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LDR resistance is not generally very precisely specified or linear. LDRs do not output anything, you need to pass a current through them and measuire the voltage they drop to calculate their resistance. From the resistance you should be able to extract the irradiance using figures on the data sheet but the only practical way to eliminate non-linearity is to use a look-up table to convert resistance to light intensity.
The problem with attempting to generate your own lookup table is that you require an uniform light source which can be adjusted over a range of 40,000:1 without affecting its color temperature, and a calibrated luxmeter whose optical response mimics that of the human eye.
This is not a trivial laboratory setup.
If you require a reasonably accurate lux meter (Please Note, we are talking luxes, no lumens), your best be is to obtain one of the many available sensor ICs that provide a calibrated output according to the lux level.
Most of them still require that you perform some calculations or have a look up able to convert their output to luxes. But the formulas and methods are clearly spelled on their datasheets.
I've used many sensors from TAOS (now AMS) and they are straightforward to use. Other IC vendors also produce them.
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