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    Lux, Lumen Etc PDF

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    Syed Abubakar
    The document defines and describes several units used to measure characteristics of light: - The steradian is the unit of solid angle measurement, with one steradian representing the solid angle that subtends an area equal to the radius squared on a sphere. - The candela measures luminous intensity per steradian and depends only on the angle of emitted light, not the distance from the source. - The lumen is the unit of total luminous flux and is calculated by multiplying the intensity in candelas by the solid angle in steradians over which light is emitted. - Other units described include the lux (illumination of a surface), luminance (light emitted from a surface), and irradiance/ill

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    Lux, Lumen Etc PDF

    Uploaded by

    Syed Abubakar
    262 views8 pages
    The document defines and describes several units used to measure characteristics of light: - The steradian is the unit of solid angle measurement, with one steradian representing the solid angle that subtends an area equal to the radius squared on a sphere. - The candela measures luminous intensity per steradian and depends only on the angle of emitted light, not the distance from the source. - The lumen is the unit of total luminous flux and is calculated by multiplying the intensity in candelas by the solid angle in steradians over which light is emitted. - Other units described include the lux (illumination of a surface), luminance (light emitted from a surface), and irradiance/ill

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    The document defines and describes several units used to measure characteristics of light: - The steradian is the unit of solid angle measurement, with one steradian representing the solid angle that subtends an area equal to the radius squared on a sphere. - The candela measures luminous intensity per steradian and depends only on the angle of emitted light, not the distance from the source. - The lumen is the unit of total luminous flux and is calculated by multiplying the intensity in candelas by the solid angle in steradians over which light is emitted. - Other units described include the lux (illumination of a surface), luminance (light emitted from a surface), and irradiance/ill

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
    262 views8 pages

    Lux, Lumen Etc PDF

    Uploaded by

    Syed Abubakar
    The document defines and describes several units used to measure characteristics of light: - The steradian is the unit of solid angle measurement, with one steradian representing the solid angle that subtends an area equal to the radius squared on a sphere. - The candela measures luminous intensity per steradian and depends only on the angle of emitted light, not the distance from the source. - The lumen is the unit of total luminous flux and is calculated by multiplying the intensity in candelas by the solid angle in steradians over which light is emitted. - Other units described include the lux (illumination of a surface), luminance (light emitted from a surface), and irradiance/ill

    Copyright:

    © All Rights Reserved
    Download as PDF, TXT or read online from Scribd
    Download as pdf or txt
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    The key takeaways are the definitions and relationships between various units used to measure light and illumination such as steradian, candela, lumen, lux, luminance, irradiance and illuminance.

    Luminous intensity is the density of luminous flux leaving a source in a particular direction per steradian, measured in candela. Luminous flux is the total quantity of visible light emitted by a source per unit of time, measured in lumens.

    Irradiance is the radiant flux or power received per unit area and is measured in watts per square meter. Illuminance is the luminous flux or power received by the retina of the eye per unit area and is measured in lux.

    Characteristics of Light

    Steradian:
    • The steradian (symbolized sr) is the Standard International (SI) unit of
    solid angular measure. There are 4 pi, or approximately 12.5664,
    steradians in a complete sphere.
    • A steradian is defined as conical in shape, as shown in the illustration.
    Point P represents the center of the sphere. The solid (conical)
    angle q, representing one steradian, is such that the area A of the
    subtended portion of the sphere is equal to r2, where r is the radius of
    the sphere.
    Candela
    • The candela measures the luminous intensity per steradian of light
    originating from a specific direction or a real source as if it were emerging
    form a point source at that origin.
    • The number of “Candelas” from a real source varies with direction not with
    distance.
    • Since the luminous intensity is described in terms of an angle, the distance
    at which you measure this intensity is irrelevant.
    • In this picture, screen B would catch exactly the same amount of light rays
    (emitted from the light source) as screen
    A —provided that screen A were removed to not
    obscure screen B. This is because screen B covers
    the same angle as screen A.( i.e Candela is only
    concern with angle not with the distance from the source)
    LUMEN
    • The basic unit of light energy is the lumen, which measures the total flow of
    photons or light energy produced by a light source.

    • The lumen (unit lm) gives the total luminous flux of a light source by multiplying
    the intensity (in candela) by the angular span over which the light is emitted.
    With the symbol Φv for lumen, Iv for candela and Ω for the angular span in
    steradian, the relation is:
    Φv=Iv⋅Ω
    • If a light source is isotropic (meaning: uniform in all directions), Φv=4π Iv. This is
    because a sphere measures 4π steradians.

    • Power is the flow of energy. One joule per second is 1 watt of power, so a 1
    lumen = 1/683 Joule/sec = 1/683 watt.
    • A good example demonstrating the difference between lumens and
    candlepower is the laser. Lasers have a very high candlepower
    because the light is focused into a single point, but it would have a
    very low lumen value because it does not give off a substantial
    amount light. You certainly would not want to go on a walk in the
    woods at night with a laser pointer.
    Lux
    • Lux (unit lx) is a measure of illumination of a surface.
    • The difference between lux and candela is that lux measures the
    illumination of a surface, instead of that of an angle.
    • The net result is that the distance of that surface from the light source
    becomes an important factor (in case of lux): the more distant that the
    surface is from the light source, the less it will be illuminated by it. In the
    picture at the right, screen A has the same size as screen B.
    Luminance
    • Luminance is a measure for the amount of light emitted from a surface (in a
    particular direction).
    • Luminance is the amount of light energy emitted or reflected from an object
    in a specific direction. Luminance is measured in candela per square metre
    (cd/m2) or nit (these two terms being interchangeable)
    • Luminance and illumination ("Lux") are related,
    in the sense that luminance is typically used for
    light-emitting surfaces and illumination for
    surfaces that are being lit.
    Irradiance and Illuminance:
    • Irradiance is a measure of radiometric flux per unit area, or flux
    density.
    Irradiance is typically expressed in W/cm2 (watts per square
    centimeter) or W/m2 (watts per square meter).
    • Illuminance is a measure of photometric flux per unit area, or visible
    flux density.
    Illuminance is typically expressed in lux (lumens per square meter) or
    foot-candles (lumens per square foot).

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