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Homework Help: Highschool electronics- Illumination

  1. May 28, 2007 #1
    1. The problem statement, all variables and given/known data
    I have done an experiment at school, where the voltage drop across an LDR was measured at varying light intensities. The light intensity was determined by the distance of a light globe from the LDR (the light globe was part of an external circuit). Also the current flowing through the LDR circuit was measured at each distance.

    Bacically I have voltage drop across LDR, current in circuit, distance of globe from ldr and resistance of LDR using ohm's law.
    oh yeh and i have the wattage of the globe and its supply voltage
    Is it possible to calulate the light intensity in lux by using an equation or did this have to be measured using a lux meter?

    I realise the distance of the light globe from the LDR is effectively the light intensity, but i need it to be in lux.....

    2. Relevant equations

    3. The attempt at a solution
    Last edited: May 28, 2007
  2. jcsd
  3. May 28, 2007 #2


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    Homework Helper

    This is the age old problem of calibrating an instrument. Meaning that you have to draw a calibration curve of illumination in lux against the resistance of you device.

    This can either be done with sources of known illumination (not a recommended procedure) or using another measurement device that will give you the readings in lux (not very helpfull is it!).

    Try and borrow a lux meter from a photographer to calibrate your resistor.
    Last edited: May 29, 2007
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