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Laws of illuminations

Illumination Laws

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Laws of illuminations

  1. 1. There are two laws of IlluminationsA. Law of Inverse SquaresB. Lambert’s Cosine Law Vijay Balu Raskar (B.E.Electrical)
  2. 2. Light – Radiant energy that providesvisual sensation• Human eye can sense – 380nm(violet) to 700nm (red)• Maximal relative energy contentof sunlight• Maximal luminosity of humaneye. Vijay Balu Raskar (B.E.Electrical)
  3. 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. 4. TRANSPARENT•Common unit of light intensity is candela.•It is Luminous intensity in the Perpendicular direction of asurface, 1 / 600,000 of a black body at temperature ofsolidification or Freezing of Platinum under StandardAtmospheric pressure.•It is abbreviated as Cd.•It is indicative of Light Radiating Capacity of a source ofLamp.•Now we define Luminous intensity on a surface. It isknown 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. 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 1steradian solid angle will be 1 lumen. Vijay Balu Raskar (B.E.Electrical)
  6. 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 amountperceptible.•Gives same change in sensation.•Sensation produced by optic nerves have logarithmic dependence or relationship toLight Radiation producing the sensation. Vijay Balu Raskar (B.E.Electrical)
  7. 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 wastaken as standard by comparison with aPentane Lamp.•Thing to be kept in mind is Primary Standardshould be reproducible. Vijay Balu Raskar (B.E.Electrical)
  8. 8. Inverse Square Law•Intensity of Illumination produced by a point source variesinversely 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. 9. Vijay Balu Raskar (B.E.Electrical)
  10. 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 naturallydecreses with the square of the distance as the size of theradiative spherical wavefront increses with distance.Illumination of a surface is inversely proportional to thesquare of the distance between the surface and the lightsource provided that the distance between the surface andthe source is sufficiently large so that the source can beregarded as a point source. Vijay Balu Raskar (B.E.Electrical)
  11. 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 eachdirection.For Center of hollow large radius,Light spreads over a large area proportional to the squareof the radius.Conclusion:- As radius increases, it will be inverselyproportional to the square of the distance.For Parallel Surface, (Cone or Pyramids)Light spreads over a surface inversely proportional to thesquare of the source. Vijay Balu Raskar (B.E.Electrical)
  12. 12. Lambert’s Cosine Law Vijay Balu Raskar (B.E.Electrical)
  13. 13. Sometime surface is not normal to the direction ofLight.But, inclined by some angle.The area over which the light is spread is thenincreased in the ratio.The illumination decreases in the ratio cos 1According to this law, Illumination at any point on asurface is proportional to the cosine of the anglebetween the normal at that point and the direction ofluminous flux. Vijay Balu Raskar (B.E.Electrical)
  14. 14. Lambert’s Cosine Law of Incidence I . cosE 2 I Vijay Balu Raskar (B.E.Electrical) I . cos D
  15. 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. showsthe variation of Illuminance on the floor. It is well known that Illuminance ismaximum under the lamp at ‘B’. Vijay Balu Raskar (B.E.Electrical)
  16. 16. Vijay Balu Raskar (B.E.Electrical)
  17. 17. Vijay Balu Raskar (B.E.Electrical)
  18. 18. •To measure the candle power of the lamp. Typical measurement can be done using aphotometric 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 oneither side.•Applying inverse law one can arrive at the value of IX.This introduced the primary standard and other terminology related to measurement of lightflux. Vijay Balu Raskar (B.E.Electrical)