You may be able to use the LED as the sensor as well. I don't have references handy, but they are photodiodes themselves, and may even be matched to the spectrum they emit.
From: etienne garbaux <email@example.com>
Sent: Thu, December 3, 2009 3:50:02 PM
Subject: Re: UV-densitometer
> Years ago I built my own visible light transmission densitometer.Now I need to build an UV-densitometer. * * * Can anyone tell me what UV-sensor and what UV- light source is used in your UV-densitometer ?
These days, a UV LED is the preferred source. Most emit between 350 and 395 nm, which matches most UV-sensitive photo systems well.
Here is a review of several products:
They are available down to 200 nm or so:
You can buy individual UV LEDs from most electronics suppliers (Digi-Key, Newark, Allied, Mouser in the US), or UV flashlights containing UV LEDs from lots of sources (check Amazon or do a Google search, but make sure you get an LED flashlight -- some UV flashlights use filtered xenon bulbs). The flashlights are sold for detecting blood and urine stains, verifying bank notes, viewing flourescent materials, etc.
Silicon photodiodes ("SPDs"), often fitted with blue filters to block infrared sensitivity and then known as "silicon blue photodiodes," have useful sensitivity in the 400 nm region. You will need to filter an SPD to block visible wavelengths if there is any ambient light.