LIGHT

The Requirements of good Light

Good light is essential for efficient vision.

Poor lightening lead to straining and eye fatigue.

The following light factors are essential:1. SUFFICIENCY

2. DISTRIBUTION

3. ABSENCE OF GLARE

4. ABSENCE OF SHARP SHADOWS

5. STEADINESS

6. COLOR OF LIGHT

7. SURROUNDINGS.

1. SUFFICIENCY

Sufficient light is essential to recognize the surroundings details without eyes straining.An illumination of 15-20 foot candles is accepted as a basic minimum for satisfactory vision.

2. DISTRIBUTION

For efficient vision, lighting should be a uniform and of the same distribution all over the area without contrast; if not, eyes straining and fatigue occur.

3. ABSENCE OF GLARE

Glare is excessive contrast.Glare may be from the direct light source or reflected from another object such a table tops and polished furniture.Glare causes annoyance.The eye can’t tolerate glare because it causes acute discomfort and reduces critical vision.

4.ABSENCE OF SHARP SHADOWS

Slight shadows are inevitable; but sharp and contrasting shadows are disturbing.Shadows causes confusion to the eyes and shouldn’t be present in the vision field.

5. STEADINESS

The source of the light should be constant; and it shouldn’t flicker, because flickering causes eye strain and may lead to accidents.

6. COLOR OF THE LIGHT

The colour of the light is not very important so long as the intensity is adequate.Since the natural light has a comforting effect on the eye, the artificial light should be as far as possible approximate the daylight colour.

7. SURROUNDINGS

For efficient vision the colour schemes in rooms is very important.

ROOM ITEM REFLECTION FACTOR

Roofs 80 %Walls 50 - 60 %Furniture 30 -40 %Floor ≤ (10 – 20 %)

MEASUREMENT OF LIGHT

Light contains electromagnetic radiation waves.Light containing all visible waves is perceived as white.There are 4 major measures for light:

1. Luminous intensity االضاءة شدة2. Luminous flux االضاءة تدفق3. Illumination illuminance مضوائية

االضاءة4. Brightness luminance االنارة سطوع

NATURAL LIGHT

Natural daylight comes partly from the sky and partly from reflection.Efficient utilization of daylight calls for the proper designs of building and towns (civil engineering planning).Suggestions for improving daylight illumination:

1. ORIENTATION2. REMOVAL OF OBSTRUCTION3. WINDOWS4. INTERIOR OF THE ROOM

1. ORIENTATION:

Building facing east and west are not lighted equally with natural daylight in the tropics because they are parallel with rising and setting of the sun. So, the buildings should be constructed facing north and east; esp. the schools, in order to equalize light orientation temperature distribution.

2. REMOVAL OF OBSTRUCTIONS:

Any obstructive object that can decrease daylight illumination should be removed totally or partially e.g., excessive trees, hills, etc.

3. WINDOWS:

Windows should be well located.

Tall windows allow greater amount of light entrance.

Broad windows allow greater light diffusion.

As a rule, the space of windows shouldn’t be less than 10% of the space of the floor.

4. INTERIOR OF THE ROOM:

In order to obtain the full benefit of the natural illumination, the roofs should be white in colour, the upper portions of the walls should be light-tinted, the lower portions should be a little bit darker so as to give comfortable contrast to the eyes.

MEASUREMENT OF DAYLIGHT

Since the intensity of daylight is changing, it couldn’t be measured by the unit of foot candle. So, we measure it using a ratio called daylight factor (D.F).

• certain given point illumination D.F = X 100 simultaneous whole hemisphere exposed point illumination (500 candles)

ARTIFICIAL LIGHTING

Artificial lighting should be as close as possible to daylight in composition.

There are 5 systems of artificial lighting:

1. DIRECT LIGHTING: 99-100 % of light is directed towards the working area.

2. SEMI-DIRECT: 10 – 40 % of the light is projected upwards.

3. INDIRECT: 99 – 100 % of the light is directed towards the roof.

4. SEMI-INDIRECT: Here 60 – 90 % of the light is directed upwards and the rest downwards.

5. DIRECT –INDIRECT: Here the light is directed equally upwards and downwards.

Methods of artificial illumination:

1) FILLAMENT LAMBS:

Generate heat. the more heat, the more light. Utilize much electric current.

2) FLOURESCENT LAMBS:

Utilize low electric current, their light simulate day light, they are cool & efficient

LIGHTING STANDARDS

There are no fixed standards.

The general rule is that the illumination level should be 30 times higher than the level at which the task can just be done.

BIOLOGICAL EFFECTS OF LIGHT

The daylight can cause the in vitro degradation of Bilirubin; so it is used as a therapeutic measure in premature infants with hyperbilirubinemia.

The light effects the biological rhythm of body temperature & daily physical activity.

Light stimulates Melanin synthesis, activation of precursors of vitamin D synthesis, adrenocortical secretion and food consumption.