Photometry
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Questions and Answers

What is the unit of illuminance?

  • Candela (Cd)
  • Lumen per square meter (lm/m²)
  • Lumen per steradian (lm/sr)
  • Lux (lx) (correct)

    • A parallel beam of light has an intensity of illumination that varies with distance as:

  • I ∝ r
  • I ∝ 2πr
  • I ∝ 1/r (correct)
  • I ∝ 1/r²

    • What is the difference between illuminance and luminance?

  • Illuminance is the luminous flux reflected, while luminance is the luminous flux incident.
  • Illuminance is the luminous flux, while luminance is the luminous intensity.
  • Illuminance is the luminous flux incident, while luminance is the luminous flux reflected. (correct)
  • Illuminance is the luminous intensity, while luminance is the luminous flux.

    • What is the relation between luminous intensity (L) and illuminance (I) for a unidirectional point source?

    <p>I = L/r²</p> Signup and view all the answers

    What is Lambert's Cosine Law of Illuminance related to?

    <p>The inverse square law of illuminance</p> Signup and view all the answers

    What is the relationship between illuminance and the angle of incidence of light?

    <p>Illuminance is proportional to the cosine of the angle of incidence</p> Signup and view all the answers

    What is the principle of photometry?

    <p>Comparing the illuminance of two sources</p> Signup and view all the answers

    What is the relationship between illuminance and distance from a point source?

    <p>Illuminance is inversely proportional to the square of the distance</p> Signup and view all the answers

    Why does a 40 watt fluorescent tube give more light than a filament bulb of the same wattage?

    <p>Because the bulb emits ultraviolet and infrared radiation</p> Signup and view all the answers

    What is the purpose of a photometer?

    <p>To compare the illuminance of two sources</p> Signup and view all the answers

    What is the ratio of illuminance on the slide to the illuminance on the screen if there is negligible absorption of light by the aperture?

    <p>10 : 1</p> Signup and view all the answers

    If the illuminance at a point A on the center of a table is IA, what is the illuminance at a point B on the corner of the table, given that the intensity of illumination follows the inverse square law?

    <p>IA / 13</p> Signup and view all the answers

    What law relates the luminous intensity of a light source to the illuminance at a point on a surface?

    <p>Lambert's Cosine Law</p> Signup and view all the answers

    If the luminance of a light source is L, what is the illuminance at a point on a surface at a distance r, according to the inverse square law?

    <p>L / r^2</p> Signup and view all the answers

    What is the ratio of the intensities of illumination at a point on the center of the edge and on the corner of the table, given that the intensity of illumination follows Lambert's Cosine Law?

    <p>(17 / 13)^(3/2)</p> Signup and view all the answers

    What is the relationship between illuminance and distance for a line source?

    <p>I ∝ 1/r</p> Signup and view all the answers

    What is the solid angle subtended by the whole sphere at its centre?

    <p>4π radians</p> Signup and view all the answers

    A surface is oriented such that the central ray of light makes an angle θ with the normal to the surface. What is the maximum illuminance of the surface?

    <p>I = 2L cos θ/r²</p> Signup and view all the answers

    What is the relationship between luminous intensity and illuminance for a unidirectional point source?

    <p>I = L cos θ/r²</p> Signup and view all the answers

    What is the luminous flux of a source of 1/685 watt emitting monochromatic light of wavelength 5500 Å?

    <p>1 lumen</p> Signup and view all the answers

    A photometer is used to compare the illuminance of two sources. If the distances of the sources are adjusted so that their illuminance is equal, what can be said about the luminous intensities?

    <p>L₁ r₁² = L₂ r₂²</p> Signup and view all the answers

    What is the unit of luminous intensity?

    <p>Lumen per steradian</p> Signup and view all the answers

    What is the relationship between luminous intensity and illuminance at a point on a surface, according to the inverse square law?

    <p>Illuminance is inversely proportional to the square of the distance from the source</p> Signup and view all the answers

    What is the units of illuminance in photometry?

    <p>lumen per square meter</p> Signup and view all the answers

    What is the luminous efficiency of a 40 W tungsten bulb, given that its luminous flux is 465 lumen?

    <p>12 lumen/watt</p> Signup and view all the answers

    If the illuminance at a point on a surface is 4 lux, what will be the illuminance at a point 2 times farther away from the source?

    <p>1 lux</p> Signup and view all the answers

    What is the ratio of illuminance at two points on a surface, if the distance of the second point from the source is 3 times that of the first point?

    <p>1:9</p> Signup and view all the answers

    A surface has a luminance of 20 candelas per square meter. If the luminous flux incident per unit area of the surface is 10 lumens per square meter, what is the illuminance of the surface?

    <p>10 lux</p> Signup and view all the answers

    What is the relation between the luminous intensity (L) and the illuminance (I) of a unidirectional point source of light?

    <p>I = L/r^2</p> Signup and view all the answers

    If the luminous flux of 1 lumen is falling on an area of 1 square meter of a surface, what is the illuminance of that surface?

    <p>1 lux</p> Signup and view all the answers

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