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MarathonSeriesFA/FR 3
Anotherbenefitisthat2‐colorsensorsmeasureclosertothehighesttemperaturewithinthemeasured
spot(spatialpeakpicking)insteadofanaveragetemperature.A2‐colorsensorcanbemountedfarther
away,evenifthetargetdoesnotfilltheresultingspotsize.Theconvenienceisthatyouare
notforced
to install the sensor at some specific distance based upon target size and the sensor’s optical
resolution.
2.1.1PartiallyObscuredTargets
The radiatedenergy from a target is,in most cases, equally reduced whenobjects or atmospheric
materialsblocksomeportionoftheopticalfieldofview.Itfollowsthattheratiooftheenergiesis
unaffected,andthusthemeasuredtemperaturesremainaccurate.A2‐colorsensorisbetterthana
1‐
colorsensorinthefollowingconditions:
• Sightingpathsarepartiallyblocked(eitherintermittentlyorpermanently).
• Dirt,smoke,orsteamisintheatmospherebetweenthesensorandtarget.
• Measurementsaremade throughitemsorareas that reduceemittedenergy, suchasgrills,
screens,smallopenings,orchannels.
•
Measurements are made through a viewing window that has unpredictable and changing
infraredtransmissionduetoaccumulatingdirtand/ormoistureonthewindowsurface.
• Thesensoritselfissubjecttodirtand/ormoistureaccumulatingonthelenssurface.
1‐colorsensorsseepollutedatmosphereanddirtywindowsandlensesasareductionin
energyandgivemuchlowerthanactualtemperaturereadings!
2.1.2TargetsSmallerThanFieldofView
Whenatargetisnotlargeenoughtofillthefieldofview,orifthetargetismovingwithinthefieldof
view,radiatedenergiesareequallyreduced,buttheratiooftheenergiesisunaffectedandmeasured
temperatures remain accurate. This remains true as long as the background temperature
is much
lowerthanthetarget’s.Thefollowingexamplesshowwhere2‐colorsensorscanbeusedwhentargets
aresmallerthanthefieldofview:
• Measuring wire or rod — often too narrow for field of view or moving or vibrating
unpredictably.Itismucheasiertoobtainaccurate
resultsbecausesightingislesscriticalwith
two‐colorsensors.
• Measuring molten glass streams — often narrow and difficult to sight consistently with
single‐wavelengthsensors.
2.1.3LoworChangingEmissivities
Iftheemissivitiesinbothwavelengths(colors)werethesame,astheywouldbeforanyblackbody
(emissivity=1.0)orgraybody(emissivity<1.0butconstant),thentheirratiowouldbe1,andtarget
emissivitywouldnotbeaninfluence.However,innaturethereisnosuchthingasa
greybody.The
emissivityofallrealobjectschangeswithwavelengthandtemperature,atvaryingdegrees,depending
onthematerial.
When emissivity is uncertain or changing, a 2‐color sensor can be more accurate than a 1‐color
instrument aslong as theemissivity changes by the samefactor in both
wavelength bands. Note,
however,thataccuratemeasurementresultsaredependentontheapplicationandthetypeofmaterial
beingmeasured.Todeterminehowtouse2‐colorsensorswithyourapplicationwhenuncertainor
changingemissivitiesareafactor,pleasecontactyoursalesrepresentative.