FAQ

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For some time now Datadisc has been able to measure the radiation (irradiance) from the sun at any spot in the UV, Visible and IR bands. New sensors from ScienceScope can now do this even more accurately.

Up till now it has not been possible to make meaningful comparisons between the measurements because visible light is measured in lux (which takes account of the human eye's response in making the measurement) and the UV and IR are measured in the more simple-to-understand watts per square metre.

The derived sensor feature of Datadisc now adds the possibility to record visible radiation from a light sensor measured in Watts per square metre allowing direct comparison. (The conversion to the new unit does depend on the assumption that the light is pretty much "white".)

Another complication is that the IR sensor in use up till now actually measures total radiation including visible. the new ScienceScope sensor filters out the visible and reads IR only.

But what sort of ratios do we expect anyway ?

These data were taken from a 1980s edition of the CRC handbook and are for "standardised" data relating to the sun's radiation at the "top" of the atmosphere. There will be some reductions at ground level that depend on wavelength and atmospheric conditions. (Principally the removal of the shortest UV by ozone and IR absorptions by water and carbon dioxide.)

The areas which have been plotted using the Data: Calculate: Function option show us the relative proportions quite easily. (Datadisc doesn't know yet that integrating W m-2 nm-1 against nm should have the units W m-2, but we will make sure future editions know this.)

It's quite a surprise isn't it ? Most of us I am sure from O-level / GCSE Physics would expect IR to be far the greatest but actually it's a near tie between visible and IR.

This example illustrates some of the power of Datadisc you may not be aware of:

  • Logarithmic scaling (the graph would be almost useless with a linear x scale)

  • Ability to measure and plot areas under graphs (if you are not wanting precision as above then an area cursor is available for quick estimation of areas)

  • The ability to input "non-sensor" data

    (In the above analysis we have taken the following to be the definitions of the various bands:)

    Ultra Violet

     120 to 390 nm

    Visible radiation

     390 to 760 nm

    Infra-Red

     760 to 30000 nm

    Far Infra-Red

     30000 nm to 1000000 nm (ie 1 mm where radio is considered to start)

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