?url_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Adc&rft.relation=http%3A%2F%2Fmiis.maths.ox.ac.uk%2Fmiis%2F260%2F&rft.title=Interference+Filters+for+Thermo-photovoltaic+Applications&rft.creator=Borne%2C+Thomas&rft.creator=Cross%2C+Danielle&rft.creator=Du%2C+Gang&rft.creator=Edwards%2C+David+A.&rft.creator=Haus%2C+Joe&rft.creator=King%2C+John&rft.creator=Lacey%2C+Andrew&rft.creator=Monk%2C+Peter&rft.creator=Please%2C+Colin&rft.creator=Tran%2C+Hoa&rft.subject=Energy+and+utilities&rft.description=The+overall+intention+is+to+use+a+source+of+black-body+radiation%2C+perhaps+some+object+being+heated+inside+by+burning+fuel%2C+to+produce+electricity+from+a+thermophotovoltaic+(TPV)+cell+by+the+correct+sort+of+radiation+arriving+at+the+cell.+Only+light+below+a+certain+wavelength+%CE%BB_g+will+generate+a+voltage+in+the+cell%3B+such+high-frequency+radiation+may+be+termed+%E2%80%9Cuseful%E2%80%9D+in+contrast+to+the+%E2%80%9Cuseless%E2%80%9D+lower-frequency+radiation.+To+increase+efficiency%2C+ideally+the+otherwise+useless+long-wavelength+light+is+returned+to+the+radiator+by+reflecting+back+from+the+TPV+cell.%0D%0A%0D%0AA+key+part+of+the+device+currently+under+investigation+is+an+%E2%80%9Cinterference+filter%E2%80%9D.+This+consists+of+a+large+number+(say+around+60)+of+thin+layers+(the+whole+device+is+about+2+%CE%BCm+thick)+with+differing+refractive+indices.+The+combination+of+multiple+reflections+and+refractions%2C+with+associated+change+of+phase%2C+results+in+interference+so+that+light+of+certain+frequencies+is+mostly+reflected+back+while+other+light+is+mostly+transmitted+through+to+the+other+side+of+the+filter.+In+the+ideal+case%2C+all+the+good+frequencies+will+pass+through+the+filter+to+the+TPV+cell+lying+behind+it+while+the+bad+ones+are+reflected+away%3A%0D%0A-+overall+reflection+coefficient+(by+power)+%3D+1+for+%CE%BB+%3C+%CE%BB_g+%2C+0+for+%CE%BB+%3E+%CE%BB_g+%3B%0D%0A-+overall+transmission+coefficient+(by+power)+%3D+0+for+%CE%BB+%3C+%CE%BBg+%2C+1+for+%CE%BB+%3E+%CE%BBg+.%0D%0AUnfortunately%2C+because+of+the+effect+of+direction+on+optical+lengths+and+hence+on+the+level+of+interference%2C+the+transmission+and+reflection+coefficients+depend+strongly+upon+the+angle+of+incidence.+For+black-body+radiation+the+dependence+of+the+normal+component+of+power+upon+the+angle+of+incidence+%CE%B1+exhibits+a+maximum+at+%CE%B1+%3D+%CF%80%2F4%2C+although%2C+of+course%2C+the+power+density+(with+respect+to+angle)+is+positive+for+all+0+%3C+%CE%B1+%3C+%CF%80%2F2.+This+means+that+%E2%80%93+assuming+that+black-body+radiation+is+impinging+on+the+TPV+cell+%E2%80%93+it+makes+sense+to+try+to+optimize+the+filter%E2%80%99s+performance+for+%CE%B1+%3D+%CF%80%2F4%2C+but+then+most+radiation+will+be+incident+along+sub-optimal+directions.+Indeed%2C+because+the+optical+lengths+inside+the+filter+are+essentially+given+by+cos+%CE%B1+(or+sec+%CE%B1)%2C+the+effectivenesss+of+the+filter+depends+least+strongly+upon+angle+for+%CE%B1+near+0+(normal+incidence).+For+this+reason+attention+must+also+be+focused+on+trying+to+direct+the+light+from+the+radiator.+With+light+travelling+in+(roughly)+one+direction%2C+it+is+then+possible+to+orient+the+cells+so+that+light+falls+on+them+normally+and+the+interference+filters+should+be+designed+to+work+optimally+with+%CE%B1+%3D+0+(or+small).%0D%0A%0D%0AA+second+design+improvement+is+also+worthy+of+mention.+It+was+raised+during+our+deliberations%2C+but+constraints+on+time+prevented+us+from+further+examining+this+research+topic.+The+black-body+radiation+spectrum+is+not+immutable%3B+it+is+modified+by+using+a+multiple+dielectric+layer+design+for+the+emitter.+This+has+the+advantage+of+controlling+both+the+angular+emission+pattern+and+the+emission+spectrum.&rft.date=1999&rft.type=Study+Group+Report&rft.type=NonPeerReviewed&rft.format=application%2Fpdf&rft.language=en&rft.identifier=http%3A%2F%2Fmiis.maths.ox.ac.uk%2Fmiis%2F260%2F1%2Flockheed99.pdf&rft.identifier=++Borne%2C+Thomas+and+Cross%2C+Danielle+and+Du%2C+Gang+and+Edwards%2C+David+A.+and+Haus%2C+Joe+and+King%2C+John+and+Lacey%2C+Andrew+and+Monk%2C+Peter+and+Please%2C+Colin+and+Tran%2C+Hoa++(1999)+Interference+Filters+for+Thermo-photovoltaic+Applications.++%5BStudy+Group+Report%5D+++++