laws of illuminations
DESCRIPTION
Illumination LawsTRANSCRIPT
Vijay Balu Raskar (B.E.Electrical)
There are two laws of Illuminations
A. Law of Inverse SquaresB. Lambert’s Cosine Law
Vijay Balu Raskar (B.E.Electrical)
•
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)
• 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)
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)
•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)
•Thus we have sensitivity given by the equation ,
•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.
0
log.I
ICS
Vijay Balu Raskar (B.E.Electrical)
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)
Inverse Square Law •Intensity of Illumination produced by a point source varies inversely as square of the distance from the source.
•Where, I is intensity and•d is Distance
2d
IE
Vijay Balu Raskar (B.E.Electrical)
Vijay Balu Raskar (B.E.Electrical)
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.
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.
2
1
dE
Vijay Balu Raskar (B.E.Electrical)
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)
Lambert’s Cosine Law
Vijay Balu Raskar (B.E.Electrical)
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
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.
1
cos
Vijay Balu Raskar (B.E.Electrical)
Lambert’s Cosine Law of Incidence
2
cos.
D
IE
cos.II
Vijay Balu Raskar (B.E.Electrical)
•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)
Vijay Balu Raskar (B.E.Electrical)
Vijay Balu Raskar (B.E.Electrical)
•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.