Introduction
The Science of Light is very important to understand for light metering even if you are a beginner in cinematography or an expert cinematographer. In this article, we will dive deep into the concepts.
Table of Contents – Science Of Light
What is Light ?
Light is a portion of the electromagnetic spectrum that can be detected by the human eye.
Electrically charged particles such as electrons and protons create electromagnetic fields when they move, and the energy these fields transport is called electromagnetic radiation.
Light behaves both as a particle and as a wave
Like a wave it has different wavelengths giving the sensation of colours to our eyes.
Like energized particles they cause photochemical changes in film and photoelectric response in the digital camera sensor.
Colour of Light
Our eyes are only sensitive to a narrow band between wavelengths 400 nm and 700 nm approximately. This limited span of wavelengths is known as the visible spectrum.
When a relatively even mixture of all the visible wavelengths is produced by a light source the illumination looks ‘white’
But if only some wavelengths are present the light appears coloured.
Wavelengths between about 400 nm and 450 nm are seen as dark purple-violet. This alters to blue when wavelengths are changed to between 450 nm to 500 nm. Between 500 nm and 580 nm the light appears to be more blue-green, and from about 580 nm to 600 nm we see yellow. The yellow grows more orange if the light wavelengths become longer and at 650 nm it looks red, becoming darker as the limit of response is reached at 700 nm.
The human eye contains two kinds of light receptors
Rods: which detect dim light and are used for night vision
Cones: which detect different colours but require brightly lit environments
Cone cells are of three different kinds
- S-cones: For short-wavelengths
- M-cones: For medium wavelengths
- L-cones: For longer-wavelengths
The combination of their responses is responsible for colour vision.
When the receptors are stimulated equally by an object we tend to see it as white.
Colour Temperature
It’s a method to describe the colour of a light source.
Colour temperature is a parameter describing the colour of a source of visible light by comparing it to the colour of light emitted by a Black Body.
A blackbody is an ideal object that absorbs all of the radiation that it receives. It does not reflect any light, nor does it allow any light to pass through.
When you heat up a black body it will emit its own radiation. As the temperature rises the colour of light emitted by a black body change.
The corresponding colour of light are measured by Kelvin
We generally think Orange is Warm colour and Blue is Cool.
But on the colour temperature scale the opposite is true.
The hotter the temperature, cooler the light.
White Balance
White balance is a way for the camera to compensate for the colour temperature of various light sources.
How does it do that ? It adds the opposite colour to the image in an attempt to bring the colour temperature back to neutral.
Its purpose is to ensure accurate and natural looking colours.
Intensity of Light
Light intensity is the amount of light either emitted or reflected by a source
The closer an object is to the light source, the brighter it will be. The level of illumination will drop as the object is moved away.
So, more the distance less is the amount of light.
This phenomenon is known as Falloff
Inverse Square Law
I = 1/d2
I here is the intensity of light
d is the distance
So, the intensity of light is inversely proportional to the square of the distance
Stabilization of light falloff
- At one foot from the source suppose we are at f/22
- At two feet away we will be at f/11
- At four feet away we will be at f/5.6
- At eight feet away we will be at f/2.8
- At sixteen feet away we will be at f/1.4
- from one foot to eight feet the falloff is 6 stops
- from eight to sixteen feet the falloff is only 2 stops
- From sixteen to twenty-four feet the falloff will be only one stop
Measurement of light
Lumen: Lumen is a measure of the total amount of visible light emitted from a light source.
Watts measure how much power is consumed by a light source; lumen measures the brightness.
In practical situations we measure light by Lumens/Area
Lux: Lumens per square meter
Foot Candle: Lumens per square feet
Light meters
A light meter is a device used to measure the amount of light.
In Photography, an exposure meter is a light meter coupled to either a digital or analog calculator which displays the correct shutter speed, aperture and ISO required for the correct exposure in a given lighting situation.
Simply put, Metering is to take a measurement of the amount of light, so your camera knows how to make the correct exposure.
Exposure meters are of two kinds
- Incident
- Reflective
An incident meter simply reads the intensity of the light that falls onto the subject
A Reflective Meter measures the amount of light hitting the subject and bouncing back to the camera
Reflected reading is affected by the colour or brightness of the subject itself.
- If the subject is bright, more light bounces back to camera.
- If the subject is dark, less light bounces back to the camera.
So, it’s basically is the measure of relative brightness of different objects in frame.
Shadow
A shadow is the absence of light
Light travels in a straight line until it hits an object. If the object blocks or partially blocks the light from passing through it, that creates an area of darkness behind the object – the shadow.
Umbra and Penumbra
The umbra (Latin for “shadow”) is the darkest part of a shadow, where the light source is completely blocked by the object.
The penumbra (from the Latin paene “almost, nearly”) is the region in which only a portion of the light source is obscured by the object.
| Basis of Comparison | Umbra | Penumbra |
| Definition | The central, darkest region of a shadow where the light source is completely blocked | The area in a shadow where the light source is partially blocked |
| Formation | Formed when an object completely blocks the path of light from a source | Occurs when the object is unable to completely block off the light source, or in the presence of multiple light sources |
| Darkness | Complete absence of direct light, resulting in profound darkness | Partial shading with some illumination present |
| Edges | Well-defined and sharp | Soft and gradual transition between light and shadow |
Shadows depend on four factors.
- The angle of the source of light
- The size of the source of light
- The shape of the object on which the light falls
- The relative position between the object and the screen
Angle of Light
Size of source
The smaller the source, deeper and sharper the shadow
The larger the source, softer the shadow
Shape of objects
The relative position between the object and the screen
The Zone System
Chiaroscuro is a stylistic art term meaning “light-dark”
Chiaro meaning bright and Scuro meaning dark
The term Chiaroscuro emerged during the Italian Renaissance
The technique employed directional light and high tonal contraststo create volume, modelling, depth and atmosphere.
How Light Behaves
- Absorption
- Reflection
- Transmission
- Refraction
- Dispersion
- Scattering
- Diffraction
- Polarization
Absorption of light is a process by which light is absorbed and converted into energy.
When visible light is absorbed by an object, the object converts the short wavelength light into long wavelength heat.
Photosynthesis is a process by which plants and certain other organisms transform light energy into chemical energy.
In a digital camera light energy is transformed into electrical energy
Some materials are opaque to some wavelengths of light, but transparent to others.
Glass and water are opaque to ultraviolet light, but transparent to visible light.
Reflection is the change in direction of light at an interface between two different media so that the light returns into the medium from which it originated
Specular reflection, which occurs with smooth surfaces like mirrors, causes light rays to reflect at the same angle as they hit the surface.
Diffuse reflection, which occurs with rougher surfaces, scatters light rays in different directions.
Light and colours
Colour is reflected light. When we see colour, we are actually looking at reflected light. If a material or matter absorbs light of certain wavelengths (or colours) of the spectrum, an observer will not see these colours in the reflected light. On the other hand, if certain wavelengths of colours are reflected from the material, these are the colours that the observer will see.
Transmission refers to the passage of light through a medium.
Refraction is the bending of light as it passes from one medium to other.
It happens because speed of light is different in different medium
The refractive index of a medium is a number that indicates the light-bending ability of that medium.
n=c/v
n is the refractive index of the medium
c is the speed of light in vacuum
v is the speed of light in the medium
Total internal Reflection is the phenomenon in which waves arriving at the boundary from one medium to another completely reflected back into the first medium.
Dispersion is the spatial separation of visible light into its constituent spectrum
Each colour has distinct wavelengths.
Red light has the longest wavelength in this range of seven colours, while violet light has the shortest.
In vacuum, all wavelengths of light travel at the same speed. However, in any transparent material, such as glass or water, different wavelengths move at different speeds.
Various colours’ bend through different angles due to differences in their velocity. Red light travels the fastest through any transparent medium, whereas violet light travels the slowest.
As a result, red light bends the least while violet light bends the most.
Thus, the dispersion of white light into seven colours happens when various colours bend at different angles while passing through a glass prism.
Scattering
When light passes through a medium, some portion of the light is absorbed by the medium’s particles and some part of it defected to different directions based on the wavelength of the constituent light. This phenomenon is known as a scattering of light
Higher frequencies with shorter wavelengths scatter more due to the fact that they are wavier than the lower frequencies with longer wavelengths.
Also,
- Tiny particles scatter light of a shorter wavelength.
- Large particles scatter light of a longer wavelength.
Diffraction is the slight bending of light as it passes around the edge of an object. The amount of bending depends on the relative size of the wavelength of light to the size of the opening. If the opening is much larger than the light’s wavelength, the bending will be almost unnoticeable. However, if the two are closer in size or equal, the amount of bending is considerable, and easily seen with the naked eye
Through diffraction light spreads out and illuminates’ areas where a shadow is expected
Light is the interaction of electric and magnetic fields travelling through space. The electric and magnetic vibrations of a light wave occur perpendicularly to each other.
The electric field moves in one direction and the magnetic field in another ‘perpendicular to each other. So, we have one plane occupied by an electric field, another plane of the magnetic field perpendicular to it, and the direction of travel is perpendicular to both.
These electric and magnetic vibrations can occur in numerous planes.
A light wave that is vibrating in more than one plane is known as unpolarized light.
Polarization is the process of transforming unpolarized light waves to polarized light waves.
Article Credits/Mentor
Prof. Sh. Joydeep Bose
Cinematography
