Photoresistor Calibration

I've developed a program and system to calibrate light sensors of any sort in order to convert resistance in ohms into luminosity in Lux. Here's the formula I measured:

The measurements and the fits are found in the file manager (thanks Alex) under StudentGroups/tech/Calibration. A brief explanation is found below.

A Brief Explanation:

Purpose:

The purpose of doing this project was to find the constants of the fitted curve produced by the graph of photoresistor resistance against the luminosity in the area.

Materials I used:

• Data Collector
  • 3 Photoresistors (2 were plastic, one was without plastic)

• A lux meter
  • I used a Weston Master V with an exposure-to-lux calculator.

Procedure:

Set the photoresistors up as described above, and normalize the light coming into the photoresistors. I used a paper tube to normalize the light. Use the program (found in the filemanager with the tables). I made to collect the mapped resistance values. Make sure you know which resistor is reading out where. The program will take 100 samples of the light and read it out as a number from 0-1023. Copy and paste the data into a spreadsheet so you can convert it later. Run this program in at least 4 significantly distinct light regions in order to create an accurate curve.

After collecting the data, I averaged the analog values and converted those into resistance. I used a formula to convert analog values to resistance.

Essentially:

I had a 10kOhm reference resistor, so the Constant Resistor Value (or reference resistor) was 10000. After I converted each average analog value to an average resistance, I plotted those points against the lux meter approximate values.

For darker areas, use a higher reference resistor to make the photoresistor more sensitive to more subtle changes in light, and for brighter areas, use a lower reference resistor to make the photoresistor able to output a higher luminosity.

I got the slope and intercept of the 3 regression curves using a log-log best fit line in base 10. I ended up using the average slope and intercept between the two plastic covered resistors as the calibration factors.

The formula for luminosity after the best-fit line was found is:

Which is in the form of:

Additional Notes on Photoresistors

• Information on different types of photoresistors: subclub.org/field/expose1e.htm