Sean,

Wow, that paper is a difficult read! (or is it just me???)

Yes, lambda is ?, but its not too clear what lambda actually is. From the paragraphs just above eqn 14 it does look like it is something to do with the target signal. And if you click on the link to the info on the Poisson distribution then lambda is defined as "a positive real number, equal to the expected number of occurrences that occur during the given interval. For instance, if the events occur on average 4 times per minute, and one is interested in the probability of an event occurring k times in a 10 minute interval, one would use a Poisson distribution as the model with ? = 10×4 = 40." And that sounds like the faint target signal level to me.

But if you go down to the bottom of the paper to the final paragraph before the Conclusions, then it talks about lambda in a different way. It looks like its a factor that you can enter depending upon how close you want to be to a normal Poisson distribution.....with a value of 15 being a "good" value. IF this is correct, then lambda is effectively a constant....one that you choose at the begining.

I've got this feeling that what the author is saying is something like this......the maths analysis assumes a Poisson distribution. A Poisson dist becomes normal when the faint signal becomes > 5, with 15 being a 'good' number. So if you put 15 into the equations, then they will be valid and will give you the optimum subexposure for the faint parts of the image that have a signal level of 15. However, if you want to get the optimum subexposure time for parts of the image that have a signal level of 10 (or even 5) then it will still hold true.

This value of 15 is the signal level of the faint part that we are concerned about during the exposure, i.e. it is not the value per minute, but the value per minute multiplied by the subexposure time.

IF any of the above is correct then it still doesn't make much sense. The equation seems to be dimensionally incorrect....S seems to be in units of sqrt(T)?

I have no idea if any of this rambling helps at all?

Cheers

Simon