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MkrGeo

An original point of view

Optics, Photography

What is a light transition? What examples of it can we see?

Starburst effect red light and white light

Starburst effect comparison between white and red light.

 

The light is omnipresent in the Universe and the environment. This is electromagnetic radiation, that can be detected by the human eye. I have written more about the light and the ways of measurement it in this article. Now I would like to describe a very common process, occurring both in Universe and on Earth’s atmosphere, which is a light transition.
This article will be a far simplification of this process. I am not going to bombard you some formulas and heavy scientific stuff. I would like to show you this common process practically. The transition of light refers much more to medium rather than to vacuum. The transition of light is a kind of molecular electronic transition, where electrons in a molecule are excited from one energy level to a higher energy level. The energy change associated with the transition determines many molecular properties such a colour. Relevant information in the Landau Theory, which covers a general phase transition mechanism. The light transition is the part of this mechanism, that happens abruptly and is associated with a sudden light level difference. This process is related to the difference between illuminated and shaded areas in conjunction with human eye perception.

  1. LIGHT AND SHADOW – A TYPICAL EXAMPLE

Basically, we have light and shadow. That’s a simple issue. If something is not illuminated, is shaded. However, we have to figure out, that despite the distance between illumination source and object shaded we always have the half-shadow zone. Every half-shadow has a bright and dark edge near the transition of light from full illuminated area to half shadow and from half-shadow to shadow (Pic. 1).

Light transition between fully shaded and illuminated area

Pic. 1 The light transition between the fully shaded and illuminated area (Gilles, Minnaert, 1954).

This is a human eye perception effect, which is quite easy to spot, especially on plane coloured surfaces (Pic. 2).

Light transition between shaded and illuminated area white effect

Pic. 2 A light transition between fully shaded and fully illuminated areas. In each case, this is pretty much plain coloured surfaces. Red arrows mark the brightest “line” between an illuminated part of the surface and the half-shadow area. This “line” is an external half-shadow border at once. A blue arrow shows another “line” located at the half-shadow and shadow border, which is the darkest. The inner part of the shadow appears to be slightly brighter than towards the edge.

This illusion will appear everywhere, where the beam of light encounters some hindrances. These obstacles can hide an illumination source totally or partially. It depends on the size of this obstacle against the (angular) dimensions of the light source. Sometimes, when the hindrance (seen from some surface) is too thin or small to cover totally the illumination source, then only a half-shadow occurs. In this case, we can also spot this aforementioned brightest “line” between fully illuminated and partially shaded area. Another “line” is drawn at the border between a half-shadow and shadow. Maybe you will able to notice, that inner part of the fully shaded areas appear to be a bit brighter, than towards the shadow edges. It’s good to mention, that sometimes we can see more than one shadow (Pic. 3, 4), which not necessarily must cover the light source totally.

Double shadow pattern

Pic. 3 The double shadow mechanism, which is the most popular for leafless trees, when 2 twigs hinder the solar beam at once (Gilles, Minnaert, 1954).

Double shadow example

Pic. 4 The double shadow mechanism is to be seen i.e. under the trees. There are 3 major areas marked by blue arrows: 1 – completely shaded, where 1 or more shadow covers the illumination source totally, because is close enough to the surface, 2 – one or more shadows casted by twigs and foliage produce a half-shadow only, because they are too far from the surface and smaller than illumination source at once, 3 – fully illuminated area.

When we have more shadows covering the illumination source at once, with beams still coming through, then only half-shadow is produced. In this case, the light transition is less pronounced.

A similar contrast phenomenon we can observe on undulating ground, where this bright illusion is observed just above the ground is closer. This is agreed with the Machs bands – an optical Illusion, where the contrast between edges of the slightly different shades is exaggerated. It is an effect of human eye perception, triggering edge-detection in this case. In the result, the contrast-band always appear to be an exaggeration of the curvature (Gilles, Minart, 1954) (Pic. 5, 6).

Mach bands appearance pattern

Pic. 5 The theory of Mach bands appearance on-site, when looking far. Undulating area causes a contrast illusion. When you screen off the image to dotted line square only you got a better effect of it. Looking on one exemplary hill range we can see, that is slightly brighter on the bottom likewise the left part of the image, where the boundary between adjacent shades, where darker area falsely appears even darker and brighter area falsely appear even brighter (Wikimedia.org/Gilles, Minnaert, 1954).

Mach bands live example when watching from Bryce Canyon national Park towards rand Staircase-Escalante National Monument, Utah, USA

Pic. 6 A live Mach bands example, which often merges with a bigger haze intensity in the lowest part of the planetary boundary layer. The Mach bands illusion is the best to see during the transparent air conditions. Bryce Canyon National Park – looking towards Grand Staircase-Escalante National Monument, Utah, USA.

This example can be also observed during a partial solar eclipse, when shadows cast by Sun disappear and reappear asymmetrically as eclipse progress. These changes can be explained as due to changes in the shape of the prenumbra of shadows as the visible portion of the Sun forms crescents of different orientation (Ross, Diamond, Badcock, 2003).

     2. HUMAN PERCEPTION AND POWER OF VISION

A light gathering power is the most important ability for observing the light by the human eye. Another feature of the human eye is the ability to adjust to the illumination level on the stupendous range between the faintest stars and the brightest light, being about 100 trillion times stronger (Schaaf, 2002). Even a full Moon is actually 465000 times dimmer than the Sun  (Schaaf, 2002).
All this shows how easy the human eye adapts from day to night conditions and another way round. The eye works with two fundamentally different  (though partly overlapping) systems in bright light and dim, referred to my previous article about light around us. There I mentioned about the photosensitive retinal cell, which responds well in these different light conditions. Everyone is familiar with the changes in pupil size due to changing light intensity: the pupils are small in bright light, large at dim. The pupil diameter can dilate from about 2mm in full light to 8mm in full darkness, however, as person ages, the pupil dilation ability is lower leading to decrement the light-gathering power. Fortunately, this defect is not significant for most of our life as an observational experience.
The ability to see faint objects to the naked human eye is the dark adaptation. As I mentioned both above and also in my previous article a human eye has two major kinds of photosensitive cell: rods and cones. Cones work well in bright light, giving the ability to distinguish colors. Rods work well in dim light and cannot distinguish colors. The adaptation to a particular level of light takes a while and depends on the level of transition. In extremal cases, when you go outdoors at night from a brightly lit interior you are not able to see faint stars for a while. The rods need to redevelop in darkness in a matter of minutes. For the biggest difference in illumination the improvement of the ability to see faint objects may take from 15 to 30 min to be mostly accomplished (Schaaf, 2002). The ability to see faint objects to the naked human eye is the dark adaptation (Pic. 5).

Cones and rods adaptation curve

Pic. 7 A dark adaptation curve (Pirenne MH. 1962)

This dark adaptation must be also considered in terms of pre-adapting light conditions, which level can vary depends on the illumination difference between the bright and dark area. The intensity and duration of pre-adapting light are strongly associated with a light transition area. When this light transition is better pronounced, what is caused both by bigger illumination difference between two separate areas and the width of the half-shadow area, then the intensity of pre-adapting light will be bigger. In this case, when the level of pre-adapting luminance is higher, the duration of cone mechanism dominance extends, while the rod mechanism switch over is more delayed. In addition, absolute threshold takes longer time to reach. The opposite is true for decreasing the levels of pre-adapting luminances (Barlett, 1965) (Pic. 8).

Dark adaptation curve depends on the length and light level

Pic. 8 Dark adaptation curves following different levels of pre-adapting luminances (Barlett, 1965).

The illuminance impinging on the human eye is measured with the Troland (Td) units, where we have a product of photopic (or scoptic) luminance and pupil area (Pic. 8).
If the half-shadow area is less pronounced, what happens basically when the obstacle has much bigger angular dimensions, than a source of light or is too close to the illuminated surface, which in this case is a human pupil. Then the pre-adapting light have a much shorter duration and the decrease in dark adaptation is more rapid (Pic. 9). For extremely short pre-adaptation periods only a single rod curve is obtained unlike to opposite situation, when long pre-adaptation period incurs both rod and cones reaction.

Dark adaptation curve following different durations of a pre-adapting luminance

Pic. 9 Dark adaptation curves following different durations of a pre-adapting luminance (Barlett, 1965).

There are also other factors, which affect the dark adaptation like: size and location of the retina, a wavelength of the threshold light and rhodopsin regeneration.
The wavelength threshold light plays an important role in dark adaptation curve affection. Basically, long wavelengths such an extreme red create the absence of a distinct rod/cone break as the rod and cone cells have similar sensitivities to light of long wavelengths. Conversely, at short wavelengths, the rod/cone break is more prominent, because the rod cells are much more sensitive than cones once the rods have dark-adapted (Barlett, 1965).
The process of dark adaptation means also changes in the dark adaptation threshold. The dark adaptation threshold indicates the illumination level, where some point or object becomes visible for the observer at this moment. It changes as the eye adapts more for darkness or as some bright light around becomes stronger. Basically, the dark adaptation threshold is an effect of weak light presence against some stronger light available in the vicinity. Usually, a stronger light beat up the weaker one, making a dimmer object less visible or even invisible. A good example is a comparison between a single white light from torch and stars seen on the sky. Using a torch to shine a flashlight in the eyes of the observers may change the dark adaptation threshold rapidly, making some dimmer celestial objects temporarily invisible for the observer (Pic. 10).

The starbusrt effect comparison between white and red light

Pic. 10 The starburst effect caused by torchlight using during the stargazing. Comparison between white and red light. A white light after only 66s exposure appears to look brighter than red light captured with 93s exposure for the following parameters: f/5.6 ISO 12800. For stargazing purpose is much better to use a red light rather than white, because a red light affects much less on the dark adaptation threshold, being highly important at this exercise. Near Horsehoe Bend, AZ, USA.

Fortunately, the eye can recover very rapidly from a very brief exposure to bright light, obviously when the light is not bright. In this case is much better to use red light instead of white, because the rods of the eyes are hardly sensitive to very red light at all  (because rods do not distinguish any colors as colors but very red light they do not even respond to as illumination (Schaaf, 2002)).
The adaptation to a particular level of light takes place also during the day, however, last much quicker than during the night. It can be noticed when a thunderstorm approaches. Usually a thick cumulonimbus cloud blocks most of the direct sunlight, turning the daylight into the evening or even twilight conditions  (especially for interiors) (Pic. 11), which was also discussed in the light scattering issue.

Light transition when cumulonimbus approaches

Pic. 11 The light transition moment for cumulonimbus cloud approaching. Look carefully on the cloud base, which is seen from a few kilometers and above your head. When this cloud underside is far enough, then a bigger amount of (scattered) light coming to the observer’s field of view. In the result, the details of the cloud bottom are less visible to the observer unlike to situation on the picture 2nd, where this cloud base moves above the observer’s head. Then, as you can see a rightmost part of the picture is much brighter, however, the amount of scattered light is much less, giving the rough details of the cloud base structure. Odrzykoń, Poland.

Another example can be found when we enter i.e. to the basement, where a light level is really low and remains nocturnal conditions. A total solar eclipse is also an example of significant light changes. In this case, the human eye adaptation is still no longer than a few seconds. A key role in light transition plays a huge difference of luminosity between one area and another. A much more illuminated area ordinary draw out the details from the less illuminated area, which initially appears to look like a “dark or grey wall”. When an observer is closer to the transition area then some details behind this “wall” become visible (as long as the light has been scattered). This is strongly associated with the contrast phenomena issue, that is to be developed in the next section.
The moment, when some details of the less illuminated region depend on the illumination difference between the bright and darker area as well as the medium, in which the light is scattered. Analyzing the pictures made by Apollo at Moon (Pic. 12) I guess, that this process doesn’t work in a vacuum, where is co conditions for light scattering.

NASA Apollo landing site

Pic. 12 Apollo landing site. See the shaded hills beyond with no details at all. due to this, I can guess, that light scattering doesn’t exist or is at least strongly restricted in vacuum conditions (NASA).

By the other hand, the light will be reflected from illuminated objects giving any illumination for shaded ones. This factor can be also considered in medium, where depends on the object properties and kind of medium will be stronger or weaker than scattering.

3. CONTRAST AND VISUAL AQUITY

Next to the dark adaptation, the main role in the perceiving of the light transition plays also a contrast. Contrast is a difference in luminance or colour, that makes an object distinguishable. In visual perception of the real world, contrast is determined by the difference in the color and brightness of the object and other objects within the same field of view.

Contrast amplitude

Pic. 13 The image presents a different contrast amplitude, which varies between the upper and lower part of the image. When the amplitude is bigger, then the contrast ratio is higher and we can see more details of the surface (Wikimedia.org).

The human visual system is more sensitive to contrast than absolute luminance. An observer can perceive the world similarly regardless of the huge changes in illumination over the day or from place to place. However, in any given moment of time, the eye can only sense a contrast ratio of 1000 (Kitsinelis, 2012).
The contrast ratio is a property of a display system, defined as the ratio of the luminance of the brightest color  (white) to that of the darkest color  (black), that the system is capable to produce. A retina in the human eye has a static contrast ratio of around 100:1. It means, that the brightest white is 100 brighter than the darkest black. As soon as the eye moves rapidly to acquire the target, it re-adjust its exposure by adjusting the iris, which adjust the size of the pupil.

Contrast sensitivity chart common test for eyes

Pic. 14 Pelli Robson contrast sensitivity chart – the most common test, which measures your ability to distinguish between the finest increments of light versus dark (Psych.nyu.edu).

This process is associated with a dark adaptation, described above. Again, the light transition area divides brighter and darker area. When the brighter area is far, then only a “grey or black wall” can be visible instead of the details inside the darker region,  because the pupil is already adjusted to illuminated, brighter area. Then our eye has a much better visual perception of objects being in an illuminated area unlike to another area being shaded, where the visual perception is really poor. Situation changes, when the shaded, darker area approaches and illumination level of the brighter area in the field of view start to decrease. Then the details of the shaded region become visible. This is a rough moment, when human eye adjusts to a new level of illumination. An analogue situation occurs when in our field of view are two different brightness regions close to each other. Then our pupils open and close adjusting the view dynamically like a video camera. In conclusion, when illuminance difference between two separate regions is lower, then the contrast ratio is bigger. In the effect, we can have more distinguishable details on both regions. Conversely, a huge light level difference between two regions (like the clear and cloudy sky) decreases a contrast ratio. The result is a lack of discernable details in one of the region, depends on its prevailing presence in our field of view. For instance, when looking on the bright region, you can distinguish only the bright region features, the darker one appears to be featureless (Pic. 15).

Contrast of cloud deck and ground underneath as seen from the plane

Pic. 15 Usually clouds reflect a bigger amount of light than ground do. In the result the could cover see from the aircraft cruise altitude appear to look bright grey or even whitish. The shaded ground is much darker underneath. The contrast ratio is better pronounced for clouds being brighter and occupying most of the field of view. The shaded ground plays a minor role in this picture (also the observer eye field of view), so contrast ratio is really squeezed without any details discernable. OSL-OAK.

The human eye can sense a contrast ratio up to 1000. The eye adapts its definition of what is black. Is worth to mention the contrast sensitivity, which measures the ability to discern between luminances of a different level in a static image. To make it simple contrast sensitivity describes the ability of the visual system to distinguish bright and dim components of a static image, how much contrast is needed to perform a particular visual task. The contrast sensitivity is equal to 1/Contrast-threshold. It means, that contrast sensitivity defines the threshold between the visible and invisible (Pelli, Bex, 2013). The contrast threshold is about 1% for a wide range of targets, independent of size and luminance (Pelli, Bex, 2013).

4. LIGHT TRANSITION IN PHOTOGRAPHY. CONTRAST AND DYNAMIC RANGE TECHNIQUE.

The light transition phenomenon is to be noticed also in photography and movies. Before I start to describe it I would like to bring some information about a dynamic range, which has similarities with the contrast ratio. The dynamic range is commonly used in photography when the big luminance range scene is being photographed. A dynamic range technique is used also for the limits of luminance range that a given digital camera or film can capture  (Myszkowski, Mantiuk, Krawczyk, 2008).
A basic aim of the dynamic range technique is to make the image capabilities comparable to the capabilities of the human eye. Dynamic range is the range of information between the lightest and darkest part of an image, also known as an image luminosity. We can distinguish a Standard Dynamic Range  (SDR) and High Dynamic Range (HDR). The basic difference between these two dynamic ranges is the ability to show the details and colours of the scene in the picture in comparison to the ability of the human eye. Standard dynamic range  (SDR) is the current standard for video and cinema displays. It represents only a fraction of the dynamic range, that HDR is capable of. The High Dynamic Range allows you to see more of the detail and color in scenes. The 6-stops range of Standard Dynamic Range has a limited dynamic range (Borer, Cotton, 2015). It results in a big contrast of the image with lost details of the darkest and the brightest areas. In the result, shaded areas appear to look dark without any details and sky (especially cloudy) appears to be completely white, also without any details  (white sky effect)(Pic. 16).

White sky effect caused by lack of HDR function Amsterdam

Pic. 16 A white sky effect is common in the pictures taken with Standard Dynamic Range, when the majority of the image is occupied by much darker areas. Vondelpark, Amsterdam, Netherlands.

Conversely, a High Dynamic Range  (HDR) is able to gain almost triple than Standard Dynamic Range  (SDR), with an average approximate total of 17.6 stops. In the effect, in the place wherein SDR image, we could see almost nothing, HDR allows us to see much more details. HDR images can represent a greater range of illuminance level, than can be achieved using more traditional methods unlike to SDR providing limited exposure range with loss of detail in highlights or shadows. The best moment for comparison between SDR and HDR image technique is to capture the sunrise or sunset (Pic. 17).

Sunrise and sunset SDR-HDR comparison Merzouga and Phuket

Pic. 17 2 random examples of photos, where we can compare an SDR and HDR technique. Picture 1 – SDR technique, sunrise above the desert, 2 – HDR – sunset above the sea. Despite the different surfaces and slightly different weather conditions, both pictures differ from each other quite a lot.

The digital cameras as well as the human eye needs adaptation to rapid changes of illumination level caused the transition from illuminated to the shaded area. A single photo cannot show us this difference unlike to footage. To experience it we need to place the camera or camcorder near the border of two areas with different illumination i. e cloudy bright sky and the building wall. You will see the moment of light adaptation of your instrument. This example clearly refers to the light transition between the illuminated and shaded area in our field of view.

The same exercise can be done with single images, where your field of view moves between the single images, covering darker or brighter areas (Pic. 18 – 22). Stacking all pictures together you can see every step of light transition recorded by your camera. The human eye responds on light transition is analogous, however, the pupil is less sensitive on light changes than the camera sensor does.

Light transition - camera field of view

Pic. 18 An example of less pronounced light transition on the wall due to a big distance from the photographed object. Comparing 1 and 2 to 4 you can see what is the difference within the shaded region. Nikon D5300.

Light transition - field of view a detailed example

Pic. 19 A rough example of light transition on the bricked wall from a short distance, made by stacking a few pictures including a different part of illuminated and shaded areas. As shaded area covers more picture frame, the photographed surface become brighter. At sequence 1 we can see roughly the details of the illuminated part of the wall, conversely, sequence 7 shows the same details for the shaded region. Nikon D5300.

Light transition - camera field of view an example of a white surface

Pic. 20 A rough example of light transition as given by white surface, where every single picture varies within illuminated and shaded areas. Nikon D5300.

Light transition - Nikon D5300 field of view and landscape

Pic. 21 Light transition within shaded area, made by stacking a few pictures with a different part of brighter and darker areas. Sequence 1 shows perfectly the sky, with very little details of the ground objects. Sequence 6 is a contradiction of the sequence 1, where the ground is clearly visible with white sky effect. Nikon D5300, Corrie Road, Cambridge, UK.

Light transition - camera field of view Cambridge

Pic. 22 An example of light transition, as we can observe between illuminated and shaded areas. The illumination source can be Sun or another considerable object like Moon or street lamp. The mechanism is the same. When this source disappears behind some obstacle or object, then we become able to see more details of shaded areas.

The light transition in photography aspect we can understand as a balance between a highlight and shadow, that is to be adjustable in common graphic software like Gimp, Photoshop, etc.

      5. LIGHT TRANSITION EXAMPLES IN THE SURROUNDING ENVIRONMENT

Knowing all major elements, that are associated with the phenomena of rapid illumination changes could be good to bring several examples as an essence of this article
The common examples of the transition phase of light have been listed below.

   A. TOTAL SOLAR ECLIPSE – the best example of light transition in macroscale, which occurs in a brief moment. The transition phase, that occurs between about 30s before totality to around 20s “inside” the umbra causes a multitude of phenomena, that is going to be described soon. A total solar eclipse seen from the space looks like a dark dot surrounded by an area of gradually diminishing light (prenumbra or half-shadow) making the shadow’s edge soft and gradual (Pic. 23, 24).

Light transition 2017 total solar eclipse shadow in sky

Pic. 23 The light transition example during 2017 total solar eclipse at about 25s before 2nd contact (marked green arrow). This is a rough moment, when details in the background become visible. The rough moment of transition is driven by solar position on the sky and backward scattering (credits: Marek Substyk footage).

Light transition during 2017 total solar eclipse shadow out sky

Pic. 24 The light transition example during 2017 total solar eclipse at the moment of 3rd contact (marked yellow arrow). The rough light transition moment has been marked by forward scattering (credits: Piotr Chabior footage).

   B. LUNAR ECLIPSE – is a very smooth and slow example of light transition, which occurs in nature. Basically, a transition area is the shadow edge. Similarly to the total solar eclipse, there is partially (prenumbra) and totally (umbra) shaded area. The transition area is marked by shadow edge, which gradually moves across the lunar disk. At the prenumbral phase, we can see only a diminishing lunar disk, that is less bright towards the side of Earth’s shadow approaching. Once the shadow edge becomes visible the observer can see nothing beyond the transition line. This is because the pupils are still adapted to the brighter part of the eclipsed Moon, thus only in the highlight region, we can see the details. The situation changes as the eclipse progresses. About halfway through the partial eclipse observer cannot see the details neither a shaded part of the Moon nor the most illuminated one. Only visible details are located near to the transition area, where the sunlight is deeply eclipsed. On the other side, the limb of the shaded part of the Moon starts to be visible. At the deep partial lunar eclipse, a human eye is adapted to dim light reflected by eclipsed Moon’s surface, thus some details “behind” the transition area is visible unlike to still highlighted area, which appears to look completely white and featureless (Pic. 25).

Total lunar eclipse light transition

Pic. 25 Light transition example as seen during a total lunar eclipse on 4.03.2007. Here are 12 single images captured, where some of them has been marked as a major moments of the light transition process: 2 – U3 contact, where light transition process begins. Comparing to sequence 1 a right side of the Moon is less visible. 6 – Next step of the light transition process, when an umbral region becomes invisible for the human eye and illuminated regions of the Moon starts to reveal some details, especially near to the umbral edge. 9 – As umbra recedes, the illuminated part of the lunar disk shows more surface details, initially near the umbral border and next across a whole shined surface (Slooh.com).

   C. EARTHSHINE – is a dull glow, which lights up the unlit part of the Moon as a result of the sunlight reflected from the Earth. The best time to see earthshine is up to 5 days before or after the new Moon. The best conditions to see earthshine are when the Sun is below the horizon. In spite of considerable brightness (-3 mag) it cannot be seen during full daylight, because is ordinary drown out by the Sun being in angular vicinity. The phenomena become visible when the Sun is close to set. In the case of Earthshine, the light transition area is determined by the terminator line. At the very first days after new Moon, earthshine has the best appearance, because is bright enough against a thin crescent illuminated by Sun. Earth’s atmosphere impairs its appearance effectively, because the Sun is not deep enough, making a strong light scattering. As Moon moves further from the Sun earthshine faints. A basic reason is a change of the Earth’s phase from full to waxing or waning, that reduces an area of light reflection. Weaker earthshine successively yields a lunar crescent, which becomes wider and brighter at once. When Moon is close to first or last quarter then earthshine is drowned out completely by the much brighter, illuminated part of the Moon. An observer can only spot a faint shaded lunar limb, which at first or last quarter become invisible. In this case, a light transition occurrence overlaps with a weakening of reflected light, coming from the Earth (Pic. 26).

Earthshine sequence and light transition

Pic. 26 The light transition in case of earthshine. When Moon is closer to be full, then earthshine disappears, which is a reason for weakening reflected light coming from Earth, as it becomes waning and thinner. Usually, we are not able to see earthshine after the first quarter and before the last quarter of the Moon (Flickr.com).

   D. THUNDERSTORM CLOUDS AND CLOUD DECKS – are the most massive clouds as we can see on the sky. The biggest is cumulonimbus capillatus with a remarkable top. However, this is not the aim of my consideration, because I would like to point out at the cloud base. The effect, that I will describe below can refer also to other clouds, which are thick enough or at least uniform. When the cloud is far from the observer, he can see only the illuminated parts of the cloud both from edges and from the gaps inside. When the cloud appears as a uniform layer then an observer is not able to see the shaded details. For a person standing in a distance to cloud the cloud base will look like a “grey wall” spreading down to the horizon. This is because the area of the sky, being illuminated directly by Sun plays the main role. Moreover, the light reflected from the upper parts of the cloud is also strong, much stronger than light reflected by inner parts of the cloud base. The situation changes when the cloud approaches closer to an observer. The gaps or areas being directly illuminated by the Sun become more discernable. On the other hand, a brighter part of the shaded cloud base becomes visible. Everything happens due to the decrement of the illumination in our field of view, which incur an eye adaptation to darker conditions. A light transition area definitely leads to change the contrast of cloud base, making all its details clearly visible (Pic. 27 – 30).

Chmura burzowa Cumulonimbus widziana z Trzech Koron

Pic. 27 When cumulonimbus cloud is far away from the observer, he can see only its parts directly illuminated by Sun. The shaded regions of the cloud are featureless for the human eye. Trzy Korony, Pieniny Mts, Poland.

Light transition cumulonimbus cloud above Kraków

Pic. 28 A thunderstorm cloud approaching results in a light transition, that allows the observer to see the details within the cloud shadow, such a cloud gaps (red arrows). A blue arrow marks a border between sunlight and shadow, that causes a light transition in the local area. Na Błonie street, Kraków, Poland.

Light transition on the remote cloud example, a grey wall

Pic. 29 When the cloud deck is far from the observer, then he cannot distinguish details beneath the cloud, which appears as a “grey wall”. AGH Student Campus, Kraków, Poland. 

Light transition on thin cloud layer example, Niegocin lake Mazury Poland

Pic. 30 Sometimes, when a cloud deck is thin enough an observer can see some cloud gaps inside the earlier mentioned “grey wall”. Niegocin lake, Poland.

In this case, the light transition phenomenon can overlap with the Beer-Lambert law, especially under a high haze concentration in the air.

E. CLOUD’S SHADOW – observed mostly with the cumulus or cumulonimbus clouds, especially shifting closely to the Sun, where forward light scattering takes hold. Under the shadowed part of the sky, the strong blue color comes through. It reveals a transparent-looking part of the atmosphere caused by shaded aerosols (or thin cloud layers), which lost the ability to scatter direct sunlight. Utter lack of directly scatter sunlight in the shadowed area near the cloud borders can sometimes remain a sky visible at cloud deck altitude, outside of the planetary boundary layer turbidity (Pic. 31, 32).

Cloud shadow on the sky

Cloud shadow on the sky 2

Pic. 31, 32 A classical example of cloud’s shadow observed with the cumulus cloud (credits: Ali Majeed Al-Hajari, Spaceweathergalery.com).

The cloud’s shadow can be also visible on another cloud deck, however, is usually a bit less pronounced.

H. CREPUSCULAR & ANTICREPUSCULAR RAYS – has been mostly described here. They are generated by multiple cloud shadows sweeping together, where single shafts of light reach the surface. Within these narrow illuminated paths, the details of the vista are impaired by haze, scattering this light.

Crepuscular rays over the ocean

Pic. 33 An example of the crepuscular rays above the ocean (credits: Craig Joiner/ Craigjoiner.com).

   G. EARTH’S SHADOW (BELT OF VENUS) – is also a good example of eye adaptation. When the twilight wedge is low above the horizon, then a highlighted section of sky prevails. In the result, the observer sees only a grey plain-coloured area, just under the transition line. As the Belt of Venus grows up, it changes a tint. Sun is deeper below the horizon, thus illumination of the sky is lower. Moreover, the shaded part of the sky prevails on our field of view. As a result of the illumination decrement, the observer’s eye begins to see the different hues of the shaded portion of the sky (Pic. 34).

Light transition belt of Venus example

Pic. 34 A progress of Earth’s shadow across the evening sky at Edinburgh. For sequences 1, 2 and 3 it appears as a “grey wall” with one tint. Sequence 4 correspond to the Belt of Venus position when Sun is about 3,5-4 degrees below the horizon. This is a moment of light transition, when a shaded part of the sky starts to appear in various hues, which are better visible in sequences 5 and 6 (Deckchair.com).

F. SUNRISE & SUNSET TRANSITION – is the same common phenomenon like an Earth’s shadow appearance, but refers to the pollutants and haze concentration, on which the terminator line can be visible by the observer. This is quite easy to spot, when looking at remote mountains located close to solar point. Usually, shortly after sunset or just before sunrise the mountain’s shadows are visible on the sky due to some haze and atmospheric molecules concentration. The sunset and sunrise transition is a terminator line visible on the sky then (Pic. 32, 33). An opposite phenomenon can be seen, when standing on the top of alone high mountain or volcano at the sunrise and sunset.

Sunset transition above Tatra Mts

Just after sunset transition above Tatra Mts, Carpathians

Pic. 35,36 An sunset transition seen above Tatra Mts as seen from about 1. The mountain shadows are visible on the sky due to atmospheric molecules, which scatter the light  (Credits: Damian Bryndza).

Another example of sunrise or sunset transition can be a dawn’s warm glow phenomena, which has been described previously.

G. COMMON DAY-TO-DAY EXAMPLES – for example when you want to see some object towards the solar direction you need to shade your eyesight by thumb for example to see it. It refers to any strong source of illumination, i.e. street lamp or when looking towards the solar direction (Pic. 34).

Light transition - shined object

Pic. 37 When looking towards solar direction usually you got much more illumination, than towards antisolar direction. in the result, some deeply shaded areas can hide their details.

When the street is illuminated by the street lamp we can see the illuminated area perfectly, but the region behind the street lamp remains invisible or barely visible for an observer. The details emerge at the light transition area, when the moment of eye adaptation occurs. Always stronger illuminated source eliminates a weaker one, being in the vicinity or behind. A good example, that has been mentioned above is watching dimmed areas during the day (Pic. 35). For instance, when you look on some big hole under full sunlight conditions, like a cave, well or a basement then you can see nothing, because your eye is adapted to full light conditions instead (Pic. 35). On top of that, the ambiance is illumination directly by solar beams, reflecting the light much stronger than objects being inside this shaded hole, which reflects only light scattered in the atmosphere. As our sight approaches closer to this shaded area, our view of sight loses the regions with stronger illumination and the interior objects become visible. In this case, a transition area is a moment, when our sight range likewise in cameras begin to cover a shaded area only.

Light transition - shined object, bushes and shrubs

Pic. 38 A dimmed and deep shaded areas can be seen only, when excluding a high illumination level from your field of view. Corrie Road, Cambridge, UK.

Light transition tunnel Croatia

Pic. 39 A good example of a big illumination level difference under the full daylight conditions. A deep shaded tunnel, where: 1 – seen from distance with less amount of details to be discerned by the observer, 2 – seen from the vicinity with the interior details better visible. Duba, Croatia.

6. CONCLUSIONS

In this article, I have presented live examples of light transition, which occur everywhere in space and our environment on the border between highlighted and shadowed areas. There wasn’t a better way to show it as taking pictures and preparing some relevant movies. Unfortunately, these materials don’t reflect perfectly a real light transition phenomena as a human eye is able to see. The human mind is able to intelligently interpret the information from our eyes, unlike the cameras, that does it manually and sequentially. The basic and well-known principle is, that a stronger source of light draws out the weaker ones, especially when situated in the vicinity or beyond. The light transition area cannot be noticed abruptly, because a human eye needs the time to adapt. The duration of adaptation depends on the difference of illumination level between the highlighted and shaded area. The eye adaptation, as well as a digital camera, can be strongly associated with a specific form of light transition, unique for every field of view. It happens anywhere, when illumination level in a field of view drops rapidly. Moreover, the light transition area itself is not a rough line. This is a transition area between the light and shadow, that we call a half-shadow. It is typical for all distances, from i.e. the paper sheet located a front of your eyes to the interplanetary distance. The most significant moment in transition is the border between a half-shadow and shadow, which is because the illumination level drops logarithmically. An important factor is the atmosphere reach in aerosols and molecules, that scatter the light additionally making the transition phase more smooth. Except for the natural issues a key role plays a human eye adaptation to rapid changes of illumination level as well as modern photography techniques, giving a chance to extend the contrast range in the field of view. This topic is much wider than the issues raised in this article. I hope, that this material, as well as references, provided give you a way to find more about this occurrence also in terms of different fields.

 

Mariusz Krukar

 

References:

  1. Barlett N.R., 1965, Dark and light adaptation (in:), Graham C.H., Vision and visual perception, John Wiley and Sons, New York.
  2. Borer T., Cotton A., 2015, A “Display independent” High Dynamic Range television system, (in:) BBC Research & Development
  3. Caelli T., 1981, Visual perception: theory and practice, Pergamon Press, Ontario
  4. Gilles M., Minnaert M., 1954, The nature of light & colour in the open air, Dover publications Inc., New York
  5. Kitsinelis S., 2012, The right light: Matching technologies to needs and applications, CRC Press, Boca Raton-London-New York
  6. McAuriffe Marisha, 2016, The perception of light – understanding architectural lightning design, Oxford Global Press, London
  7. Myszkowski K., Mantiuk R., Krawczyk G., 2008, High Dynamic Range video, Morgan & Claypool Publishers, Berkeley
  8. Olmsted P. D., 2000, Lectures of Landau Theory of phase transitions, University of Leeds, Department of Physics and Astronomy
  9. Pelli D.G., Dex P., 2013, Measuring contrast sensitivity, (in:) Vision Research, v.90,  p.10-14.
  10. Pirenne MH, 1962, Dark adaptation and night vision (in:) Dawson H., The Eye, vol. 2, Academic press, London.
  11. Reis J.K., et all., 2016,  Pollutant dispersion simulation during sunset transition time using an analytical Eulerian model, (in:) American Journal of Environmental Engineering, vol.6 (4A), p.40-45.
  12. Rogers K., 2011, The eye: The physiology of human perception, Britannica Educational Publishing, New York
  13. Ross J., Diamond M., Badcock D., 2003, Mach bands change asimmetrically during solar eclipses, (in:) Perception v. 32(6), p. 767-770
  14. Schaaf F., 2002, The starry room: Naked eye astronomy in the intimate Universe, Dover Publications Inc., New York.
  15. Stoppe B & J., 2009, Stopees’ guide to photography & light, Elsevier Inc., Burlington, Oxford

 

Links:

  1. Landau theory of a first order phase transition
  2. A phase transition of light
  3. Mach band illusion.html
  4. Spatial frequency channels.html
  5. Pelli Robson contrast sensitivity chart
  6. Contrast sensitivity testing.htm
  7. Contrast ratio – what does it really mean?
  8. Contrast sensitivity background
  9. Contrast sensitivity – explanation
  10. Cameras vs human eye
  11. Interactive SDR-DHR comparison
  12. What is HDR? HDR-SDR compared
  13. HDR vs SDR comparison
  14. What is Earthshine?

 

Wiki:

  1. Contrast_(vision)
  2. Contrast_ratio
  3. Dark_adaptation_threshold
  4. Dynamic_range photography
  5. Field_of_view
  6. High-dynamic-range_imaging
  7. High dynamic range_video
  8. Landau_theory
  9. Mach_bands
  10. Molecular_electronic_transition
  11. Night_vision
  12. Standard dynamic range_video
  13. Terminator_(solar)
  14. Troland
  15. Visual_perception

Youtube:

 

 

Read also:

  1. Light around us and how to measure it

 

 

 

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