Sunday, March 8, 2015

Optics


(This article is a modified republishing of my own article which I wrote for Digital Academy's website, for my students, when I used to teach there.

The versions written for DA could be found in these two links.

https://dafilmschool.wordpress.com/2012/05/28/cinematographic-learning-part-4/

http://dafilmschool.in/cinematographic-learning-part-4/)

Any kind of image or impression is visible through contrast. And any contrast is primarily created by light.

Just as a painter needs some functional knowledge of the chemical and physical properties of his paints, a photographer needs to know the basics of his paint – the light. The more he knows how light interacts with nature and with his eyes, the more he can have the guidance of light to create beautiful images.

It is all cinematographers’ dream to be one with the light, to see the basic fabric of light. The moment he sees it, nature reveals herself to him. All great Masters of cinematography knew light as their greatest kin in the world. Keeping this in mind, it is no more surprising to know that the Maestro Satyajit Ray scolded his Cinematographer, another Maestro, Subrata Mitra, for taking too much time to light up the set. He plainly said on Mitra’s face, “You are light’s slave!”

Ray was wrong. Cinematographers are not slaves to light, but kins. They seek oneness with light to materialize the sparks on the screen that they have in mind.
 


Subrata Mitra’s lighting for Ray’s Charulata (1964)



It is mandatory for a genuine cinematographer to know as much about light as possible, exactly the way someone tries to understand his best friend. The formal study of light is known as optics.


clip_image002


The first thing to know about light may come as a big surprise. Light shows everything, but it is invisible itself. Light shows up matter in its path of travel. When the path of travel consists of smoke or dust in the air, or cloud, streaks or beam can be seen. However, when it travels through clean air, if someone stands perpendicular to the rays and look at the supposed beam in the air, he sees nothing.


clip_image004

We can see the light beam as the source, or as coming through a glass. But, eyes perpendicular to the beam, she cannot see the light. It is complete darkness for her.








Perhaps the most important feature of light is it always travels in straight line. This is not difficult to understand when one keeps the phenomenon of shadow in mind. When a light bulb is lit up, light radiates in full sphere all around it.


clip_image006

All the individual light rays travel in their respective paths, in straight lines. However, if some object is placed in a light ray’s path, any one of three phenomena may be observed depending on the nature of the blocking object.

If light passes through the blocking object, mostly unaffected, the object is called transparent. Glass, water, air and many other minerals and synthetic materials are transparent.


clip_image008


If light is blocked, and cannot pass through, the object is known as opaque. Later, we shall see that opaque objects are of different types too.


clip_image010


There are objects, such as smoked glass, cellophane paper or plastic bottles, which block some rays and let others pass, but spreading them more outwards. These are called translucent objects.

In cinematography, translucent objects play a major role, as we shall see later in this article.
When light is blocked by an opaque object, the block is delineated by the shape of the object, otherwise known as the shadow.

Hence, shadow is the absence of light, and just like any other contrast it is visible only when there is a lack of light in some part in an otherwise lit up area. It sounds paradoxical, but to see shadow prominently, light is needed.


clip_image012


Even the most transparent object in the nature casts a shadow as nothing is purely transparent. But, the more opaque an object is, the shadow is darker (ie, there is no light in the shadow area. The shadow area is totally dark unlike the image above.)


clip_image014


What happens to the light rays that cannot pass through?

 Again any one of three things happens. They can be reflected off the blocking surface of the opaque object in a very regular manner, just like mirror. They can be reflected off the surface in an irregular manner that spreads them out at the time of reflection, just like any visible object other than mirror. Or, they can be totally absorbed.

Just like any ideal physical phenomenon, pure mirror reflection, irregular reflection or absorption never happens. There is always a mix of everything, with one of the three dominating.


clip_image016

One thing is sure, and no cinematographer can afford to ever forget that – whatever happens, an individual light ray always travels in a straight line, and can never turn around a blocking object unlike water.

So, it can be seen when light rays meet something in their path they are absorbed, or transmitted through the object or reflected off the object surface. When light rays are mostly absorbed, the object looks dark. In other words, its details cannot be seen.

When the object surface reflects light rays, a very interesting natural rule is followed.


clip_image017

As it can be seen in the diagram above, a light ray always gets reflected in the same angle in which it falls on a surface. Nothing in the universe causes an exception to this. This is a fundamental property of light, and does not depend on the source, color or brightness of the light, or on any property of the reflecting surface.

However, this does not say how a beam of light would behave while getting reflected on an uneven surface.

For all cinematographic purposes, light can be considered as a cylindrical pack of individual light rays – a beam - in common parlance.

Each ray in the light beam reflects off in the angle of incidence (i.e, the angle in which they hit the object surface). But, if different areas in the surface are themselves at angles (i.e, if the surface is not a plane), even two parallel light rays in a hitting beam, will get reflected in angles to each other, depending on the angles between two points on the reflecting surface.

In another word, light rays will fan out, or be concentrated.

Normally, visible surfaces fan out light rays very irregularly. The reflection they perform is known as diffuse reflection, as opposed to the very regular mirror reflection.


clip_image018

Sometimes, cinematographers use a technical term – specular reflection. That is same as the mirror reflection (Latin word for mirror is speculum.)

In a mirror reflection the light source is seen while the reflecting surface is not noticeable. But, a diffuse reflection shows the reflecting surface, and not the light source. Human eyes see most objects because of diffuse reflection only.

Even in a diffuse reflection, a significant portion of the beam can cause mirror reflection at a particular angle. So, the light source is partially reflected on a wall, or on a polished door. Such a reflection is known as a hotspot, and is always to be avoided in any shoot. Any composition with a hotspot looks clumsy.


clip_image021





Hotspot on the door
                                   






Sometimes the hotspot can be made less prominent by making the glossy area matte, with the help of spray.

When the light is allowed by the transparent or translucent object, light rays are always bent. Just like mirror reflection, two parallel rays remain parallel after bending, because they bend in the same angle, if the transparent object is a plane (or flat).

However, if the transparent object is curved, like part of a transparent sphere, light rays bend more as one goes up or down from the center.


clip_image022


The bending can be towards one another, or away from one another. Accordingly, light rays converge to a focus, or diverge away.

Anyone can sense that such curved transparent objects are known as lens. They magnify or reduce an image forming through them.

Bending of light caused by any transparent object is known as refraction. The bending can be regular or accelerated in both the ways. However, in the case of lenses, the acceleration itself is regular.

What happens when the acceleration is not regular? That means, light rays cross one another randomly after transmission through the object and spread. Translucent materials produce such an effect.


clip_image024

As the light rays are more spread out through a translucent material, in effect it is same as increasing the size of the light source. That in turn makes the light create a softer shadow.


This is why cinematographers use different translucent materials before light sources, and at different distances from sources, to create different levels of shadows. In nature also it is a very regular phenomenon. On a clear day, the sun casts very hard, well-defined, fine edged shadows. But, as the sky turns cloudy the shadows also turn to fuzzier, to a practically shadowless situation when the sky is completely overcast.


Besides reflection and refraction, another important, but less noticeable behavioral property of light is its polarization.

It is necessary to go through a few fundamental facts about light before one tries to understand polarization.

What is light? What is it made of? A beam of light can be visualized as a pack of rays, emitted from the source and passing through space to fall somewhere.

But what are those individual rays made of? Particles? If they are particles, overlapping of two different particles should always create more energy. After all that is what one expects when one canon ball strikes another to give more energy to the last one in the row for a greater impact.

However, such interference of light creates bands of darker and brighter lights. Brighter light is a commonsense expectation if light is mixed. But darker? That makes no sense.


clip_image025


Such an interference pattern makes perfect sense if light is visualized as series of waves. When opposite waves cancel each other out, the effect visible is darkness.

However, light visualized as wave cannot satisfactorily explain why light fails to turn around opaque objects.

Human beings have been studying light for the last two millennia, or more. But, only in the last century, a break through to this question came.

Light is dual in nature. When it travels in space it is mostly like wave. But, the moment it interacts with matter it behaves like particle.

No one really knows exactly how light is. But the approximate knowledge gathered so far is interesting to take a lifetime of study.

Coming back to polarization, this property of light deals with its wave nature. Light is the visible spectrum of electromagnetic radiation. The full spectrum contains cosmic rays, gamma rays, x-rays and ultra-violet rays on the more energetic side, and infra-red rays (associated with heat waves), mobile waves and radio waves of different types on the other.

In between, a miniscule portion, seen by the human eye, is called light.


clip_image027

A light wave can be visualized like a sine wave, and it is multi-dimensional in reality unlike a sea-wave. For the practical purpose, one can imagine a light wave bloated all around like a sphere.


clip_image029

In diagrammatic representation like above, the distance between the two highest energy level (both in the positive and negative, or up and down directions) is known as wavelength. In physical manifestation, this denotes the light’s color. How high is the energy level from a zero position, is known as the wave’s amplitude. That denotes the light’s brightness.

From the diagram above, it can be seen the wavelength is different for red, green and blue light each. The shorter the wave length the more energy is packed in that light.

clip_image031

Light rays travel in space in such waves. However, the wave is not a unidimensional, linear one as mentioned above. Besides, each wave has two wave-components perpendicular to each other.

There are many natural and synthetic materials, like cloud, glass and plastic, that reflect or transmit one wave blocking the other completely or partially. This phenomenon can be used very creatively for cinematographic purpose, when an already polarized light is blocked by putting another polarizer material on the camera lens, thus making only a portion of the cinematographic frame darker.


clip_image033


Image without polarizer Image with polarizer

There are many other interesting sides to light. As said earlier, a lifetime is not enough just to churn through the superficial layers of light’s behavior.

Van Gogh wanted to paint the ultimate source of all earthlight – Sun. He staked his life at this inhuman pursuit. He succeeded.

A cinematographer keeps studying about light and in the course knows more about his art and himself, throughout his life.


Post a Comment