Types of UV detectors: DAD, PDA, UV, VWD, SWD, MWD
This is the simplest of the detectors as it only measures one wavelength that the scientist configures using the software. It's confusing because DAD and MWD also measure UV absorbance, but when saying UV detector people usually mean exactly the fact that it can work with just a single wavelength.
This detector first filters out the right wavelength, then it directs the light to the sample. Since the light that passes through the sample is of just one wavelength, it's called monochromatic light.
Variable Wavelength Detector (VWD)
This is an improvement over the UV detector that allows switching between the measured wavelengths. So the detector does the measurement, then some mechanical motor changes an angle to another wavelength and makes another measurement.
So the key difference between the UV detector and VWD is you either get a single chromatogram or you can get multiple.
Diode arrays (DAD & PDA)
Diode array detector and Photodiode detector are 2 different names that mean the same thing. Instead of measuring just a single wavelength, they measure the whole spectra. E.g. they could measure light absorbance at 200 to 400 nm - all at the same time.
As opposed to the UV detector, the light of broad spectrum is directed through the sample. The sample absorbs different wavelengths differently. After the light passed through the sample, the ray is divided into many monochromatic rays - each falling on its own mini-detector (photodiode). There could be many photodiodes (e.g. 1024) which allows to measure many wavelengths at the same time.
MWD & SWD
MWD (Multiple wavelengths detector) and SWD (Single wavelength detector) are actually the same as DAD/PDA detectors. But while the physical device is the same, vendors can restrict how many wavelengths are recorded at the firmware/software level. This is probably done for marketing/sales reason and doesn't have anything to do with the science.
So while the device is just a DAD, what users get as a result is more similar to UV detector: it's just instead of just one measurement/chromatogram they get multiple (e.g. 8).
Here's some weirdness. If MWD/SWD is just a DAD that doesn't give the result from selected diodes, why some (many? all?) MWD/SWD associate the signal with different retention times? It seems like the detector measures one wavelength, then switches to measuring the next one. While that shouldn't be the case with the diodes - we can measure both of them at the same time.
Similarities to Mass Spectrometry
We can draw parallels between the data produced by these detectors. UV and VWD detectors are similar to the Single Ion Monitoring (SIM) done in Mass Spec. While DAD/PDA detectors are similar to Scan Mode. There doesn't seem to be any equivalent of MWD thought, but that's a weird concept in the first place.
Chromatogram names in Peaksel
Since at the basic level all these detectors give us UV absorbance,
Peaksel always refers to the signal from any of these detectors as UV.
So we end up with chromatograms like
UV Total (aka TWC, total wavelength chromatogram) and
UV xxx for extracted chromatograms
(see DAD/PDA: Extracted and Reference wavelengths).