Electronic – Accuracy of manufactured photocells (LDR) with respect to light intensity

accuracylightmanufacturingphotodiodephotoresistor

I was looking through photocell (photoresistors) datasheets, like for instance the NORPS-12, especially the graphic on resistance value versus light intensity.

In no datasheet could I find information on the accuracy, due to the manufacturing process (that is between different single component of the same type, say different NORPS-12 photocells.)

The only information I found from a blog was: "can vary quite a bit from photocell to photocell (perhaps as much as 50%.)" That is quite a lot.

  • Why is this information never(?) provided?
  • What is the typical variation between single components? 50% variation of the resistor values for the same light intensity, really?
  • Are there photocell types whose variability is less than others?
  • Does it imply that each photocell has to be calibrated for each device with respect to light measurement applications?

Thank you very much fo sharing your experience and knowledge about this issue.

Best Answer

Why is this information never(?) provided?

Photoresistors are usually low cost binary sensors used to determine if it is light or dark in the environment. If you want to know the difference between day and night you do not require a lot of accuracy.

Are there photocell types whose variability is less than others?

Yes, which is why for light measurements photodiodes are used. If you put them in a reverse biased configuration they are highly linear and and able to precisely measure huge differences in the number of photons per second hitting the diode with great accuracy. There will still be variation in the sensitivity of individual diodes (due to slightly different quantum efficiency and dark current), but these are typically very small differences that can be ignored or calibrated out if necessary.

When you say "small differences", what is the order of magnitude ? A few percents? Less?

Thorlabs has a plot of the minimum and maximum responsivity for 104 tested photodiodes here:

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=2822

You can see the extreme values can be fairly different. For products from the same production run (or better yet, same wafer) the variation tends to be a lot smaller. For matched photodiodes used in balanced detection, DC rejection of 30-40 dB is pretty common. Dark current also varies a lot between batches. In my random experience, I've seen most of the parts in a batch come back with 1% of each other, but when I ordered from a second batch I got almost a factor of two difference (although this was a difference between very small numbers).

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