Illumination Laws

1. There are two laws of Illuminations
A. Law of Inverse Squares
B. Lambert’s Cosine Law
Vijay Balu Raskar (B.E.Electrical)

2. Light – Radiant energy that provides
visual sensation
• Human eye can sense – 380nm
(violet) to 700nm (red)
• Maximal relative energy content
of sunlight
• Maximal luminosity of human
eye.
Vijay Balu Raskar (B.E.Electrical)

3. • Good efficient lighting obtained by
combining luminescence &
fluorescence.
• Light – Radiant energy that provides
visual sensation
• • Human eye can sense – 380nm
(violet) to 700nm (red)
Vijay Balu Raskar (B.E.Electrical)

4. TRANSPARENT
•Common unit of light intensity is candela.
•It is Luminous intensity in the Perpendicular direction of a
surface, 1 / 600,000 of a black body at temperature of
solidification or Freezing of Platinum under Standard
Atmospheric pressure.
•It is abbreviated as Cd.
•It is indicative of Light Radiating Capacity of a source of
Lamp.
•Now we define Luminous intensity on a surface. It is
known as Illuminance.
•It is Luminous Flux per unit area or lumens per sq m. =
lumen / m2 = lm / m2 = lux (lx).
Vijay Balu Raskar (B.E.Electrical)

5. •Consider a transparent sphere of radius 1m shown in Fig.
•If we place a 1 Cd source at the centre then light flux coming out through an area of 1m2 over 1
steradian solid angle will be 1 lumen.
Vijay Balu Raskar (B.E.Electrical)

6. •Thus we have sensitivity given by the equation , I
S C . log
I0
•Here I0 is the threshold intensity.
•This is known as Frechner’s Law or Weber Law.
•The same percentage change in stimulus Calculated from the least amount
perceptible.
•Gives same change in sensation.
•Sensation produced by optic nerves have logarithmic dependence or relationship to
Light Radiation producing the sensation.
Vijay Balu Raskar (B.E.Electrical)

7. LAWS of Illumination
•The original standard of light was Wax Candle,
which is highly unreliable.
•It was replaced by a Vaporized Pentane Lamp.
•This is equal to10 original Candles.
•In the year 1909, Incandescent Lamp was
taken as standard by comparison with a
Pentane Lamp.
•Thing to be kept in mind is Primary Standard
should be reproducible.
Vijay Balu Raskar (B.E.Electrical)

8. Inverse Square Law
•Intensity of Illumination produced by a point source varies
inversely as square of the distance from the source.
I
E 2
d
•Where, I is intensity and
•d is Distance
Vijay Balu Raskar (B.E.Electrical)

9. Vijay Balu Raskar (B.E.Electrical)

10. As light radiates from a point source, the intensity of Light
(I) is inversely proportional to the square of the distance (d)
from the source.
1
E 2
d
As intensity is the power per unit area (W/m2), it naturally
decreses with the square of the distance as the size of the
radiative spherical wavefront increses with distance.
Illumination of a surface is inversely proportional to the
square of the distance between the surface and the light
source provided that the distance between the surface and
the source is sufficiently large so that the source can be
regarded as a point source.
Vijay Balu Raskar (B.E.Electrical)

11. A Source of light which emits light equally in all direction,
For Center of hollow sphere,
Light spreads uniformly means light spreads same at each
direction.
For Center of hollow large radius,
Light spreads over a large area proportional to the square
of the radius.
Conclusion:- As radius increases, it will be inversely
proportional to the square of the distance.
For Parallel Surface, (Cone or Pyramids)
Light spreads over a surface inversely proportional to the
square of the source.
Vijay Balu Raskar (B.E.Electrical)

12. Lambert’s Cosine Law
Vijay Balu Raskar (B.E.Electrical)

13. Sometime surface is not normal to the direction of
Light.
But, inclined by some angle.
The area over which the light is spread is then
increased in the ratio.
The illumination decreases in the ratio cos
1
According to this law, Illumination at any point on a
surface is proportional to the cosine of the angle
between the normal at that point and the direction of
luminous flux.
Vijay Balu Raskar (B.E.Electrical)

14. Lambert’s Cosine Law of Incidence
I . cos
E 2
I
Vijay Balu Raskar (B.E.Electrical)
I . cos
D

15. •The variation of Illuminance on arbitrary surface inclined at an angle of α.
•Fig. shows a lamp placed at A, bm above the floor. For this scheme Fig 6. shows
the variation of Illuminance on the floor. It is well known that Illuminance is
maximum under the lamp at ‘B’.
Vijay Balu Raskar (B.E.Electrical)

16. Vijay Balu Raskar (B.E.Electrical)

17. Vijay Balu Raskar (B.E.Electrical)

18. •To measure the candle power of the lamp. Typical measurement can be done using a
photometric bench shown in Fig. below.
•where IS represents standard lamp. IX represents test lamp.
•There is a screen at the centre called photometer head, adjusted for equal brightness on
either side.
•Applying inverse law one can arrive at the value of IX.
This introduced the primary standard and other terminology related to measurement of light
flux.
Vijay Balu Raskar (B.E.Electrical)