Colour & thermal radiation

About colour & temperature

The surface colour of objects and their thermal temperature can be distinguished as follows.

Surface colour
  • The surface colour of an object seen by an observer is dependent on:
    • The light that falls upon it.
    • The sensitivity of the human eye to the range of wavelengths that correspond to the colours of the visible spectrum.
    • The physical and chemical properties of an object, so its material composition. These determine how it interacts with incident light, including how it absorbs, reflects or scatters light.
  • In terms of the difference between surface colour and thermal radiation, an apple that appears red at 5 degrees Celsius will still appear red at 85 degrees Celsius, but the thermal radiation it emits will be different at the two temperatures.
Thermal radiation
  • Thermal radiation is a measure of the electromagnetic radiation emitted by an object due solely to its temperature, in the absence of incident light.
  • The colour and brightness of most objects that we see in daily life are due to the reflected light such as sunlight or artificial light.
  • Reflected light is typically much brighter than the thermal radiation emitted by the same object at room temperature.
  • The amount of thermal radiation emitted by an object at room temperature is relatively low compared to the amount of radiation it will emit at higher temperatures.
  • However, the amount and distribution of thermal radiation emitted by an object can be affected by factors such as the composition of the object, the properties of its surface, and the ambient temperature and humidity of the surrounding environment.
  • The concept of thermal radiation typically encompasses a broad range of wavelengths across the electromagnetic spectrum, including infrared radiation, visible light, and ultraviolet radiation.
  • At room temperature, most objects emit low levels of thermal radiation in the infrared region of the electromagnetic spectrum.
  • An iron rod would need to be heated to a temperature of around 1000 to 1200 degrees Celsius to emit thermal radiation that is visible to the human eye.
    • At this temperature, the rod would glow red, and the colour of the glow would become brighter and shift towards yellow and then white as the temperature increases further.
    • It’s worth noting that the precise temperature at which an iron rod starts to emit visible thermal radiation can vary depending on  the specific rod and its environment.

Colour & visual perception

About colour & visual perception
  • Colour is not a property of electromagnetic radiation, but rather a characteristic of visual perception.
  • The human eye, and therefore human perception, is sensitive to the range of light wavelengths that constitute the visible spectrum, including the corresponding spectral colours from red to violet.
  • Light, however, is rarely of a single wavelength, so when an observer notices a red ball they are probably seeing a range of similar wavelengths of light within the visual spectrum.
  • Perception of colour is a subjective process as our eyes respond to stimuli produced by incoming light but each of us responds differently.

Colour perception

About the perception of colour
  • The perception of colour is a highly subjective experience.
  • The colour of nearby objects can influence colour perception.
  • The environment in which colours are observed and the type of object can influence colour perception.
  • The physical properties of light, including wavelength and intensity, can affect how colours are perceived.
  • An observer’s physical and mental state can affect their perception of colour. Health, medications, mood, emotions, or fatigue can all affect an observer’s eyes, vision, and perception.
  • Different observers may perceive colours differently based on their life experiences, linguistic backgrounds, and educational, social, and cultural factors.
  • The term “observer” has distinct meanings in various scientific fields, such as special relativity, general relativity, quantum mechanics, thermodynamics, and information theory.

Light & colour

About light and colour
  • Light and colour are related but distinct concepts. Light is a form of electromagnetic radiation, while colour is a perception that results from how the human eye and brain respond to different wavelengths of visible light.
  • The human eye can perceive only a small part of the electromagnetic spectrum, known as visible light, which includes wavelengths between about 400 and 700 nanometres.
  • The perception of colour depends on the wavelengths of light that stimulate the cones in the retina.
  • The perception of colour can vary among individuals and living organisms.
  • Even if humans had never evolved, electromagnetic radiation would have been emitted by stars since the formation of the first galaxies over 13 billion years ago.
  • Colour perception in humans primarily depends on the design of our eyes and the wavelength, frequency, and energy of the visible light that strikes the retina at the back of our eyes.
  • Colour is a visual experience unique to each of us at any given moment because of our different points of view and perspectives on the world. So we share our experiences of colour using language to share our experiences of colour.

Light, colour & vision

About light, colour & vision
Light
  • The human eye and human vision are adapted and responsive to the visible spectrum, which includes wavelengths of light corresponding to colours ranging from red to violet..
  • Light is the electromagnetic radiation that enables us to perceive colour. It consists of a spectrum of wavelengths, and it is the interaction of these wavelengths with our visual system that gives rise to the perception of different colours.
  • The visible spectrum is the range of wavelengths of light that the human eye can detect, typically spanning from approximately 400 nanometers (nm) for violet to 700 nm for red.
  • Light is seldom composed of a single wavelength, so an observer is typically exposed to a range of diverse wavelengths or a combination of wavelengths from various parts of the visible spectrum.
  • Visible light does not possess any properties that set it apart from other segments of the electromagnetic spectrum.
Colour
  • Colour is not an inherent property of electromagnetic radiation but rather a characteristic of vision and the visual perception of an observer.
  • Colour is not an inherent property of electromagnetic radiation but rather a characteristic of vision and the visual perceptions of an observer.
  • Colour is what human beings perceive when light is present.
  • Objects appear to have different colours to an observer based on the wavelengths and intensity of light when it reaches the retina at the back of the eye.
Vision
  • When light enters the eye, it interacts with specialized cells called cones in the retina. Cones are responsible for detecting and processing different wavelengths of light, which contribute to our perception of colour.
  • The three types of cones, commonly referred to as red, green, and blue cones, respond to different ranges of wavelengths. The combined activity of these cones allows us to perceive a wide range of colours.
  • The brain plays a crucial role in the perception of colour. It processes the signals received from the cones and interprets them to create our conscious experience of colour.
  • Colour perception is influenced by various factors, including the intensity and quality of light, the surrounding environment, and individual differences in vision.
  • Our ability to perceive and differentiate colours provides important cues about the world around us, helping us recognize objects, navigate our environment, and experience the richness of visual stimuli.

Observation of colour

About the observation of colour
  • The human eye is sensitive to the visible spectrum, which includes all the spectral colours ranging from approximately 400 to 700 nanometers.
  • The sensitivity of the eye to the visible spectrum enables us to perceive colours when light interacts with objects.
  • The visual perception of colour by an observer is associated with words such as red, blue, yellow, etc., which are commonly used to describe hue or dominant wavelength.
  • The colour an observer sees depends on:
    • The wavelengths of visible light present in the environment.
    • The wavelengths absorbed, transmitted, or reflected by an object or medium.
  • The perception of colour can be affected by factors such as brightness, contrast, and saturation, which are related to the amount of light present in a stimulus and its interaction with the eye and brain.
  • The observed colour of light is determined by its wavelength, not its frequency. However, as light travels from one medium to another, such as from air to glass, the colour seen by an observer may change due to refraction causing colours to disperse in different directions.

Saturation & colour

About saturation & wavelength
  • Saturation is one of the three primary properties of colour, alongside hue and brightness.
    • A colour looks saturated when made by a single or a small range of wavelengths.
    • A colour made by one wavelength of light is often referred to as a pure spectral colour.
    • Unsaturated colours appear faded due to a wider range of wavelengths.
    • Saturation is linked to the complexity of light.
Light complexity
  • Light complexity, linked to saturation, refers to the quantity and range of wavelengths of light used to create a colour.
    • Spectral colours are simple because they consist of just one wavelength of light.
    • Bands of colour are relatively simple because they are composed of a continuous range of wavelengths.
    • Non-spectral colours can be produced from a mix of many wavelengths from different parts of the spectrum, making them the most complex.
  • In reality, colours are often produced by complex combinations of wavelengths.
  • The greater the number and spread of wavelengths across the visible spectrum present in a colour, the lower the saturation.
  • The human eye can perceive millions of different colours due to the complex interactions of wavelengths and the eye’s colour receptors.