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ASNT Level III IR study materials
ASNT Level III IR study resources

ASNT Level III IR practice questions

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Question 1Infrared Testing (IR) Method Fundamentals and Physics

What is the approximate wavelength range of the long-wave infrared band commonly used for thermal imaging of electrical equipment?

  1. 8 to 14 micrometers Correct answer.
  2. 0.1 to 1.0 nanometers
  3. 0.4 to 0.7 micrometers
  4. 0.9 to 1.7 micrometers

Explanation

The long-wave infrared band spans roughly 8 to 14 micrometers and is widely used because it overlaps the peak emission of objects near ambient temperature. Shorter bands correspond to visible, near-infrared, or X-ray regions.

Question 2Infrared Testing (IR) Method Fundamentals and Physics

A blackbody at a higher absolute temperature emits its peak energy at what wavelength compared with a cooler blackbody?

  1. A shorter wavelength Correct answer.
  2. A longer wavelength
  3. The same wavelength
  4. A wavelength independent of temperature

Explanation

Wien's displacement law states that peak wavelength is inversely proportional to absolute temperature. Raising temperature shifts the peak toward shorter wavelengths.

Question 3Infrared Testing (IR) Method Fundamentals and Physics

Which quantity describes the fraction of incident radiation that a real surface reflects?

  1. Transmissivity
  2. Conductivity
  3. Emissivity
  4. Reflectivity Correct answer.

Explanation

Reflectivity is the ratio of reflected radiant flux to incident radiant flux. Emissivity relates emitted to blackbody emission, and transmissivity relates transmitted to incident flux.

Question 4Infrared Testing (IR) Method Fundamentals and Physics

For an opaque surface in thermal equilibrium, the sum of its emissivity and reflectivity equals what value?

  1. Zero
  2. Twice its absorptivity
  3. Its transmissivity
  4. One Correct answer.

Explanation

For an opaque body transmissivity is zero, and Kirchhoff's law links absorptivity to emissivity, so emissivity plus reflectivity is unity. This relationship underpins reflected-apparent-temperature corrections.

Question 5Infrared Testing (IR) Method Fundamentals and Physics

A surface has an emissivity of 0.85. What is its reflectivity if the material is opaque?

  1. 0.85
  2. 1.85
  3. 0.85 times the view factor
  4. 0.15 Correct answer.

Explanation

For an opaque surface, reflectivity is one minus emissivity. Subtracting 0.85 from unity gives 0.15.

Question 6Infrared Testing (IR) Method Fundamentals and Physics

Which detector type responds to incident infrared by a change in electrical resistance without requiring a chopper for basic response?

  1. Microbolometer Correct answer.
  2. Photon photovoltaic detector
  3. Quantum well detector
  4. Photon photoconductive detector

Explanation

A microbolometer is a thermal detector whose resistance changes with temperature rise from absorbed radiation. It does not depend on photon excitation across a bandgap.

Question 7Infrared Testing (IR) Method Fundamentals and Physics

Why are many photon infrared detectors cooled during operation?

  1. To reduce thermally generated noise carriers Correct answer.
  2. To shorten the atmospheric path
  3. To increase the emissivity of the target
  4. To raise the detector bandgap

Explanation

Cooling suppresses thermal excitation of carriers that would otherwise produce noise and dark current. This improves signal-to-noise for narrow-bandgap photon detectors.

Question 8Infrared Testing (IR) Method Fundamentals and Physics

Which atmospheric constituent is the principal absorber causing strong attenuation in the 5 to 8 micrometer infrared band?

  1. Nitrogen
  2. Water vapor Correct answer.
  3. Helium
  4. Argon

Explanation

Water vapor produces broad absorption bands that strongly attenuate mid-wave infrared transmission. Nitrogen, argon, and helium are essentially transparent to infrared.

Question 9Infrared Testing (IR) Method Fundamentals and Physics

What does the instantaneous field of view of a single detector element represent?

  1. The dynamic range of the detector
  2. The total spectral bandwidth of the system
  3. The angular subtense of one detector element through the optics Correct answer.
  4. The frame rate of the imager

Explanation

Instantaneous field of view is the angle subtended by one detector element as seen through the lens. It sets the spatial resolution limit for a given lens and detector pitch.

Question 10Infrared Testing (IR) Method Fundamentals and Physics

A lens has a focal length of 50 mm and the detector element pitch is 25 micrometers. What is the approximate instantaneous field of view in milliradians?

  1. 0.5 Correct answer.
  2. 1.25
  3. 25
  4. 2.0

Explanation

Instantaneous field of view equals detector pitch divided by focal length. Dividing 25 micrometers by 50 millimeters gives 0.5 milliradians.

Question 11Infrared Testing (IR) Method Fundamentals and Physics

Which term describes the minimum temperature difference a thermal imager can resolve at a stated target temperature and spatial frequency?

  1. Spectral responsivity
  2. Spatial resolution
  3. Noise equivalent temperature difference Correct answer.
  4. Emissivity uncertainty

Explanation

Noise equivalent temperature difference quantifies the smallest resolvable temperature change against system noise. It is a key performance metric for thermal cameras.

Question 12Infrared Testing (IR) Method Fundamentals and Physics

Total radiant exitance from a blackbody is proportional to which power of absolute temperature?

  1. The fourth power Correct answer.
  2. The inverse square
  3. The square
  4. The first power

Explanation

The Stefan-Boltzmann law states that total exitance scales with absolute temperature to the fourth power. This strong dependence explains why small temperature changes produce measurable radiance changes.

Question 13Infrared Testing (IR) Method Fundamentals and Physics

A blackbody at 300 K emits its peak spectral radiance in which region?

  1. Ultraviolet
  2. Long-wave infrared Correct answer.
  3. X-ray
  4. Visible

Explanation

Applying Wien's displacement law to 300 K yields a peak near 10 micrometers, which lies in the long-wave infrared. Visible and shorter regions correspond to much hotter bodies.

Question 14Infrared Testing (IR) Method Fundamentals and Physics

Which quantity is defined as the ratio of radiance emitted by a real surface to that emitted by a blackbody at the same temperature and wavelength?

  1. Spectral emissivity Correct answer.
  2. Absorptance
  3. Radiosity
  4. Reflectance

Explanation

Spectral emissivity compares a real surface's emission to blackbody emission at the same conditions. It is the property used to convert measured radiance into temperature.

Question 15Infrared Testing (IR) Method Fundamentals and Physics

When measuring the temperature of a low-emissivity metallic busbar, which condition most strongly biases the apparent temperature toward the surroundings?

  1. Short integration time
  2. High detector cooling
  3. High frame rate
  4. High reflected background radiation Correct answer.

Explanation

Low-emissivity surfaces reflect a large fraction of ambient radiation, so the apparent temperature is pulled toward the reflected background. Emissivity correction and background evaluation are needed.

Question 16Infrared Testing (IR) Method Fundamentals and Physics

Which material property is most desirable for a high-quality infrared reference standard used in calibration?

  1. Low thermal conductivity
  2. High reflectivity
  3. High transmissivity
  4. High and stable emissivity Correct answer.

Explanation

Reference standards require high, stable emissivity so their apparent temperature reliably tracks their true temperature. High reflectivity would introduce background contamination.

Question 17Infrared Testing (IR) Method Fundamentals and Physics

Which statement best describes the difference between radiometric and thermal imaging?

  1. Radiometric imaging only displays relative brightness without calibration
  2. Radiometric imaging reports calibrated radiance or temperature values per pixel Correct answer.
  3. Thermal imaging requires no detector
  4. Thermal imaging uses only visible light

Explanation

Radiometric imaging provides calibrated quantitative values per pixel, while non-radiometric thermal imaging gives qualitative contrast. Calibration data are required for radiometric output.

Question 18Infrared Testing (IR) Method Fundamentals and Physics

What is the primary purpose of a reference blackbody source in an infrared measurement setup?

  1. To cool the detector
  2. To provide a known radiance for calibration Correct answer.
  3. To filter out long wavelengths
  4. To reflect background radiation

Explanation

A reference blackbody provides a known, calculable radiance at a controlled temperature, enabling calibration of the imaging system. It anchors measured signal to physical radiance.

Question 19Infrared Testing (IR) Method Fundamentals and Physics

Which effect explains why a hot object appears blurred at its edges in a thermal image beyond the optical resolution limit?

  1. The modulation transfer function of the system Correct answer.
  2. The Stefan-Boltzmann law
  3. Kirchhoff's law
  4. Wien's displacement law

Explanation

The modulation transfer function characterizes how spatial frequencies are attenuated by optics and detector sampling, causing edge softening. The other laws describe radiometric relationships, not spatial response.

Question 20Infrared Testing (IR) Method Fundamentals and Physics

An object at 400 K is imaged with a camera whose spectral band is 8 to 14 micrometers. Compared with an object at 300 K, the detected signal will be approximately how many times greater, assuming constant emissivity and using total blackbody exitance?

  1. About 1.0
  2. About 3.2 Correct answer.
  3. About 10
  4. About 1.3

Explanation

Total exitance scales with the fourth power of absolute temperature. The ratio (400/300)^4 is approximately 3.16, so the signal is about 3.2 times greater.

Question 21Infrared Testing (IR) Method Fundamentals and Physics

Which detector material system is commonly used for cooled mid-wave infrared photon detection?

  1. Indium antimonide Correct answer.
  2. Silicon photodiode optimized for visible light
  3. Gallium phosphide visible emitter
  4. Germanium visible photodiode

Explanation

Indium antimonide has a bandgap suited to mid-wave infrared photon detection and is typically cooled. Visible-optimized photodiodes respond at much shorter wavelengths.

Question 22Infrared Testing (IR) Method Fundamentals and Physics

Which statement about the emissivity of a polished metal compared with a painted metal surface is correct?

  1. The polished metal has higher emissivity and lower reflectivity
  2. The polished metal has lower emissivity and higher reflectivity Correct answer.
  3. Both surfaces have equal emissivity
  4. The painted surface has zero emissivity

Explanation

Polished metals are poor emitters and good reflectors, giving low emissivity. Painted surfaces typically have high emissivity and low reflectivity.

Question 23Infrared Testing (IR) Method Fundamentals and Physics

Which factor most limits the accuracy of temperature measurement through an infrared-transmitting window?

  1. Detector frame rate
  2. Window transmission losses and emission Correct answer.
  3. Camera battery level
  4. Lens focal length only

Explanation

A window attenuates target radiance and may add its own emission, so transmission and window temperature must be accounted for. Frame rate, power, and focal length do not directly create this bias.

Question 24Infrared Testing (IR) Method Fundamentals and Physics

What is the main consequence of using a detector with a narrow spectral band centered on an atmospheric absorption region?

  1. Increased spatial resolution
  2. Higher frame rate
  3. Reduced received signal from the target Correct answer.
  4. Greater detector cooling

Explanation

Operating within an absorption band attenuates target radiance before it reaches the detector, lowering signal. Spatial resolution, frame rate, and cooling are governed by other system parameters.

Question 25Infrared Testing (IR) Method Fundamentals and Physics

Which concept explains why a small hot target may be undetectable even when its radiance is high?

  1. The detector bandgap is too wide
  2. The atmosphere is fully transparent
  3. The target has zero radiance
  4. The target subtends less than one resolution element Correct answer.

Explanation

When a target occupies less than a resolution element, its energy is averaged with background, reducing contrast. This is a spatial resolution limitation rather than a radiance deficiency.

Question 26Infrared Testing (IR) Method Fundamentals and Physics

Which statement correctly describes the relationship between radiant exitance and radiance for a Lambertian surface?

  1. Radiant exitance equals radiance multiplied by the solid angle of the detector.
  2. Radiant exitance equals radiance multiplied by pi. Correct answer.
  3. Radiant exitance equals radiance multiplied by the cosine of the emission angle.
  4. Radiant exitance equals radiance divided by pi.

Explanation

For a Lambertian (diffuse) surface, the radiance is constant with angle, and integrating over the hemisphere yields the relation M = pi * L, where M is radiant exitance and L is radiance.

Question 27Infrared Testing (IR) Method Fundamentals and Physics

A blackbody is at a temperature of 1000 K. According to Wien's displacement law, its peak spectral radiant exitance occurs at approximately which wavelength?

  1. 29 micrometers
  2. 290 micrometers
  3. 0.29 micrometers
  4. 2.9 micrometers Correct answer.

Explanation

Wien's displacement law states that the peak wavelength in micrometers multiplied by the absolute temperature in kelvin is approximately 2898, so at 1000 K the peak is near 2.9 micrometers.

Question 28Infrared Testing (IR) Method Fundamentals and Physics

Which statement best describes the concept of emissivity for a real surface?

  1. It is the ratio of the radiation transmitted through the surface to that incident upon it.
  2. It is the ratio of the radiation absorbed by the surface to that emitted by a blackbody at the same temperature.
  3. It is the ratio of the radiation reflected by the surface to that incident upon it.
  4. It is the ratio of the radiance emitted by the surface to that emitted by a blackbody at the same temperature and wavelength. Correct answer.

Explanation

Emissivity is defined as the ratio of the spectral radiance emitted by a real surface to that of a blackbody at the same temperature and wavelength; it is a measure of how efficiently a surface emits thermal radiation.

Question 29Infrared Testing (IR) Method Fundamentals and Physics

In infrared thermography, what is the primary effect of the atmosphere on the measured temperature of a distant object?

  1. It changes the object's emissivity, making it appear more like a blackbody.
  2. It attenuates the object's infrared signal and adds its own emitted radiation, causing the apparent temperature to be lower than the true temperature. Correct answer.
  3. It amplifies the object's infrared signal, causing the apparent temperature to be higher than the true temperature.
  4. It shifts the peak wavelength of the object's emission to shorter wavelengths.

Explanation

The atmosphere absorbs and scatters infrared radiation and also emits its own radiation; the net effect is that the apparent temperature of a distant object is typically lower than its true temperature unless corrected for atmospheric transmission and path radiance.

Question 30Infrared Testing (IR) Method Fundamentals and Physics

Which detector type is commonly used in uncooled thermal imagers and operates by sensing temperature changes?

  1. Mercury cadmium telluride (MCT) photoconductor
  2. Microbolometer Correct answer.
  3. Platinum silicide (PtSi) Schottky barrier detector
  4. Indium antimonide (InSb) photodiode

Explanation

Microbolometers are thermal detectors that change electrical resistance with temperature and are widely used in uncooled thermal imagers operating in the long-wave infrared.

Question 31Infrared Testing (IR) Method Fundamentals and Physics

For a photon detector, what is the effect of cooling the detector on its performance?

  1. It increases the detector's quantum efficiency.
  2. It increases the detector's spectral response range.
  3. It decreases the detector's response time.
  4. It reduces thermally generated noise, improving the signal-to-noise ratio. Correct answer.

Explanation

Cooling a photon detector reduces the thermal generation of charge carriers, which lowers dark current and noise, thereby improving the signal-to-noise ratio.

Question 32Infrared Testing (IR) Method Fundamentals and Physics

Which of the following best describes the purpose of a cold shield in a cooled infrared detector assembly?

  1. To limit the detector's field of view and reduce the amount of background radiation reaching the detector. Correct answer.
  2. To increase the detector's sensitivity by concentrating incoming radiation.
  3. To filter out specific wavelengths of infrared radiation.
  4. To protect the detector from physical damage during operation.

Explanation

A cold shield is a cooled aperture that restricts the detector's field of view, thereby reducing the background photon flux and associated noise, which improves the detector's performance.

Question 33Infrared Testing (IR) Method Fundamentals and Physics

In infrared imaging, what does the term 'NETD' stand for?

  1. Noise Equivalent Thermal Detectivity
  2. Normalized Emissivity Temperature Difference
  3. Noise Equivalent Temperature Difference Correct answer.
  4. Nominal Energy Transmission Density

Explanation

NETD is the noise equivalent temperature difference, a measure of the smallest temperature difference that can be resolved by a thermal imager, limited by the system noise.

Question 34Infrared Testing (IR) Method Fundamentals and Physics

A thermal imager has a detector array with 640 by 480 elements and a field of view of 30 degrees horizontal. What is the approximate instantaneous field of view (IFOV) per detector element in milliradians?

  1. 0.082
  2. 82
  3. 8.2
  4. 0.82 Correct answer.

Explanation

The IFOV is the field of view divided by the number of detector elements in that direction: 30 degrees = 0.5236 radians, divided by 640 gives approximately 0.000818 radians, or 0.82 milliradians.

Question 35Infrared Testing (IR) Method Fundamentals and Physics

Which of the following statements about the spectral response of a photon detector is correct?

  1. It is independent of the detector material.
  2. It is uniform across all infrared wavelengths.
  3. It has a long-wavelength cutoff determined by the material's bandgap energy. Correct answer.
  4. It is limited only by the optics and not by the detector material.

Explanation

Photon detectors respond to photons with energy greater than the bandgap, so there is a long-wavelength cutoff corresponding to the bandgap energy; longer wavelengths do not have enough energy to excite carriers.

Question 36Infrared Testing (IR) Method Fundamentals and Physics

What is the primary advantage of using a thermal detector over a photon detector in infrared thermography?

  1. It has a faster response time.
  2. It is more sensitive to short-wavelength infrared.
  3. It has higher detectivity.
  4. It can operate at room temperature without cooling. Correct answer.

Explanation

Thermal detectors respond to the heating effect of absorbed radiation and generally do not require cryogenic cooling, which simplifies system design and reduces cost.

Question 37Infrared Testing (IR) Method Fundamentals and Physics

A blackbody cavity is often used as a reference source in infrared radiometry. What is the key property that makes it useful for this purpose?

  1. Its emissivity is nearly unity and is relatively insensitive to wavelength and viewing angle. Correct answer.
  2. Its surface is highly reflective, providing a stable signal.
  3. Its temperature can be measured with a thermocouple, ensuring accuracy.
  4. Its emissivity varies strongly with wavelength, allowing spectral calibration.

Explanation

A blackbody cavity approximates an ideal emitter with emissivity close to 1, and its emission is well described by Planck's law, making it a reliable reference for calibration.

Question 38Infrared Testing (IR) Method Fundamentals and Physics

Which of the following materials is commonly used as a window for infrared cameras operating in the 8 to 14 micrometer band?

  1. Germanium Correct answer.
  2. Acrylic plastic
  3. Borosilicate glass
  4. Fused quartz

Explanation

Germanium is transparent in the long-wave infrared (8–14 micrometers) and is commonly used for IR windows and lenses; it is opaque to visible light.

Question 39Infrared Testing (IR) Method Fundamentals and Physics

When using an infrared camera to measure the temperature of a target through an infrared-transmitting window, which factor most significantly affects the accuracy of the temperature reading?

  1. The window's temperature and its own emission and reflection. Correct answer.
  2. The window's refractive index.
  3. The window's surface roughness.
  4. The window's thickness.

Explanation

An infrared window emits and reflects radiation depending on its temperature and emissivity; these contributions add to or subtract from the target's signal, causing errors if not accounted for.

Question 40Infrared Testing (IR) Method Fundamentals and Physics

In infrared thermography, what does the term 'apparent temperature' refer to?

  1. The temperature of the camera's detector array.
  2. The temperature indicated by the camera without correction for emissivity and atmospheric effects. Correct answer.
  3. The temperature measured by a contact thermometer at the same location.
  4. The temperature of the background surrounding the object.

Explanation

Apparent temperature is the temperature that the camera computes assuming a blackbody target and no atmospheric attenuation; it differs from true temperature when emissivity is less than 1 or when the atmosphere affects the signal.

Question 41Infrared Equipment, Detectors, and System Performance

Which detector type responds to incident infrared radiation by a change in electrical resistance without requiring a bias voltage to generate its signal?

  1. Microbolometer Correct answer.
  2. Thermopile
  3. Photon (quantum) detector
  4. Pyroelectric detector

Explanation

A microbolometer is a thermal detector whose active material changes resistance as it warms from absorbed IR; the resistance change is read by bias current but the signal originates from the temperature rise. Photon detectors produce electron-hole pairs directly from photon energy, and thermopiles and pyroelectric devices use thermoelectric or polarization effects rather than a simple resistance change.

Question 42Infrared Equipment, Detectors, and System Performance

A photon detector must generally be cooled because it operates by:

  1. thermoelectric voltage that reverses sign above room temperature
  2. direct conversion of absorbed photons into charge carriers, which thermal excitation can swamp Correct answer.
  3. pyroelectric polarization that vanishes above room temperature
  4. mechanical deflection of a bimaterial strip, which expands with heat

Explanation

Photon detectors rely on incident photons exciting carriers across a band gap; at elevated temperature, thermally generated carriers create noise that masks the signal, so cooling reduces thermal excitation. Thermal detectors such as bolometers and thermopiles generally operate uncooled.

Question 43Infrared Equipment, Detectors, and System Performance

A thermal imaging system has an IFOV of 1.0 mrad and is viewing a target 500 m away. What is the approximate projected pixel size on the target?

  1. 0.50 m Correct answer.
  2. 5.0 m
  3. 0.05 m
  4. 50 m

Explanation

The projected spot size equals IFOV times range: 1.0 mrad times 500 m equals 0.5 m. IFOV is expressed in radians, so the result is in the same length unit as the range.

Question 44Infrared Equipment, Detectors, and System Performance

The Noise Equivalent Temperature Difference (NETD) of a thermal imager is best described as:

  1. the target-to-background temperature difference at which the signal equals the system noise Correct answer.
  2. the maximum scene temperature the detector can record before saturating
  3. the angular resolution of a single detector element
  4. the total number of detector elements in the focal plane array

Explanation

NETD quantifies temperature sensitivity: it is the temperature difference producing a signal equal to the RMS noise, so lower NETD means better sensitivity. It is a temperature-difference figure, not a saturation limit, array count, or angular measure.

Question 45Infrared Equipment, Detectors, and System Performance

In a thermal imaging system, the function of the cold stop (cold shield) in a cooled detector assembly is to:

  1. mechanically protect the detector from vibration
  2. limit the detector's field of view to cold surroundings to reduce background noise Correct answer.
  3. increase the spectral bandwidth reaching the detector
  4. filter out visible light while passing long wavelengths

Explanation

The cold stop restricts the cone of radiation reaching the cooled detector so it sees only cold surfaces inside the dewar, minimizing stray background flux and noise. It is an aperture/field-limiting element, not a spectral filter or mechanical damper.

Question 46Infrared Equipment, Detectors, and System Performance

A staring focal plane array with 320 by 240 pixels and a 20-degree horizontal field of view has an approximate horizontal IFOV of:

  1. 1.09 mrad Correct answer.
  2. 2.5 mrad
  3. 0.0625 mrad
  4. 0.625 mrad

Explanation

Horizontal IFOV equals the horizontal field of view divided by the number of horizontal pixels: 20 degrees (0.349 rad) divided by 320 equals about 1.09 mrad. Dividing by the wrong axis or misplacing the decimal gives the other choices.

Question 47Infrared Equipment, Detectors, and System Performance

Which statement correctly distinguishes a cooled photon detector from an uncooled microbolometer in terms of response speed?

  1. Microbolometers respond faster because they have no thermal mass
  2. Photon detectors generally respond faster because carrier generation is nearly instantaneous Correct answer.
  3. Uncooled detectors respond faster because they avoid dewar thermal lag
  4. Both respond at identical speeds because both are limited by the optics

Explanation

Photon detectors convert absorbed photons to carriers on very short timescales, enabling high frame rates, while thermal detectors such as microbolometers are limited by the thermal time constant of the sensing element. Thermal mass slows, rather than speeds, the response of microbolometers.

Question 48Infrared Equipment, Detectors, and System Performance

The spectral response band of an indium antimonide (InSb) photon detector is approximately:

  1. 8 to 14 micrometers
  2. 0.4 to 0.7 micrometers
  3. 15 to 25 micrometers
  4. 1 to 5 micrometers Correct answer.

Explanation

InSb is a mid-wave infrared photon detector material responding in roughly the 1 to 5 micrometer band, typically cooled with liquid nitrogen. The 8 to 14 micrometer band corresponds to long-wave materials, and the shorter and longer options lie outside InSb response.

Question 49Infrared Equipment, Detectors, and System Performance

A thermal imager's minimum resolvable temperature difference (MRTD) is measured by:

  1. observing a series of standard four-bar patterns of decreasing size while adjusting target-to-background temperature Correct answer.
  2. recording the maximum scene temperature before pixel saturation
  3. counting the number of dead pixels in the focal plane array
  4. measuring the detector output with a uniform blackbody and computing RMS noise

Explanation

MRTD is a subjective system-level figure obtained by an observer resolving four-bar targets of varying spatial frequency as the temperature difference is reduced to threshold. It combines resolution and sensitivity, unlike pure noise or saturation measurements.

Question 50Infrared Equipment, Detectors, and System Performance

An infrared detector's D-star (specific detectivity) figure of merit is used primarily to:

  1. define the maximum operating temperature of the detector
  2. express detector sensitivity normalized for area and electrical bandwidth Correct answer.
  3. state the physical size of the detector element
  4. specify the optical aperture diameter of the system

Explanation

D-star normalizes detectivity to unit detector area and unit bandwidth, allowing comparison of detectors of different sizes and readout bandwidths. It is a sensitivity figure of merit, not a dimensional, thermal-limit, or aperture specification.