Introduction: Infrared (IR) detectors convert infrared radiation into measurable electrical signals and are essential in pharmaceutical analysis, instrumental spectroscopy, and quality control. This overview compares main thermal and pyroelectric detector types — Golay cell, bolometer, thermocouple, thermistor and pyroelectric detector — describing principles, materials, sensitivity, response time, noise-equivalent power (NEP) and spectral range. B. Pharm students will learn how detector selection affects mid‑IR and near‑IR spectroscopy, drug polymorphism studies, content uniformity, coating analysis and non‑destructive testing. Emphasis is on practical aspects: cooling requirements, need for modulation, detector linearity and maintenance. Understanding these detectors strengthens interpretation of IR instrumentation and analytical method validation. Now let’s test your knowledge with 30 MCQs on this topic.
Q1. What is the primary sensing principle of a Golay cell?
- Change in electrical resistance of a thin film
- Pressure change in a gas causing a membrane deflection measured optically
- Thermoelectric voltage generated at a junction of two metals
- Change in pyroelectric polarization with temperature
Correct Answer: Pressure change in a gas causing a membrane deflection measured optically
Q2. Which detector type relies on a temperature-dependent resistor element for detection?
- Pyroelectric detector
- Golay cell
- Thermistor
- Photodiode
Correct Answer: Thermistor
Q3. Why do pyroelectric detectors typically require a chopped (modulated) IR source for measurement?
- They only respond to changes in temperature, not steady radiation
- They need cooling to cryogenic temperatures
- Their spectral response is limited to visible light without chopping
- Chopping increases thermal capacity and stability
Correct Answer: They only respond to changes in temperature, not steady radiation
Q4. Which parameter describes the smallest input power that produces a signal equal to the detector noise?
- Responsivity
- Noise-equivalent power (NEP)
- Time constant
- Bandwidth
Correct Answer: Noise-equivalent power (NEP)
Q5. In a bolometer, what physical property of the sensor element changes with absorbed IR radiation?
- Optical reflectance
- Electrical resistance
- Magnetic susceptibility
- Dielectric constant at high frequency
Correct Answer: Electrical resistance
Q6. Which detector is most likely to require cryogenic cooling for optimal sensitivity in mid-IR spectroscopy?
- Uncooled microbolometer
- Golay cell
- Cooled photoconductive detector (e.g., HgCdTe)
- Thermistor operated at room temperature
Correct Answer: Cooled photoconductive detector (e.g., HgCdTe)
Q7. Which material is commonly used in pyroelectric detectors for robust room‑temperature operation?
- Triglycine sulfate (TGS)
- Silicon photodiode
- HgCdTe
- Germanium carbide
Correct Answer: Triglycine sulfate (TGS)
Q8. For pharmaceutical IR applications like tablet coating analysis, which detector attribute is most critical?
- High optical throughput at visible wavelengths
- Fast response time and adequate sensitivity in mid-IR
- Magnetic shielding
- Operation only at cryogenic temperatures
Correct Answer: Fast response time and adequate sensitivity in mid-IR
Q9. Which detector type typically exhibits the fastest time constant among thermal detectors?
- Large-volume bolometer
- Golay cell
- Miniaturized thermocouple junction
- Bulk thermistor
Correct Answer: Miniaturized thermocouple junction
Q10. Detectivity (D*) is a figure of merit normalized to detector area and bandwidth. High D* indicates:
- Lower sensitivity to small signals
- Better sensitivity per unit area and bandwidth
- Higher thermal inertia
- Faster time response
Correct Answer: Better sensitivity per unit area and bandwidth
Q11. Which noise source often limits performance of thermal IR detectors at room temperature?
- Photon shot noise from visible light
- Johnson (thermal) noise
- Cosmic ray background
- Magnetic hysteresis
Correct Answer: Johnson (thermal) noise
Q12. A thermocouple IR detector generates a voltage because of:
- Change in dielectric constant with temperature
- Seebeck effect at dissimilar metal junctions heated by IR
- Optical interference in a cavity
- Pyroelectric charge separation
Correct Answer: Seebeck effect at dissimilar metal junctions heated by IR
Q13. Which detector is inherently broadband and well-suited for FTIR bench use when cooled detectors are not available?
- Golay cell
- Silicon photodiode
- Lead sulfide photoconductor
- CCD array
Correct Answer: Golay cell
Q14. What is a key disadvantage of Golay cells compared to solid-state thermal detectors?
- Very short wavelength cutoff limiting mid-IR use
- Mechanical fragility and slower response
- Inability to detect wideband radiation
- Requires cryogenic cooling
Correct Answer: Mechanical fragility and slower response
Q15. In IR spectroscopy, why might an analyst select an uncooled microbolometer detector?
- For highest possible D* and lowest NEP
- Cost-effectiveness and compact uncooled operation for imaging
- Because it requires a chopped source
- To measure ultraviolet absorption
Correct Answer: Cost-effectiveness and compact uncooled operation for imaging
Q16. The responsivity of a detector is usually expressed in:
- Watts per square meter (W/m2)
- Volts per watt (V/W) or amperes per watt (A/W)
- Kelvin per watt (K/W)
- Ohms per square (Ω/□)
Correct Answer: Volts per watt (V/W) or amperes per watt (A/W)
Q17. Which detector type is most sensitive to low-frequency mechanical vibrations and must be mechanically isolated?
- Thermistor bead
- Pyroelectric detector
- Golay cell
- Semiconductor photodiode
Correct Answer: Golay cell
Q18. For quantifying drug polymorphs using mid-IR spectroscopy, which detector characteristic improves detection of subtle spectral features?
- Low NEP and high spectral signal-to-noise ratio
- High mechanical resonant frequency
- Large thermal mass
- Operation only without modulation
Correct Answer: Low NEP and high spectral signal-to-noise ratio
Q19. Which detector produces an output voltage proportional to instantaneous temperature change and therefore exhibits no DC response?
- Pyroelectric detector
- Bolometer in DC mode
- Thermistor with DC bridge
- Thermocouple measuring steady-state heat
Correct Answer: Pyroelectric detector
Q20. Which factor reduces the time constant of a thermal detector, improving its speed?
- Increasing thermal mass
- Decreasing thermal conductance to heat sink
- Reducing heat capacity of the sensing element
- Using thicker absorber coatings
Correct Answer: Reducing heat capacity of the sensing element
Q21. In a thermistor-based IR detector, what is usually used to convert resistance change into a measurable electrical signal?
- Optical interferometer
- Wheatstone bridge or constant current source with amplifier
- Cryogenic pump
- Lock-in amplifier without modulation
Correct Answer: Wheatstone bridge or constant current source with amplifier
Q22. Pyroelectric detectors are particularly useful in which pharmaceutical IR application?
- Long-term DC thermal monitoring without modulation
- Time-resolved measurements like rapid scanning FTIR and pyroelectric imaging
- Measuring absolute blackbody temperature without reference
- Visible wavelength colorimetric assays
Correct Answer: Time-resolved measurements like rapid scanning FTIR and pyroelectric imaging
Q23. Which property of the absorber coating on a thermal detector directly affects responsivity?
- Magnetic permeability
- Absorptivity (emissivity) in the IR spectral region
- Electrical conductivity at DC
- Optical transparency in the visible
Correct Answer: Absorptivity (emissivity) in the IR spectral region
Q24. Which detector is most appropriate for an FTIR instrument where broad wavelength response and baseline stability are priorities?
- Uncooled microbolometer array
- Golay cell or thermopile for broadband mid-IR
- Silicon CCD
- Visible photomultiplier tube
Correct Answer: Golay cell or thermopile for broadband mid-IR
Q25. A common semiconductor material used to make room-temperature bolometers for microbolometer arrays is:
- Vanadium oxide (VOx)
- Silicon carbide (SiC)
- Gallium arsenide (GaAs)
- Lead sulfide (PbS)
Correct Answer: Vanadium oxide (VOx)
Q26. Which statement about thermocouples as IR detectors is correct?
- They provide very high spectral resolution without modulation
- They are robust, simple, and generate small voltages proportional to temperature difference
- They must operate at cryogenic temperatures
- They only detect visible light
Correct Answer: They are robust, simple, and generate small voltages proportional to temperature difference
Q27. What design consideration improves signal-to-noise in thermal detectors used for pharmaceutical FTIR?
- Maximizing thermal mass to damp noise
- Optimizing optical throughput, reducing detector noise and proper modulation
- Eliminating optical filters to increase background
- Using the highest possible bias current regardless of heating
Correct Answer: Optimizing optical throughput, reducing detector noise and proper modulation
Q28. The time constant (τ) of a thermal detector is given by C/G, where C is heat capacity and G is thermal conductance. To increase speed you should:
- Increase heat capacity C
- Decrease thermal conductance G
- Decrease heat capacity C or increase G
- Operate at lower optical power only
Correct Answer: Decrease heat capacity C or increase G
Q29. In the context of pharmaceutical analysis, which detector attribute helps detect low concentration impurities using IR spectroscopy?
- High thermal mass
- High responsivity and low NEP for better limit of detection
- Operation only under ambient light
- High mechanical resonance
Correct Answer: High responsivity and low NEP for better limit of detection
Q30. Which practical maintenance or operational point is important for long-term stability of thermal IR detectors in a pharmaceutical lab?
- Never calibrate; factory settings are permanent
- Regular optical alignment, periodic calibration, and protection from humidity and contaminants
- Always operate detectors at maximum electrical bias to reduce drift
- Use open enclosure to allow airflow over the sensing element
Correct Answer: Regular optical alignment, periodic calibration, and protection from humidity and contaminants
I am a Registered Pharmacist under the Pharmacy Act, 1948, and the founder of PharmacyFreak.com. I hold a Bachelor of Pharmacy degree from Rungta College of Pharmaceutical Science and Research. With a strong academic foundation and practical knowledge, I am committed to providing accurate, easy-to-understand content to support pharmacy students and professionals. My aim is to make complex pharmaceutical concepts accessible and useful for real-world application.
Mail- Sachin@pharmacyfreak.com