Well, if you don't have already a laser reading head (thermopile), you need to find a seller that sell both the items (or another seller that sell a thermopile that match the LPM that you are buying from the first one), if you want to have an usable instrument.
An LPM without head is, basically, unhelpful ..... is like have a DMM without lead wires
.....
So, if you buy a Molectron unit, you need to find a reading head from Molectron, that is useable with your unit, and sensitive enough for measure the lasers that you want (basically, i mean, no reasons for buy a 150W YAG head, for use it for measure 10 or 20mW pointers) ..... same for Scientech units, that have their own heads ..... same for Coherent LPMs, that usually have "smart connectors" on their heads, that "say" to the instruments what type of head is connected, and so on .....
Don't buy randomly heads and LPMs without check if they match, or you will end loosing a lot of money.
as example, in
this auction, you can see that over the meter there is a reading head (also if the seller don't know, apparently, nothing about it) .....
this also shows the reading head (overpriced unit, for the conditions that it is, but the fieldmaster is a good unit) ..... anywhere you DON'T see the reading head, or where the seller don't state the presence of a reading head, it's usually selled without it.
Also, beware the reading head types ..... where the seller don't specify the exact type of reading head, or where the seller picture the head in the way that you can't see the inside reading disk, like, as example,
here and
here, there is something that stinks, for me ..... 99% of the times, it means that is a photodiode reading head, and they needs an instrument that match perfectly, with the related wavelenghts compensation scales, and still don't measure all the laser types (photodiodes don't have a linear reading) ..... anywhere the reading surface looks like glass, it's 99% a photodiode ..... thermopiles looks non-reflecting matte black .....
