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Andre I should have replaced flux in my email by photons/sec to be clear (or at least a bit clearer). Flux in magnetism is the strength of the magnetic field per unit area so it is confusing to use the term in the way I did. Different fields use the same term for different purposes. Technical (John's statement) or confusing (mine). Take your pick! Colin From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Colin Nave Sent: 09 February 2011 14:35 To: [log in to unmask] Subject: Re: [ccp4bb] Why 0.1% bandwidth? Andre The 0.1% bandwidth is a standard for flux/mrad/bandwidth or brightness (flux/mrad^2/mm^2/bandwidth) from the synchrotron source. It is an emittance rather than an acceptance. A typical perfect crystal monochromator might take (i.e. accept) a tenth of this. However, dependent on the range of angles on the mono the actual bandpass from the mono might be back at 0.1% or higher (but flux still down by a factor of 10). Undulator beamlines generally have a low divergence and the angular divergence term for the mono bandpass is often small. The best advice is to beware of any flux, brightness or bandpass numbers which you might see. Most facilities select some terms which might show their facility in a good "light". FELs for example are keen on peak brightness. If you see brilliance it was probably a mis-translation from German in to English by a Frenchman. Yes it is all confusing! Regards Colin From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Andre Luis Berteli Ambrosio Sent: 09 February 2011 12:14 To: [log in to unmask] Subject: [ccp4bb] Why 0.1% bandwidth? Dear ccp4bb, I sometimes find the flux of x-ray sources reported in units of "photons/s/0.1% bandwidth" instead of simply "photons/s". Where does the "1/0.1% bandwidth" unit come from? I have also seen other percentages like 0.01% bw or 0.02% bw... Is it simply defining some degree of acceptance in energy (for example, the flux between 8 KeV +/- 8 eV for a given stored current)? Does it somehow have to do with energy resolution? Thank you in advance for your answers, -Andre Ambrosio