation tint, from red (Hue = 0) to orange (Hue = 100).

Halation
Hue = 0

Halation
Hue = 100

Scattering
As the name suggests, Scattering sets the power of secondary exposure of the red layer to the reflected scattered light. This is a smoother and more global effect, mostly visible in non-contrast midtone areas.

Halation
Scattering = 0

Halation
Scattering = 100

Impact
The overall impact of the effect. This parameter can be roughly treated as ‘Opacity’ because it does not change the physical emulation parameters, but the transparency of the superimposed effect only.

Mask Mode
This option replaces the actual source image with the pure black background so you can visually check the settings more clearly.

Halation

Chromatic Aberrations

Sometimes there are some chromatic aberrations or purple fringing visible near the contrasting edges. Its strength may be comparable to the desired Halation effect and therefore interfere with it, making any adjustments more difficult.

It’s possible to eliminate aberrations with two dedicated parameters in the Input group.

Halation

Defringe
Total degree of chromatic aberration suppression. The higher this value, the less noticeable the aberrations become.

Defringe Radius
The size of details considered as aberrations. With higher values the detail radius becomes bigger and the zone affected by Defringe filter grows.

Halation
Defringe = 0, Defringe Radius = 0

Halation
Defringe = 100, Defringe Radius = 100

Halation
+ Halation

The most effective way to eliminate chromatic aberrations is to apply this tool before all correction nodes in the processing chain, i.e. as close as possible to the source image. Therefore, if you need any nodes before Dehancer, we recommend to create an additional Dehancer node with the only Defringe correction enabled, then the intermediate nodes come, and the last one is the main Dehancer node with film profiles and other tools including Grain, Bloom and Halation.

For further reading we recommend this article:
Dehancer in DaVinci Resolve: node sequence

Bear in mind that in order to effectively determine chromatic aberrations, the timeline size must be at least 1920×1080 Full HD. Ideal conditions are met when the timeline resolution matches the resolution of source media.

When timeline resolution is lower than that of the source, Defringe effect on the Preview window or reference monitor may seem insufficient, but after export it will generally look better.

The Defringe tool may be used independently, with or without Halation, as well as a separate node.

Example

Halation is a subtle effect, so you have to zoom in closely to see it clearly.

Halation
Source image

Halation
+ Halation

Keep in mind that Halation rarely appears without the Bloom effect. Therefore, it is recommended to use both Halation and Bloom tools to obtain a more authentic image. In Dehancer it’s possible to fine-tune each effect independently.

Halation
+ Bloom

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Note
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