A thermal detector combines an absorber and a temperature sensor

Transcription

1 Basics A thermal detector combines an absorber and a temperature sensor window P absorber envelope under vacuum temperature sensor electrical signal out T 1

2 Absorber spectral response ABSORBANCE α or relative 0.7 detector response WAVELENGTH (µm) 2

3 Absorber thermal circuit P T d K t C t thermal balance equation: (as an RC parallel circuit) or, C t dt d /dt + (1/K t ) T d = P τ dt d /dt + T d = K t P whence f 2 = 1/2πτ =1/2πK t C t 3

4 Temperature-sensing section T d r V u R=R( ) T d C Vu (a) thermopile: R = V u /P = γk t (b) responsivity: p d bolometer R = R/P = αr 0 K t T d /dt (c) pyroelectric R = Q/P = pk t 4

5 Pole-zero cancellation in pyroelectrics in electrical circuit, derivative operator pdt d /dt (pyroelectric) cancels out integration of thermal circuit (f>>1/τ) T d =(1/C t ) Pdt then I = p dt d /dt = (p/c t )P short-circuit current is proportional to P (even at f >>1/τ) or, responsivity is R = I/P = p/c t 5

6 Frequency response 1 1/ 2 pyroelectric -1 relative responsivity R(ω)/R(0) -2 (pyr) f 1 bolometer thermocouple f 2 (RC) -3 f 2 (therm) f 2 (bol) 1 0 1k k 0k 1M FREQUENCY (Hz) 6

7 Detectivity of thermal detectors K t = dt/dp = 1/(4AσT 3 ) T n = [kt 2 /C] 1/2 NEP = T n /K t = [kt 2 /C] 1/2 / K t = [kt 2 2πB] 1/2 (4AσT 3 ) D* = AB / NEP = 1/ [8πσkT 5 ] 1/2 typical values: D*=1.8. W -1 cm Hz 1/2 at T=300K, and D*= W -1 cm Hz 1/2 at T=77 K, 7

8 Detectivity vs λ PbS 195 K DETECTIVITY 2 D* (cm Hz/W) D limit BLIP HgCdTe 77 K 290 K PbSnTe 77 K Ge:Cu 4 K TGS 290 K InSb 77 K Si:Pt K 77 K WAVELENGTH (micrometers) bolometer 290 K thermopile 290 K

9 Temperature measurements thermal baffle filter objective fov Ω T TD detector target blackbody T=3 radiant power T=300 K gray filter to TD non-gray deviation λ (µm) 9

10 Temperature signal received power: p = r(λ,t) πna 2 A λ r is blackbody radiance: r(λ,t) =2hc 2 /π λ 5 (exp hν/kt-1) p = (dp/dt) T= [dr(λ)/ T dt] π NA 2 A λ = = π NA 2 A λ κ r(λ) T/T where κ = (hν/kt) [exp hν/kt /(exp hν/kt-1)] hc/λkt= 47.88(µm) /λ for hν>kt (or, λ <48µm, the break-point between thermal and quantum)

11 Noise In a BLIP-limited detector: i n(bg) = σ NEP = σ NA AB/D BLIP = NA [2πeσ r(λ) λab] but signal is then i = σ p = σ π NA 2 A λ κ r(λ) T/T S/N = κ ( T/T) NA [πσ Ar(λ) λ/2eb] and the NEDT - noise equivalent dif ferential temperature - S/N=1: NEDT = (T/κNA) [2eB/σπ r(λ) λa] = 2kT 2 D BLIP (1/ηNA) (B/A) 11

12 Theoretical NEDT MINIMUM DETECTABLE TEMPERATURE NEDT ( C) D BLIP limit D** = 1 0 1k k η=0.5, NA=0.5 A=0.01 cm 2 D BLIP =6. (W -1 cm Hz) BANDWIDTH B - (Hz) 12

13 NED in real detectors If detector is real (not BLIP-limited): i n(bg) = σ NEP = σ NA AB/D**, signal is the same S/N = i/i n(bg) = π NA (A/B) κ r(λ) λ ( T/T) D** and NEDT = T [π NA κ r(λ) λ D**] -1 (B/A) = 2kT 2 (D 2 BLIP /D**) (1/ηNA) (B/A) 13

14 Effect of non-unity emissivity If emissivity is ε <1, total radiance is sum of : blackbody term scaled by ε, and ambient re-diffused contribution (1- ε): r tot (λ,t) = ε r(λ,t+ T)+(1-ε) r(λ,t) r(λ,t) thus, background noise is about the same, and radiant measurement of temperature supplies a signal ε T. CORRECTIONS: by a separate conventional thermometric calibration (typ. accuracy:1% or of 0.05 C for T<50 C) by estimate f ε from proprieties of surface under test (typ. accuracy: %for T<50 C) 14

15 An infrared thermometer 15

16 Correcting against non-unity emissivity If temperature T= T amb + T is large enough to make the ambient contribution negligible, i.e. r tot (λ,t) = ε r(λ,t amb + T) +(1-ε) r(λ,t amb ) r(λ,t amb ) + ε [dr(λ,t amb )/dt ] T ε [dr(λ,t amb )/dt ] T then we can correct against ε by looking at the current J = σ r tot (λ,t) found at two wavelengths λ 1 and λ 2. This is called two-color pyrometry. 16

17 Two-color pyrometry using 2 λ channels: J bg (λ 1 ) = ε 1 σ 1 λ 1 πna 2 2hc 2 /λ 15 (exp hc/λ 1 kt -1) J bg (λ 2 ) = ε 2 σ 2 λ 2 πna 2 2hc 2 /λ 25 (exp hc/λ 2 kt -1) ratio of the detected signals R = J bg (λ 2 )/J bg (λ 1 ) = (ε 1 /ε 2 )(λ 1 /λ 2 ) 5 (σ 2 λ 2 /σ 1 λ 1 ) exp[hc(1/λ 2-1/λ 1 )/kt] taking the logarithm of R, ln R= ln C 1 +C 2 /T, T is solved as: T= C (ε 1 /ε 2 )(hc(1/λ 2-1/λ 1 )/k 2 /[ln R ln C 1 ] = log [J bg (λ 2 )/J bg (λ 1 )]+5log(λ 2 /λ 1 )+log(σ 1 λ 1 /σ 2 λ 2 ) independent from emissivity (if ε const. in λ 1 λ 2 ) and is self-calibrated (in absolute temperature) 17

18 Thermovisions Spectral range of operation: MIR, λ=3-5µm or FIR, λ=8-14µm GENE RATIO NS: 0-th: pioneer s work: Spectracon (oil -f ilm camera), st: LN-cooled InSb scan camera (AGA, Huges, etc.) general purpose 2nd: LN-cooled CMT and LTT FPA, military 3rd: TEC-cooled Bolometer, portable 4th: uncooled Pt-Si and VO x bolometer camcorder 18

19 Equipment Avio LN-cooled InSb-FPA high-performance thermovision Inf rametrics uncooled CAM corder 19

20 Fields of application industrial applications: visualization of temperature pattern for application in: electronics, power engineering airconditioning, automotive, construction medical applications: early diagnosis of breast cancer, spotting necrotized areas after severe burning, reveal circulatory diseases remote sensing: several earth resource satellites carry aboard multispectral sensors (12 channels in VIS and 1-2 in MIR/FIR) to reveal pollution, crops, etc. Military: the most important driving f orce of IR techniques, as THV is a passive vision system, working at night or in the dark with no need for illumination, revealing thermal signatures 20