Opto Diode’s A Series lead sulfide (PbS) detectors are single-channel infrared detectors covering the 1–3 µm short-wave infrared band, with peak sensitivity between 2.2 and 2.7 µm depending on model and cooling. Eleven standard models span uncooled AP-series parts and one- and two-stage thermoelectrically cooled AT-series parts, in active areas from 1 mm × 1 mm to 6 mm × 6 mm. Opto Diode, a division of ITW, manufactures them in Camarillo, California; all specifications below are from the controlled A Series datasheet, Revision September 30, 2021.
Which A Series Family Do You Need?
All three families use the same PbS sensing material. Cooling is the ladder that trades complexity and drive power for detectivity, and it also shifts peak sensitivity to longer wavelengths:
AP Series
- Six models, 1 to 36 mm²
- Peak sensitivity 2.2 µm min / 2.4 µm typ
- Time constant 200 µs typ, 400 µs max
- Glass molded lens, flat glass or flat silicon windows
AT1 Series
- Two models, 4 and 9 mm²
- Peak sensitivity 2.4 µm min / 2.5 µm typ
- Time constant 800 µs typ, 1600 µs max
- Flat sapphire window
AT2 Series
- Three models, 4 and 9 mm²
- Peak sensitivity 2.5–2.6 µm min / 2.6–2.7 µm typ
- Time constant 1250–1750 µs typ
- Flat sapphire window
Full A Series Specifications
Values are per the datasheet’s specification table. D* and responsivity are quoted at the peak wavelength λp and 650 Hz; the datasheet states no separate measurement condition for the resistance and time-constant columns. The responsivity column is headed 25–50 V/mm, and its footnote specifies a bias of 50 V/mm directly across the detector with a 1 MΩ load resistor in series (part 40009 uses a 0.5 MΩ load).
| Model | Part # | Element size | Package | Area | Element temp | λp min / typ | D* min / typ | Responsivity min | Resistance | Time constant typ / max | Window |
|---|---|---|---|---|---|---|---|---|---|---|---|
| AP-15G | 40725 | 1 × 1 mm | TO5 | 1 mm² | +23 °C | 2.2 / 2.4 µm | 8×1010 / 1×1011 | 5.3×105 V/W | 0.5–2.0 MΩ | 200 / 400 µs | Glass molded lens |
| AP-25G | 40370 | 2 × 2 mm | TO5 | 4 mm² | +23 °C | 2.2 / 2.4 µm | 8×1010 / 1×1011 | 2.7×105 V/W | 0.5–2.0 MΩ | 200 / 400 µs | Glass molded lens |
| AP-35 | 40363 | 3 × 3 mm | TO5 | 9 mm² | +23 °C | 2.2 / 2.4 µm | 8×1010 / 1×1011 | 1.7×105 V/W | 0.5–2.0 MΩ | 200 / 400 µs | Glass molded lens |
| AP-20505 | 40009 | 2 × 5 mm | TO5 | 10 mm² | +23 °C | 2.2 / 2.4 µm | 7×1010 / — | 8.0×104 V/W | 0.5–2.0 MΩ | 200 / 400 µs | Flat glass |
| AP-58E | 40736 | 5 × 5 mm | TO8 | 25 mm² | +23 °C | 2.2 / 2.4 µm | 7×1010 / — | 1.1×105 V/W | 0.5–2.0 MΩ | 200 / 400 µs | Flat Si |
| AP-68 | 40368 | 6 × 6 mm | TO8 | 36 mm² | +23 °C | 2.2 / 2.4 µm | 7×1010 / — | 9.0×104 V/W | 0.5–2.0 MΩ | 200 / 400 µs | Flat Si |
| AT1-27TE | 40373 | 2 × 2 mm | TO37 | 4 mm² | −25 °C | 2.4 / 2.5 µm | 1×1011 / 1.5×1011 | 6.3×105 V/W | 1.5–10.0 MΩ | 800 / 1600 µs | Flat sapphire |
| AT1-37T | 40147 | 3 × 3 mm | TO37 | 9 mm² | −25 °C | 2.4 / 2.5 µm | 1×1011 / 1.5×1011 | 4.2×105 V/W | 1.5–10.0 MΩ | 800 / 1600 µs | Flat sapphire |
| AT2-28TE | 40028 | 2 × 2 mm | TO8 | 4 mm² | −35 °C | 2.5 / 2.6 µm | 1.5×1011 / 2.5×1011 | 6.6×105 V/W | 2.5–15.0 MΩ | 1250 / 2500 µs | Flat sapphire |
| AT2-37T | 40193 | 3 × 3 mm | TO37 | 9 mm² | −35 °C | 2.5 / 2.6 µm | 1.5×1011 / 2.5×1011 | 4.3×105 V/W | 2.5–15.0 MΩ | 1250 / 2500 µs | Flat sapphire |
| AT2S-38T | 40029 | 3 × 3 mm | TO8 | 9 mm² | −45 °C | 2.6 / 2.7 µm | 2×1011 / 3×1011 | 5.18×105 V/W | 6.0–20.0 MΩ | 1750 / 3500 µs | Flat sapphire |
D* is quoted in cm·Hz½·W−1 at λp, 650 Hz, 1 Hz bandwidth. Absolute storage and operating temperature ratings are −40 to +65 °C for AP models and −40 to +85 °C for AT models; maximum rated element temperature is 65 °C. AP element temperatures are specified at ambient 23 °C, AT at ambient 25 °C, and cooled-device specifications apply at maximum cooling with the heat sink at +25 °C.
All eleven models are listed on the PbS product-category page, with matching part numbers and active areas.
Reading the Specifications
D* (specific detectivity) is the figure of merit for how small a signal the detector can resolve, normalized for element area and bandwidth — higher is better. Across the A Series it climbs from 7–8 × 1010 minimum uncooled to 2 × 1011 minimum on the two-stage-cooled AT2S-38T, roughly a factor of 2.5 to 3, which is what cooling buys.
Responsivity is given in volts per watt because a PbS detector is a photoconductor: it changes resistance under illumination and is read as a voltage across a bias network, not as a photocurrent. Responsivity generally falls as the element grows — across the glass-molded-lens AP parts it runs from 5.3 × 105 V/W at 1 mm² to 1.7 × 105 V/W at 9 mm², and in each cooled pair the 4 mm² part is specified above the 9 mm² part. The trend is not strictly monotonic across the whole line, because window type and load resistance also matter: AP-20505 (10 mm²) is specified at 8.0 × 104 V/W, below the larger AP-58E (25 mm², 1.1 × 105 V/W), and its footnote specifies a 0.5 MΩ load resistor where every other part uses 1 MΩ. Compare the specified figure for the exact part rather than inferring from active area alone. Because D* is normalized for area and is similar across the uncooled parts, noise-equivalent power scales with the square root of active area — as a general matter, the smallest element that comfortably captures the optical spot gives the best noise performance.
Time constant sets the usable chopping or modulation rate, and it rises with each cooling stage: 200 µs typical uncooled, 800 µs at one stage, 1250–1750 µs at two. Cooling raises sensitivity and pushes peak response toward 2.7 µm, but slows the detector. The datasheet also plots signal, noise and signal-to-noise against bias voltage, and shows PbS relative D* versus wavelength alongside PbSe for comparison.
Packages and Pin-Outs
The datasheet prints dimensioned drawings and pin tables for every package. Uncooled TO5 parts use a three-terminal arrangement — pins 1 and 3 are the detector, pin 2 is case ground. The two uncooled TO8 parts differ from each other: AP-68 (40368) lists detector on pins 1 and 9 with case ground on pin 12, while AP-58E (40736) lists only the two detector pins, 1 and 9. Cooled parts add the thermoelectric cooler and a thermistor for temperature feedback: on the TO8 cooled parts, pin 1 is cooler (−), pin 4 cooler (+), pins 9 and 12 the detector, and pins 10 and 11 the thermistor, with color-coded leads. TO37 cooled parts carry the same six functions on pins 1, 2, 6, 7, 8 and 9. Plan for a cooler driver and a thermistor sense line when you specify an AT part — typical cooler power at maximum cooling is 0.8 V at 1.5 A for AT1, 0.8 V at 1.2 A for AT2, and 1.9 V at 1.2 A for AT2S.
Applications
The datasheet lists gas analysis (medical and industrial), emissions monitoring, spectroscopy, process control systems, thermal imaging, and flame monitoring and detection. The 1–3 µm band carries the absorption features used in gas sensing and in moisture and food-composition measurement, along with the emission signatures used in flame and combustion monitoring and non-contact temperature measurement of hot targets. If your target absorption sits beyond 3 µm — hydrocarbons near 3.4 µm, CO2 at 4.3 µm — the 1–5 µm PbSe B Series is the correct family; our PbS vs PbSe selection guide walks through that choice.
Frequently Asked Questions
What wavelength range do PbS detectors cover?
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The A Series datasheet specifies the highest sensitivity in the 1 to 3 micron region. Peak sensitivity depends on cooling: 2.2 µm minimum and 2.4 µm typical for uncooled AP models, rising to 2.6 µm minimum and 2.7 µm typical on the two-stage-cooled AT2S-38T.
Is cooling worth it for a PbS detector?
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Cooling raises minimum detectivity by roughly 2.5 to 3 times — from 7 to 8 times 10 to the tenth minimum uncooled up to 2 times 10 to the eleventh minimum on the two-stage AT2S-38T — and shifts peak response toward 2.7 µm. The trade is speed and complexity: the time constant rises from 200 µs typical uncooled to 1750 µs typical on the AT2S, and cooled parts need a cooler driver and thermistor feedback.
Why is PbS responsivity given in volts per watt?
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Because PbS is a photoconductor rather than a photodiode: it changes resistance under illumination and is read as a voltage across a bias network. The datasheet’s responsivity figures apply at a bias of 50 V/mm directly across the detector with a 1 megohm load resistor in series, with part 40009 using a 0.5 megohm load.
Should I choose PbS or PbSe?
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PbS covers 1 to 3 µm and PbSe covers 1 to 5 µm. Choose PbS when your absorption feature falls inside the short-wave infrared band, and PbSe when you need to reach the mid-wave region beyond 3 µm. Opto Diode manufactures both families in Camarillo, California.
How do I order A Series PbS detectors?
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Contact Opto Diode with the model number or part number from the table above. The datasheet notes that customers can choose from an assortment of standard options including element size, cooling alternatives and package size, and that custom requirements can be addressed by contacting the Opto Diode sales team. Direct orders are subject to a $3,000 minimum, and Opto Diode typically responds to online requests within 1-2 business days.
Specifying a PbS detector for gas analysis, flame detection or process monitoring? Opto Diode typically responds to online requests within 1–2 business days.
Reviewed and updated August 2026.