What units does incident light have in UV spectroscopy

[Moogie]

Hi

What units does incident light have in UV spec? Is it joules? Transmission is defined as transmitted light / incident light so any units of light simply cancel out to give a unitless number. I presume the original units were joules as light is EMR? Perhaps its more complicated than that

thanks

 

[sophiecentaur]

Watts would be more appropriate, I think, as you would normally be considering a steady incidence of power.

 

[Andy Resnick]

sometimes the wavelength is specified as 1/length (i.e. cm^-1) in spectroscopy, because that more naturally relates to the scattering of light by matter.

 

[sophiecentaur]

Agreed.

 

[Moogie]

Transmission refers to an amount of light though.

You often use wavenumber in IR spec as the units of 1/wavelength are much more amenable than wavelength. But regardless, wavelength or wavenumber is the quantity that goes along the x axis. The amount of light that passes through the solution at that wavelength is the transmission and that goes on the y-axis and that can't be measured in nm or cm^-1 (for wavenumber).

 

[ZapperZ]

But then it makes your question rather vague. What "amount" are you quantifying? There is no such thing as an "amount" here without defining a physical quantity that is being measured. Is it energy, power, number of photons, etc.?

In optical transmission measurement, while it is often that the quantity of interest is often the wave number, even this has a connection to the photon energy (see the simple conversion on the inside back cover of Ashcroft and Mermin's text).

Zz.

 

[Moogie]

I suppose that's what I was trying to ask. An amount of EMR is energy so surely its joules? But I'm not used to dealing with EMR so it just seemed unusual and I wanted confirmation

 

[ZapperZ]

I suppose that's what I was trying to ask. An amount of EMR is energy so surely its joules? But I'm not used to dealing with EMR so it just seemed unusual and I wanted confirmation

But if you are quantifying the energy in Joules, you need to specify the "window" of the measurement, i.e. how long was the detector open to receive that much energy. If not, I could do the same measurement with a different time window on the SAME light source, and get a different answer!

Most photodetector will give you a reading of "power", i.e. the rate of energy received. This makes a bit more sense, especially if you know either the light spot size or the photodetector area. Either one of these will give you all the other information (such as energy per unit time) that you might need.

Zz.

 

[Moogie]

Thanks. I believe that was what sophiecentaur suggested until he/she changed their mind