LIGHT is an important topic of Community Medicine.

2. • Light is electromagnetic radiation that can be
detected by the human eye. Electromagnetic
radiation occurs over an extremely wide range
of wavelengths, from gamma rays with
wavelengths less than about 1 × 10−11 metres
to radio waves measured in metres.

6. For efficient vision, the following light factors are essential :
(1) SUFFICIENCY -An illumination of 15 to20 foot candles (1 foot candle
= 10. 76 Lux) is accepted as a basic minimum for satisfactory vision.
(2) DISTRIBUTION-The distribution of light should be
uniform, having the same intensity, over the whole field of work.
(3) ABSENCE OF GLARE-Glare may be a direct glare from a light
source or reflected glare from sources such as table tops and
polished furniture. Glare causes annoyance.
(4) ABSENCE OF SHARP SHADOWS-Like glare, shadows cause
confusion to the eye and therefore should not be
present in the field of vision.

7. • (5) STEADINESS-The source of light should be constant. It should
not flicker because flickering causes eye strain and may lead to
accidents.
• (6) COLOUR OF LIGHT-The colour of light is not very important so
long as the intensity is adequate. Since natural light has a
soothing effect on the eye, the artificial light should as far as
possible approximate the daylight colour.
• (7) SURROUNDINGS-For efficient vision, colour schemes in rooms
are important. Ceilings and roofs should have a reflection factor of
80 per cent; walls 50 to 60 per cent; furniture 30 to 40 per cent.
There should not be much reflection from the floor, not more
than 15 · to 20 per cent. Contrasting colours are often used to
prevent accidents, e.g., culverts, bridges, etc.

9. • Natural lightinig is derived partly from the visible sky and partly
from reflection.
• In fact, much light comes to the rooms by reflection from light
coloured objects.
• Efficient utilization of natural light calls for careful design, location
and orientation of buildings and relationship between buildings
(town planning).
Natural lighting also depends upon the time of the day,
season,weather and atmospheric pollution.
• Since natural light is accompanied by radiant heat, all attempts
should be made to exclude radiant heat while admitting daylight.

10. • The following general principles are taken into consideration in planning
for the best utilization of daylight-
• (1) ORIENTATION-Buildings should be oriented, wherever
possible,towards north or south for uniform illumination. This is
particularly important in respect of schools, factories and
laboratories where uniform lighting is required in all the rooms. When a
building faces east and west, window shades are provided to protect
against the direct penetration of sunlight.
(2) REMOVAL OF OBSTRUCTIONS.
(3) WINDOWS-In modern practice, window area is correlated to the
purpose the room is intended to serve. A tall window gives greater
penetration of light; a broad window gives greater diffusion of light.

11. • (4) INTERIOR OF THE ROOMS -In order to obtain the full
benefit of the natural illumination, the ceiling should be white;
the upper portions of the walls light-tinted; and lower portions
somewhat darker so as to give comfortable contrast to the
eyes.

14. • Artificial lighting should be as close as possible to daylight in
composition.
• There are five systems of artificial lighting :
• -direct,
• -semi-direct,
• -indirect,
• -semi-indirect and
• -direct-indirect

15. • (1) DIRECT LIGHTING -In direct lighting, 99 to 100 per cent of the
light is projected directly towards the working area. Direct lighting
is efficient,economical, but tends to cast sharp shadows. It should
not fall into the eyes.
• (2) SEMI-DIRECT-Here 10 to 40 per cent of the light is projected
upwards so that it is reflected back on the object by the ceiling.
• (3) INDIRECT-light does not strike a surface directly, because 90 to
100 per cent of the light is projected towards the ceiling and walls.
• (4) SEMI-INDIRECT : Here, 60 to 90 per cent of the light is directed
upwards, and the rest downwards.
• (5) DIRECT-INDIRECT : Here, light is distributed equally.

16. • (1) FILAMENT LAMPS-These are widely used. The electric current
heats up the tungsten filament and the light emitted depends
upon the temperature. The. hotter filaments produce the bluer
light.
• 5% light// 95% Heat.
• (2) FLUORESCENT LAMPS-The lamps consist of a glass tube filled
with mercury vapour and an electrode fitted at each end. The
inside of the tube is coated with fluorescent chemicals, which
absorb practically all the ultraviolet radiation and remit the
radiation in the visible range.
• 21%light//79% Heat.

17. • The eye responds to a range of illumination ranging from
• 0.1 lux (full moonlight night) to 100,000 lux (bright sunshine).
• A useful rule of thumb is that the illumination level should be
30 times higher than the level at which the task can just be
done.

18. Visual Task Illumination (lux)
Casual Reading 100
General Office Work 400
Fine Assembly 900
Very severe tasks 1300-2000
Watch Making 2000-3000

19. 1)Hyperbilirubinaemia of premature infants - as a
therapeutic measure.
2) Biologic rhythms of body temperature
3) Physical activity
4) The stimulation of melanin synthesis
5)The activation of precursors of vitamin D
6) Adrenocortical secretion and
7)food consumption

20. • 1) Poor light causes eye strain,headache,lacrimation,interferes
with efficiency,eye-fatigue.
• 2)Glare can also cause eye strain,accidents,reduce critical
vision.
• 3)Infrared Rays can cause burn and eye cataract ,chronic
inflammation of the lids (common in glass workers )
• 4) Ultraviolet Rays can cause infalammation of the cornea &
conjunctiva,pain & running eye (common in welders ).