photoresistors

Better re-read what you wrote Isaac!

To me, reduce mean the same as decrease.

Tell us what your application is and perhaps we can suggest alternatives.

All LDRs decrease resistance as illumination rises.

what is that an oximoron?

Your question, as written, doesn't make much sense to me.

I'm assuming that what you meant to ask is if there are any materials where the resistance increases as light increases. (As someone else pointed out, "reduce resistance" and "decrease resistance" mean the same thing.)

Most materials, if their resistivity changes at all under illumination, experience a decrease in resistivity. This is commonly known as photoconductivity. (Conductivity = 1/resistivity, so conductivity goes up as light goes up.) (FWIW, to exhibit photoconductive behavior, a material has to be a semiconductor, not a metal or an insulator. And there are some pretty odd semiconductors...)

Google on "negative photoconductivity" for information on materials where the conductivity goes down (resistance goes up) as the incident light power goes up. (If you find anything by a Richard Bube or an Albert Rose, that would be worth paying attention to. I don't know if they've done any work in this area, but they're two of the leading names in photoconductivity.)

I doubt you'll find anything of use to you, because (AFAIK) the materials which exhibit negative photoconductivity generally require special preparation or special conditions.

Never-the-less, it's an interesting enough phenomenon that there's a moderate amount of research going on to better understand it.

Bob Pownall (Random trivia - If I'm not mistaken, Einstein's Nobel Prize came not for his work on special or general relativity, but for his work in explaining photoconductivity in terms of quantum mechanics.)

i guess i shouldn't drink and post!... i was interested in a photo resistor that functions the opposite of those that i purchased from radio shack... but, i've discovered that a common, and elementary circuit, has answered my question - the night light, such as here:

however, i'm assuming that because R1 in that circuit is a static value, then the light level will remain the same as long as the circuit has switched to its path?..

thanks, all..

Its quite common to see the LDR in series with a resistor across the supply to form a voltage divider, and usually feeding one or other input of an op-amp. To reverse the effect of changing light simply turn the voltage divider upside down - or use the other input on the op-amp.

The LED will typically be either fully on (with its current determined by the battery voltage divided by R) or fully off. The region where it is only partially illuminated will be quite small.