Licht-im-Terrarium: Literaturdatenbank

Proportionality of solarmeter reading and vitamin d conversion

I would expect a relation like

http://texify.com/img/%5C!%5Cfrac%7B%5Cmathrm%7BSolarmeter%5C%2CReading%7D%7D%7B%5Cmathrm%7BVitamin%5C%2CD%5C%2CPercentage%7D%7D%2520%3D%2520%5Cfrac%7B%5Cint%5Cmathrm%7Bd%7D%5Clambda%2520%5C%2C%2520S%28%5Clambda%29%5Ccdot%2520B%28%5Clambda%29%2520%7D%7B%5Cint%5Cmathrm%7Bd%7D%5Clambda%2520%5C%2C%2520S%28%5Clambda%29%5Ccdot%2520D3%28%5Clambda%29%7D.gif

so that an equation "vitamin d conversion = A * solarmeter reading" should hold.

If this correct

• Why is a line "vitamin d conversion = A * solarmeter reading + Offset" fitted to the data? What is the meaning of the offset?
• "A" is a function of the spectrum of the lamp, so why are the measurements at different distances not included into the same regression analysis? I would not expect the spectrum to change with distance.
• The mercury vapour lamps seem to have a very different output, perhaps not only with respect to intensity but also with respect to spectrum. Why are all lamps included in the same regresssion analysis?

Distance of readings

Manufacturers recommendations for the distance of the lamp to the animal are 60cm for the Powersun 160W. The lamp was tested at 150cm distance, readings of the new lamp were 3.0±3.42 - this is at the detection limit of the Solarmeter 6.2, low signal to noise ratio and hardly any vitamin d conversion can be expected. Why was the distance chosen that high?