Determining Exposure Times for Spectrographic Film

[spaghetti3451]

I am taking the spectra of lithium, carbon and mercury on a piece of film with the help of a spectrograph. One of the parameters that affect the readings is the exposure times. (How?) I have to find the correct exposure times experimentally. (Why?) That is because the exposure times depend on the quality of the source. (So what?) For instance, if a lithium spectrum is taken using an electric arc as the source (the electric arc is produced by two lithium-doped carbon electrodes), then the exposure times depends on the alignment of the electrodes, the amount of lithium used and the stability of the arc. (So how does that influence the exposure time?).

So, I have to take several spectra using different exposure times to properly expose both the strong and weak features. (Don't get this point.)

Any help would be greatly appreciated.

 

[aabottom]

I am taking the spectra of lithium, carbon and mercury on a piece of film with the help of a spectrograph. One of the parameters that affect the readings is the exposure times. (How?)

Flim exposure depends on the total accumulated energy density at the film plane, thus it is the product of irradiance (watts/meter^2) at the film plane and the exposure time. Longer exposure times allow more light energy to hit the film.

I have to find the correct exposure times experimentally. (Why?)

Likely because you don't know the radiance (watts/[steradian*meter^2]) of the source, f-number of the optical system, and the sensitivity (ISO film speed) of the film.

That is because the exposure times depend on the quality of the source. (So what?)

The quality of the source effects it's radiance.

For instance, if a lithium spectrum is taken using an electric arc as the source (the electric arc is produced by two lithium-doped carbon electrodes), then the exposure times depends on the alignment of the electrodes, the amount of lithium used and the stability of the arc. (So how does that influence the exposure time?).

A source with higher radiance (e.g. brighter) requires less exposure time to get the same amount of energy density at the film plane.

So, I have to take several spectra using different exposure times to properly expose both the strong and weak features. (Don't get this point.)

Typical film has a dynamic range of about 8 stops (2^8=256). That means the ratio of the engeries of the brightest and dimest features can be no bigger than 256.

Any help would be greatly appreciated.

It looks like you have to determine exposure time experimentally.

Have you considered using a digital camera or CCD array? With a digital camera, you see the results quickly without having to run off to the darkroom. Also, you might find a digital camera or CCD with a higher dynamic range than film.

 

[spaghetti3451]

Thanks!