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Concepts of practical light measurement
There are four basic concepts of practical light measurement: Luminous Flux, Luminous Intensity,?Illuminance, Luminance.
The cosine law
The angle of?direction of light incidence with the perpendicular to a plane
Definition of illuminance
Illuminance is the quantity of light, or luminous flux, falling on unit area of a surface (Fig). It is designated by the symbol E.
Definition of luminance
Luminance is the concept for the luminous intensity emitted per unit of area of a surface in a specific direction.
Luminous flux - symbol f, unit lumen (lm)
Luminous flux is the concept for the total quantity of light emitted per second by a light source. It is designated by the symbol f. The unit is lumen (lm).
Definition of luminous intensity
Luminous intensity is the concept for the concentration of light in a specific direction, radiated per second. It is designated by the symbol 1.
The inverse square law
Relation between luminous intensity (I) and illuminance (E)
Relation between luminous flux (f) and luminous intensity (l)
The luminous intensity in any direction of light source whose light distribution is uniform in all directions.
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Colour
Light from a thermal radiator, the sun or an incandescent lamp, can be separated into the full range of spectral colours: red, orange, yellow, green, blue and violet.
Colour Rendering
Although light sources having the same (correlated) colour temperature will also have the same colour appearance, this does not necessarily mean that coloured surfaces will look the same under them.
Lighting quality criteria
Provided the above visual requirements are satisfied, details of the object in question will be perceived by the eye.
The CIE System
The spectral colours have been plotted along the two sides of a triangle, with the primary colours - Red, Green and Violet-Blue - placed at the corners.
The electromagnetic spectrum
The spectrum of electromagnetic radiation is extremely broad. It starts from thevery long wavelengths - or quanta with a very low energy content. |
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Lighting quality and economics
In many projects, lighting is often the last thing to be considered when doing the costing.
Light color and color rendering
People tend to judge colors under what they consider to he natural or true lighting - conditions, often mistakenly using daylight for this purpose.
Lighting level
The lighting level (viz. the amount of light incident on a surface) can be expressed interms of illuminance, in lux.
Luminance distribution
The luminance distribution, (or better distribution of brightness within the field of view), is an extremely important criterion of lighting quality.
Spatial distribution of light
An important criterion of lighting quality is the way in which the light is spatially is distributed, as this determines the pattern of illuminances that will be created.?
Lighting quality criteria
Provided the above visual requirements are satisfied, details of the object in question will be perceived by the eye.
Quality of the Lighting Environment
The visual environment embraces furniture, ceiling and walls, building materials likebrick and timber. |
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