From: PO3::"ALGAE-L%IRLEARN.bitnet@IUBVM.UCS.INDIANA.EDU" "ALGAE-L : digest information on botany" 7-NOV-1994 16:17:05.29 To: Multiple recipients of list ALGAE-L CC: Subj: Re: Irradiance Standard Klaus Luening has long been trying to wean phycologists away from illuminance units, which come from instruments sensitive to human-perceived light not PAR. It's not appropriate to convert from lux, especially if you're measuring at the ends of that part of the spectrum, i.e., the red and blue that would most interest those studying photoperiodic effects. There are discussions of this in Luening's book and several others, but as long as photographic meters are cheap and irradiance meters are expensive it will be an uphill battle to standardize. Chris Lobban, Div. Natural Sciences University of Guam, Mangilao, GU 96923, USA Fax: (671) 734-1299. Voice: 734-9533 (Time: GMT+10 EST+15) From: PO3::"ALGAE-L%IRLEARN.bitnet@IUBVM.UCS.INDIANA.EDU" "ALGAE-L : digest information on botany" 7-NOV-1994 18:05:25.14 To: Multiple recipients of list ALGAE-L CC: Subj: Re: Irradiance Standard It is my understanding that, by convention(?), light flux units should be umoles (photons)/sq-m/sec, at least in the context of photosythesis. A related question is what to call such units, e.g. quantum irradiance, photon flux density (PFD), photon flux fluence rate (PFFR), etc. According to the LI-COR manual, PFD refers to measurements made with a cosine- corrected flat plate sensor, while PFFR is used for spherical sensors. They do not specify on what authority they use these terms, perhaps from physical optics. Personally, PFFR seems redundant (flux fluence), so why not just PFR? I have received comments on my arbitrary tendency to use PFD, but those criticizing it never give a definitive alternative. Another consideration is that different audiences have different traditions/ conventions. For example, I believe that the journal Photochemistry and Photobiology used to, and may still require energy flux units (Watts/sq-m) rather than quantum units. In any event, I think lux, foot-candles per fortnight, and other antiquated units are out officially, but still appear from those who (for financial reasons) only have older instruments. Bill Henley, Oklahoma State Univ. henley@vm1.ucc.okstate.edu From: PO3::"ALGAE-L%IRLEARN.bitnet@IUBVM.UCS.INDIANA.EDU" "ALGAE-L : digest information on botany" 8-NOV-1994 01:16:58.85 To: Multiple recipients of list ALGAE-L CC: Subj: Re: Irradiance Standard The reason why it is difficult (impossible at times) to convert from lux to irradiance (quantum flux ) units is that they measure different things. Most (respectable) journals now only accept units in uEinsteing/m^2/sec^-1 (or moles quanta/m^2/sec). These should of course also be measured only with a sensor active over the photosynthetic spectrum i.e. 400-700nm to measure the photosynthetically active radiation. The Li-Cor PAR sensors are quite good at this. Other sensors have different sensitivities and will give different readings for different light sources all of which have different spectra. Even fluorescent lights change in spectral emission over time. All in all, measuring light is NOT easy. Cheers, Michael Borowitzka **************************************************** Michael A. Borowitzka ! School of B.E.S. Murdoch University, Perth WA 6150 Australia Email: borowitz@possum.murdoch.edu.au Tel: +61 9 360 2333 / Fax +61 9 310 3505 **************************************************** From: PO3::"ALGAE-L%IRLEARN.bitnet@IUBVM.UCS.INDIANA.EDU" "ALGAE-L : digest information on botany" 8-NOV-1994 01:35:24.24 To: Multiple recipients of list ALGAE-L CC: Subj: Re: Irradiance Standard In my experience there has certainly been an increased use of moles of photons per area per time as the standard measure of irradiance. Hopefully as time and resources allow the majority of researchers will adopt these units. Unfortunately I would view this only as partial solution. Underwater and laboratory light spectral distributions vary enormously and I believe that significant improvements in data compatibility between studies will only be possible when this is taken into account. In most cases we wish to define the number of photons collected and used in photosynthesis which can clearly vary substantially if the predominate wavelengths are blue or green. Thus it would seem to me that the standard we will eventually have to achieve will be spectral characterization of both the irradiance and the absorption characteristics of the plants. I can only hope that some improvement in technology will make this instrumentation available at a lower cost in the near future.