Thanks for the link. I found nothing specific in the free section. One of
the pay articles did mention some numbers, but the range was rather wide.
It said that digital exposes the patient to 20 to 70% less radiation than
F speed film, but quality of image, while acceptable, is lower.

http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2591.2005.00998.x

There is different sensitivity film available. The new faster "F speed"
films are pretty close to digital as far as required radiation.
Having said that, I have digital in my office and I used to use "E
speed" film my exposure was .23 now it is .11-.13.
You still need to have enough MiliAmps and exposure time to generate
x-rays that will pass through bone, and I think with digital or film we
are in a very safe limit. You get higher exposure walking in the sun.
The advantages of digital is that
1)it becomes very easy to send x-rays via email or duplicate them
2) you can blow them up on the screen or print to a full size sheet of
paper make notes on them to communicate with others
3) less developing time, if the x-ray sensor way not positioned
correctly the xray can be retaken quite quickly.
4) no need for Environmental Protection Agency permits for having fixer
developer picked up.
5) The x-ray is always consistant, in other words you do not have the
worry as with film that the chemicals are still fresh, the quality of
the x-ray is always optimum.
6) you can change the contrast and measure the density with certain
software. You are getting 3D data from the sensor and can view the 2D
image at different settings giving you a better picture of the area.
How significant this is relative to what you can see with a good film
is questionable.
A big disadvantage to digital is sensor comfort. Patients hate the
sensor. They are impressed with the digital but after having the sensor
in their mouths, prefer the film.
I think as prices for sensors start coming down we will see more
digital. Hopefully the sensors will get more comfortable or we will be
able to do inexpensive MRI. Which really is the reason we still have
x-rays, MRI machines are very expensive for dental offices and not very
convenient for most procedures. Even though we will see a lot more with
an MRI. In a typical hospital a simple MRI runs $800, imagine having to
spend that for check-up images once a year and everytime you have a
problem, or in case of a root canal a minimum of 3 times. And with most
dental insurance maxing out at $1000/year there would be nothing left
for coverage, not to mention that insurance will not pay for MRI.