Dictionary tag: T

• The LMS colour space was one of the first systematic demonstrations of trichromatic colour theory.
• LMS describes how the three types of cone photoreceptors (L, M and S cone types) in a human eye respond given any particular light stimuli.
• The method used in the development of the LMS colour space produced a generalized representation of human colour perception.
• The underlying principle was that any colour can be described in physiological terms by measuring the response of the L, M and S cone cells in the human eye’s retina to different wavelengths of light.
• The initial source of data for the LMS colour space was taken from experiments that compared the spectral sensitivity of subjects with normal sensitivity with other subjects experiencing forms of colour blindness.
• A more recent technique used to collect data for LMS belongs to the field of visual psychophysics and is known as heterochromatic flicker photometry. It provides extensive and accurate spectral sensitivity data obtained from cellular material removed from the eye.
• The LMS colour space describes human observable colours using three parameters, known as tristimulus colour values, each component of which corresponds with the response of the L, M and S cone types.

Tristimulus colour values

• Tristimulus colour values have three components corresponding with the response of the L, M and S cone types. Each response is measured against a scale with values between 0 and 1.
• LMS tristimulus colour values for a monochromatic red, green and blue stimulus might appear as follows:
  • Red: wavelength  = 635 nanometres: L = 0.3278612, M = 0.0444877, S = 0.0
  • Green: wavelength = 520 nanometres:  L = 0.6285647, M = 0.8166012, S = 0.02920317
  • Blue: wavelength= 450 nanometres: L = 0.04986433, M = 0.08705225, S = 0.9553885
  • Data from: https://cie.co.at/datatable/cie-2006-lms-cone-fundamentals-2-field-size-terms-energy
• Tristimulus colour values can be thought of as colour-matching functions. If you know a tristimulus colour value then you can predict the corresponding colour experience.

Limitations of the LMS colour space

• The LMS colour space provides an accurate physiological description of human colour perception but has limitations related to the fact that some colours in the visible spectrum appear brighter than others.
• Whilst the achievements of the research that produced the LMS colour space underpin much of the subsequent developments within the field, LMS has been superseded by the CIE (1931) XYZ colour space.
• The CIE (1931) XYZ colour space addresses the limitations of the LMS colour space by sacrificing physiologically accurate measurements of colour perceptions in favour of a solution better suited to everyday colour management..
• The XYZ tristimulus colour parameters replace the LMS tristimulus colour parameters
• XYZ is a more convenient representation and the CIE XYZ colour plot defines all the possible colours a human observer can see. For a given luminance Y, the XZ value specifies all possible chromaticity values one can see.
• There is a simple linear transformation, via a 3 x 3 matrix, between LMS and XYZ.

• There are two main contexts to consider:
  • Circles: A tangent to a circle is a straight line that touches the circle at exactly one point, like a line just brushing against a ball. There’s also a special property – the radius drawn from the centre of the circle to the point of touch is always perpendicular (at a 90-degree angle) to the tangent line.
  • General Curves: A tangent line can also be applied to any smooth, curved shape. Here, the concept gets a bit more mathematical. We can define a tangent as a straight line that intersects the curve at exactly one point, but if we could zoom in infinitely close to that point, the curve would begin to resemble a straight line, and the tangent line would become indistinguishable from the curve itself.

• • Hot objects: Emit more electromagnetic radiation at shorter wavelengths. Imagine a hot fire burning bright with blue hues. Similarly, hot objects emit a higher proportion of their energy at shorter wavelengths, which often appear bluish.
  • Cold objects: Emit more electromagnetic radiation at longer wavelengths. Think of a dimmer fire glowing red. Colder objects emit more radiation at longer wavelengths, which tend to be perceived as redder.
• The relationship between temperature and the peak wavelength of an object’s radiation is described by Wien’s displacement law. This law states that the product of an object’s temperature and the peak wavelength of the radiation it emits is a constant.
• Black Bodies: Wien’s law applies to a theoretical concept called a black body. A black body is an idealized object that absorbs all incoming radiation and emits radiation at all wavelengths. Real objects aren’t perfect black bodies, but they still emit electromagnetic radiation based on their temperature.

Explanation of Thermal Radiation

• All matter consists of atoms and molecules in constant motion. This motion has kinetic energy, which is associated with the temperature of an object. As temperature increases, the motion of the particles becomes more agitated. This causes charged particles within the matter (like protons and electrons) to accelerate and change their energy states.

Emission of Energy

• When charged particles change energy states, they release energy in the form of electromagnetic waves.
• The frequency and intensity of this radiation depend directly on the object’s temperature.

The Spectrum of Thermal Radiation

• Thermal radiation covers a wide range of the electromagnetic spectrum. However, much of it falls within the infrared region, which we experience as heat. Hotter objects emit more thermal radiation and a higher proportion of radiation in the visible light spectrum. This is why very hot objects can start to glow red or white.

Examples

• The Sun: A primary source of thermal radiation. Its high surface temperature causes it to emit a broad spectrum of electromagnetic radiation, including infrared, visible light, and ultraviolet radiation.
• A Radiator: Designed to emit heat through thermal radiation, warming a room.
• The Human Body: Emits infrared radiation, which is why thermal imaging cameras can detect us in the dark.
• The Earth: Absorbs solar radiation and then emits thermal radiation back out into space.

Key Points

• A Constant Process: As long as an object has some internal heat, it emits thermal radiation.
• Heat Transfer Thermal radiation is one of the three main forms of heat transfer (alongside conduction and convection).
• Universal Phenomenon: Thermal radiation occurs throughout the universe, from stars to everyday objects.

• The First Law of Thermodynamics:
  • This law is a statement of the principle of conservation of energy. It states that energy can neither be created nor destroyed but only transferred from one form to another.
  • The total amount of energy in a closed system (one that does not exchange energy with its surroundings) remains constant.
• The Second Law of Thermodynamics:
  • This law deals with the concept of entropy, a measure of disorder in a system.
  • A system with high entropy is more disordered than a system with low entropy. The second law states that in an isolated system (one that does not exchange matter or energy with its surroundings), entropy always increases over time.
  • This means that usable energy tends to disperse over time into less usable forms, leading to a gradual increase in disorder.
  • Entropy can be understood as a measure of how spread out or disorganized the energy in a system is. Over time, energy tends to disperse from concentrated usable forms to more spread-out unusable forms, increasing the overall disorder.
• These two laws of thermodynamics have been extensively tested and verified through experiments.

Major contributors

• Major contributors to the laws of thermodynamics were Nicolas Léonard Sadi Carnot, James Prescott Joule and Lord Kelvin all of whom were at work during the 19^th century.
  • Nicolas Léonard Sadi Carnot (1796-1832) was a French physicist and engineer. He is best known for his work on thermodynamics, particularly his development of the Carnot cycle, a theoretical thermodynamic cycle that describes the maximum efficiency of a heat engine.
  • James Prescott Joule (1818-1889) was an English physicist and brewer. He is best known for his work on the relationship between heat and work, which led to the development of the first law of thermodynamics.
  • William Thomson, 1st Baron Kelvin (1824-1907) was a Scottish mathematician, physicist and engineer. He is best known for his work on thermodynamics, particularly his development of the Kelvin scale of temperature.

Examples of thermodynamics

• The First Law in Action:
  • Imagine throwing a ball up in the air. As the ball rises, its kinetic energy of motion is converted into potential energy due to its height.
  • When the ball falls back down, the potential energy is converted back into kinetic energy.
  • Even though the form of the energy changes (kinetic to potential and back), the total amount of energy in the ball remains constant.
  • This exemplifies the principle of energy conservation as described by the first law of thermodynamics.
• The Second Law in Action:
  • Consider a light bulb. When you turn it on, the electrical energy from the outlet is transformed into light energy and thermal energy (heat).
  • The total amount of energy is still conserved, following the first law.
  • However, the light bulb’s heat dissipates into the surroundings, making it less concentrated and usable.
  • Heat energy, in this sense, is more spread out and less usable than electrical energy, making the system more disordered according to the second law of thermodynamics. Entropy, a measure of disorder, therefore increases in this process.

• Exposure to Radiation: When a material (usually a crystalline solid) is exposed to ionizing radiation (like X-rays, gamma rays, or cosmic rays), some electrons within the material get trapped in imperfections within the crystal structure.
• Heating and Light Emission: When the material is heated, these trapped electrons gain enough energy to escape their traps. As they return to their original energy state, they release energy in the form of visible light.
• Measuring Radiation: The intensity of light emitted during thermoluminescence is proportional to the amount of radiation the material was previously exposed to. This makes it a useful technique for fields like archaeology and radiation safety.

There are two forms of thermonuclear fusion (nuclear fusion):

• Uncontrolled Fusion: This is the process where atomic nuclei merge spontaneously and release a tremendous amount of energy in an uncontrollable manner.
• It is the natural process happening within stars and the principle behind thermonuclear weapons.
• Controlled Fusion: Scientists are actively researching ways to achieve controlled fusion, where atomic nuclei are combined in a controlled environment.
• This would allow us to harness the immense energy released for constructive purposes like generating clean and sustainable power, reducing reliance on fossil fuels, and potentially powering future space exploration endeavours.

Challenges of Fusion

• Achieving controlled fusion is a significant scientific and engineering challenge. Fusion requires incredibly high temperatures and pressure to overcome the natural repulsion between atomic nuclei and force them to fuse.

Energy Release during Fusion

• Nuclear fusion reactions release energy in various forms. The primary form is in the form of high-energy photons called gamma rays. These gamma rays interact with the surrounding matter in a star, including hydrogen atoms. These interactions create other forms of light, ultimately resulting in the visible light we see radiating from stars. So, the light we perceive from stars is a product of the fusion processes happening within their cores.
• In stars, fusion primarily occurs at their core, where immense gravitational pressure and scorching temperatures provide the perfect environment for hydrogen atoms to fuse into helium. This fusion process releases the energy that powers the star and ultimately reaches us as the light we see in the night sky.

• In the context of additive colour models such as RGB or HSB, a darker tone of a hue is produced by reducing its colour brightness. The result is a desaturated, muted version of the original colour.
• In the context of subtractive colour models such as CMY and RYB, A darker tone (or shade) of a colour is achieved by adding black or a darker colour to it. The result is a desaturated, muted version of the original colour.
• In photography, tone refers to the different shades of grey that can be produced, ranging from pure white to pure black.
  • Here, tone describes the relative darkness or lightness of a specific shade of grey.
  • A greyscale image is created by discarding hue information from a range of colours. The resulting shades of grey reflect the original colours’ luminance (light intensity) but may not perfectly match their perceived brightness.
  • Whilst yellow appears to have a very light tone when converted to greyscale, blue appears to have a very dark tone.
• In the context of a greyscale, tone is used to describe the relative darkness or lightness of a specific shade of grey.
  • A greyscale is the result of removing hue from a range of colours leaving their saturation and brightness unaffected.
  • More sophisticated methods of producing a greyscale use specific algorithms to create images that better represent the perceived brightness of the original colours.
• Tone and value are closely related concepts. Tone describes the perceived lightness or darkness within a colour context, while value refers to the objective amount of light reflected or emitted, independent of colour.
  • To clarify this difference, think of value as a light meter reading that measures the amount of light reflecting off a surface. This reading would be a numerical value, independent of any colour information.

• Total Internal reflection only takes place when the first medium (where the light originates) is denser than the second medium.
• The critical angle is the angle of incidence above which total internal reflection occurs.
• The critical angle is measured with respect to the normal.
  • The normal is an imaginary line drawn in a ray diagram perpendicular to, so at a right angle to (90⁰), to the boundary between two media.

• Transmittance meanwhile is a way to measure how well a material allows light or other forms of radiation to be transmitted through it. It is essentially the fraction of incoming radiation that gets transmitted through the material.
• A high transmittance value indicates the material allows most radiation to pass through, while a low transmittance indicates most radiation is absorbed or reflected.
• The opposite of transmission is absorption, where electromagnetic radiation is absorbed by a medium and converted into other forms of energy, such as heat.
• The degree of transmission or absorption of electromagnetic radiation through a material can depend on factors such as the wavelength of the radiation, the composition and thickness of the medium, and the angle of incidence of the radiation.
• The degree of transmission can also vary depending on the type of electromagnetic radiation. For example, materials that transmit visible light well may not transmit ultraviolet light or infrared radiation as effectively.

• A transverse wave is a type of  wave in which the particles of the medium oscillate (vibrate) perpendicular to the direction of wave propagation.
• Transverse waves can be observed in various phenomena, such as waves on a string, water ripples, and certain types of seismic waves.
• Note that light and other electromagnetic waves are transverse waves that can travel through a vacuum.
• Transverse waves exhibit specific properties, including wavelength, frequency, amplitude, and wave speed.
• The motion of a transverse wave can be represented graphically using a sine wave or cosine wave, illustrating the peaks and troughs of the wave.
• Transverse waves can be polarized, meaning the oscillations are confined to a particular plane or direction, which has important implications in optics and other fields.

• When a material is subjected to mechanical stress, it creates a separation of electric charges within the material. As the material reunites, the separated charges can recombine. This recombination releases energy in the form of a burst of visible light.
• Not fully understood: While the basic mechanism is understood, the exact process of charge separation and recombination isn’t fully established and varies depending on the material.

Examples

• Sugar crystals: When sugar crystals are crushed, they produce flashes of light due to triboluminescence.
• Adhesive tape: Quickly peeling adhesive tape in a dark environment can produce light.
• Quartz: Quartz minerals exhibit triboluminescence when they are hit or fractured.

Important Notes

• Triboluminescence is distinct from other forms of luminescence as it doesn’t rely on previous absorption of energy from light or heat.
• The intensity and colour of the light produced through triboluminescence depend on the specific material.

The foundation of the trichromatic colour theory lies in understanding the physiological basis for the subjective experience of colour. It seeks to explain how our eyes and brains work together to create the rich world of colour we see around us.

• Cone cells: Unlike rod cells, which primarily detect light and darkness, cone cells are responsible for colour vision. There are three different types of cone cells in the human retina:
  • S-cones: Most sensitive to short wavelengths (blue light)
  • M-cones: Most sensitive to medium wavelengths (green light)
  • L-cones: Most sensitive to long wavelengths (red light)
• Colour perception: When light enters the eye, it stimulates the cone cells based on its wavelength. The brain then receives signals from these cones and interprets their combination as specific colours.
  • If all three types of cone cells are stimulated in different proportions, the brain perceives a mixed colour. For example, a combination of strong red and green stimulation might be perceived as yellow.
  • If only one type of cone cell is stimulated, the brain perceives the corresponding primary colour (blue, green, or red). However, due to overlapping sensitivities of the cones, pure primary colours are rarely seen in real life.
• Variations in colour vision: While trichromatic vision is considered normal, there are variations in individual sensitivities and slight differences in the distribution of cone cells. This can lead to subtle differences in colour perception between people.
• Comparison to dichromatic vision: Individuals with dichromatic vision only have two types of functional cone cells, leading to difficulties distinguishing certain colours, particularly red and green.