I just double-checked, and you're right; I've got the values backwards (I misread my notebook when I put up my previous post the other night). However, the values are correct. I measured the resistance with a multimeter, with my thumb over the photo-resistor for the dark condition (~28K ohms) and shining a reasonably bright LED flashlight on it (from about 1/4 inch away) for the light condition (~140 ohms).
So, okay, I'm not really here for an argument (this is "being hit over the head" lessons, right? : - ), but I'm learning stuff here, and someone playing with the photoresistor may find this useful someday...
The schematic at the bottom of this page http://www.ladyada.net/make/icetube/design.html
shows that the circuit is a straight voltage divider with the light sensor on the ground side. We're doing the same math, but I think of it as
V-out = V-in * [Photoresistor / (R1 + Photoresistor)]
For R = 1K, that's about .6v (light) to 4.8v (dark)
For R = 20K, that's about .04v (light) to 2.9v (dark)
I set up a simple test rig that outputs the result of the analog-to-digital comparator (ADC) and the voltage (ADCsample * (5.0 / 1024.0)). The first picture below shows the rig with a 22K ohm resistor (ignore the LEDs and DIP switches).
The next picture shows the output for the dark condition (with my finger over the sensor), at 2.35v.
The output for the bright light condition was 0.02v.
It's nice when the math sorta works out, but that would be about 19.5K ohms and 88 ohms off the photoresistor if my math is right:
Photoresistor = R1 * (Vout / (Vin - Vout))
Either that, or that's a 34K ohm resistor, but I measured it with a multimeter and got close to 22K ohms. I'm not sure what's going on there.
I did the same thing with a 1K resistor and got 4.75v and .63v, which is right in line with the calculation above. I'll spare you the pictures, but I have them : - ).
Finally, here's a shot of the light condition with the multimeter attached to the photoresistor showing 164.5 ohms.
I got about 14.3K ohms with this particular set up for the dark condition.
When I was holding the contacts to the photoresistor with my fingers (instead of the clips), I was getting the values above. I'm pretty sure this is because the clips are on dodgy cables.
So, despite a broader range of voltages (empirically as well as theoretically) with a 1K ohm resistor, you were seeing better sensitivity with a 20K ohm resistor.
My hypothesis here is that I'm testing way too far up on the bright end of the scale (probably up to 10,000 lux), and you were seeing better sensitivity in the dimmer conditions you were interested in (0.2 to 80 lux?), which would make some sense, as the resistance isn't linear.