Illuminance

The term Illuminance refers to the quantity of light that falls on a physical surface. Illuminance is typically used to define the usable light supplied by a natural or artificial light source regardless of its total luminosity.

  • If we place a book on a table, then different levels of illuminance can be observed when the sky is overcast, under the mid-day sun, in moonlight or artificial light.
  • Illuminance is independent of the surface that light falls upon. It is a description of the qualities of the light itself.
  • Illuminance is something that can be measured and so is an objective term.
  • The implication of this is that a 10-watt light-bulb is a fairly powerful source of light if it is placed next to an observer reading a book. But even a 1000-watt light-bulb will not provide enough light to read by if it is located a hundred metres away.

Illumination

Illumination or lighting is the deliberate use of light to achieve a practical or aesthetic effect.

Illumination might be provided through the use of artificial light sources like lamps and light fixtures, or natural illumination by capturing daylight.

Daylighting (using windows, skylights, or light shelves) is sometimes used as the main source of light during daytime in buildings.

Specialised forms of artificial lighting have been developed to suit every possible situation and purpose where natural light is not available.

https://en.wikipedia.org/wiki/Lighting

Incident

Incident light refers to incoming light that is travelling towards an object or medium.

  • Incident light refers to light travelling towards something but is often visualised as a single ray.
  • A ray diagram (or wave diagram) uses drawing conventions and labels to visualise the path that a single ray (or wave) of light might take as it encounters different media, materials or objects.
  • An incident ray (or wave) and its direction of travel is usually shown on a diagram as it propagates through a first medium, prior to encountering a second medium.

https://en.wikipedia.org/wiki/Ray_(optics)#Interaction_with_surfaces

Internal reflection

Internal reflection takes place when light travelling through a medium such as water fails to cross the boundary into another transparent medium such as air. The light reflects back off the boundary between the two media.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • For internal refraction to occur, the refractive index of the second medium must be lower than the refractive index of the first medium. So internal reflection takes place when light reaches air from glass or water (at an angle greater than the critical angle), but not when light reaches glass from air.
  • In most everyday situations light is partially refracted and partially reflected at the boundary between water (or glass) and air because of irregularities in the surface.
  • If the angle at which light strikes the boundary between water (or glass) and air is less than a certain critical angle, then the light will be refracted as it crosses the boundary between the two media.
  • When light strikes the boundary between two media precisely at the critical angle, then light is neither refracted or reflected but is instead transmitted along the boundary between the two media.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.
  • The critical angle is the angle of incidence above which internal reflection occurs. The angle is measured with respect to the normal at the boundary between two media.
  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.

Internal reflection

Internal reflection takes place when light travelling through a medium such as water fails to cross the boundary into another transparent medium such as air. The light reflects back off the boundary between the two media.

  • Internal reflection is a common phenomenon so far as visible light is concerned but occurs with all types of electromagnetic radiation.
  • For internal refraction to occur, the refractive index of the second medium must be lower than the refractive index of the first medium. So internal reflection takes place when light reaches air from glass or water (at an angle greater than the critical angle), but not when light reaches glass from air.
  • In most everyday situations light is partially refracted and partially reflected at the boundary between water (or glass) and air because of irregularities in the surface.
  • If the angle at which light strikes the boundary between water (or glass) and air is less than a certain critical angle, then the light will be refracted as it crosses the boundary between the two media.
  • When light strikes the boundary between two media precisely at the critical angle, then light is neither refracted or reflected but is instead transmitted along the boundary between the two media.
  • However, if the angle of incidence is greater than the critical angle for all points at which light strikes the boundary then no light will cross the boundary, but will instead undergo total internal reflection.
  • The critical angle is the angle of incidence above which internal reflection occurs. The angle is measured with respect to the normal at the boundary between two media.
  • The angle of refraction is measured between a ray of light and an imaginary line called the normal.
  • In optics, the normal is an imaginary line drawn on a ray diagram perpendicular to, so at a right angle to (900), to the boundary between two media.
  • If the boundary between the media is curved then the normal is drawn perpendicular to the boundary.