Last updated on September 7th, 2023 at 06:19 pm
Explain the term illumination in detail
illumination from a point source reduces with the square of the distance.
A source of/ candela emits a total flux of 4π/lumens. At a distance (d) this flux will be distributed over a sphere of radius, i.e. a surface of 47d². Thus the illumination at a distance d is:
E=4ml / 4md² = I / 47d²
Where En = illumination on a normal plane
Eb= illumination on a plane tilted by ß degrees
B = angle of incidence
Illumination of a surface from several sources will be the simple sum of the component illuminations:
E=E1 + E2+E3…
Illumination from a linear source of infinite length reduces in direct proportion to the distance (and not the square of distance) and from an infinitely large luminous surface (e.g. the sky) the illumination does not vary with the distance.
What do you mean by scalar illumination and illumination vector?
Lighting conditions are usually described, measured or specified in terms of illumination on a given plane, most often the horizontal working plane (taken at a desk or bench height), but possibly a vertical or inclined plane.
In other words, we usually speak of ‘planar illumination.’
This, however, does not describe all the luminous qualities of a space.
Even if the illumination on a horizontal plane is adequate, the vertical surfaces may remain dark, if the visual task is other two-dimensional, and qualities other than the planar illumination must be considered.
Scalar illumination (or mean spherical illumination) is the average illumination received on the surface of a small sphere from all directions.
It is denoted Es and measured in lux. It measures the total quantity of light present regardless of its direction.
The illumination vector is a composite quantity having both magnitude and direction.
Its magnitude is the maximum difference in illumination between two diametrically opposed points on the surface of a small sphere (denoted Emax and measured in lux).
Its direction is given by the diameter connecting the two points between which its magnitude is measured.
This direction is defined in terms of two angles: one horizontal (from a reference direction) and one vertical (from a horizontal up).
Explain the term illumination in detail
The vector/scalar ratio is a measure of the directionality of light and a good indicator of its modeling qualities.
When AEmax / Es = 4, we have a completely mono-directional light. In practice, this value is always less than 4. A value of 0 would indicate perfectly diffuse omni- directional lighting.
Discuss in detail about illumination Quantity?
The eye responds to a range of illumination levels extending over a million orders of magnitude:
from 0.1 lux (full moonlit night) to 100000 lux (bright sunshine) Practical situations and various activities (thus various visual tasks), and detailed illumination requirements are given in the publication. The following values (in lux) can provide some general guidance.
ordinary tasks, medium detail (e.g. wood machining, general office work)
severe, prolonged tasks (e.g. fine assembly, Silk weaving)
exceptionally severe tasks (e.g. Watchmaking)
As it can be seen from the graphs in Figure 5.1, visual efficiency increases with the increase of illumination but the curve flattens out at higher levels.
The law of diminishing returns applies. The decision regarding the level to be adopted depends, to a large extent on socio-cultural and economic factors in other words on how light we can afford.
A comparison of recommendations in various countries is rather revealing in the table below:
How the luminance ratio is decided for different visual fields?
With a stationary head and eyes the visual field of an average person extends to 180° horizontally and 120° vertically. Within this, the ‘central field’ is limited to 2⁰ and the immediate ‘background’ extends to about 40⁰.
Visual comfort and efficiency can be ensured by the control of luminance distribution within the visual field.
The luminance ratios should be:
Central field: background: environment
5: 2 :1
10: 3 :1 should in no case be exceeded, as this may create glare.
The eye will adjust itself to the average luminance of the visual field (adaptation). With large contrasts, this may lead to loss of seeing the less luminous areas (under-exposure) and discomfort caused by the bright areas (overexposure).
Glare may also be caused by a saturation effect, even without any contrast, when the average luminance exceeds about 25 000 cd/m² (80 000 asb).
The magnitude of glare can be indicated by the terms ‘discomfort glare’ (in a less severe case) and ‘disability glare’ (in a severe situation).
What are the various types of electric lamps used for artificial lighting? Explain.
Two types of electric lamps are generally used in electric lighting:
1. Incandescent lamps, in which a current is passed through a tungsten filament, which will thus be heated and its light emission will be due to thermo-luminescence
2. Fluorescent lamps, in which an electric discharge takes place between two electrodes through low-pressure mercury vapor (mixed with some auxiliary gases) and the excited gas molecules emit ultraviolet radiation.
This is absorbed by the fluorescent coating on the inside of the glass tube and re-emitted at visible wavelengths.
Incandescent lamps have a luminous efficacy of 10 to 16 Im/w, while fluorescent lamps give 40 to 70 Im/w. thus to achieve the same output, a much lesser lamp wattage will be necessary with fluorescent than with incandescent lamps.
For example, a 200 w incandescent lamp may give about 2500 lm, but a 40 w fluorescent tube will give almost the same output (the ballast coil necessary to the latter would give a load of about 8 w, thus the total circuit wattage would be 48 w). Or, to put it another way: the total emission of energy from the two lamps is distributed as follows:
Incandescent: 5% light 95% heat
Fluorescent: 79% heat 21% light
From a thermal point of view, the total lamp wattage is taken into account as a heat gain.
The bulk of the energy emitted is heat, but even the emitted light, when incident on surfaces in the room, will be converted into heat.
With fluorescent lamps, the circuit wattage must be taken into account, not just the tubes, as the ballast also produces heat.
If it is decided to use PSALI (permanent supplementary artificial lighting of the interiors) in a hot-dry climate, the heat produced by electric lighting will increase the indoor temperature.
It is therefore advisable to minimize such heat production by using fluorescent tubes. In a critical situation, it may be worthwhile to separate the ballasts from the lamps and put them into an isolated and independently ventilated space.
This would save the 8 w heat gain with each 40 w fluorescent tube, giving a reduction of some 17% in the heat gain due to lighting.