(Not available anywhere else)

This sheet assumes the Level 1 fundamentals are already known.
It focuses only on the additional concepts tested at Level 2.

Specific Heat

Specific heat is the amount of heat energy required to raise the temperature of:

1 gram of a material by 1°C

Different materials require different amounts of energy to produce the same temperature change.

Key concept

Higher specific heat → more energy required to change temperature.

Water has a relatively high specific heat, so it heats and cools more slowly than many metals.

Heat Energy Calculation

Use:

Q = m × c × ΔT

Where:

• Q = heat energy, Joules

• m = mass, grams

• c = specific heat, J/g°C

• ΔT = temperature change, °C

Example

200 g of water:

30°C → 90°C

ΔT = 60°C

Q = 200 × 4.18 × 60

Q = 50,160 J

Useful number

Heating 1 kg of water by 1°C:

1000 × 4.18 × 1 = 4180 J

Thermal Capacitance

Thermal capacitance refers to the amount of thermal energy an object can store.

It depends mainly on:

• mass

• specific heat

Conceptually:

Thermal Capacitance = mass × specific heat

Higher thermal capacitance means the object generally:

• requires more energy to change temperature

• changes temperature more slowly

• can store more thermal energy

Do not confuse:

Specific Heat: Property of the material.

Thermal Capacitance: Thermal storage capability of the whole object.

Thermal Resistance

Thermal resistance describes how strongly a material resists the flow of heat through it.

Higher resistance → less heat transfer.

Lower resistance → greater heat transfer.

It is affected by factors such as:

• material thermal conductivity

• material thickness

• area through which heat travels

Celsius to Kelvin

Radiation calculations require absolute temperature.

Use:

K = °C + 273

More precisely:

K = °C + 273.15

Example

90°C:

90 + 273 = 363 K

Why Kelvin matters

Infrared radiation laws depend on absolute temperature rather than relative Celsius temperature.

Wien’s Law Calculations

Wien’s Law relates temperature to the wavelength where the object emits its maximum amount of radiation.

λmax = b / T

Where:

• λmax = peak wavelength

• T = temperature in Kelvin

• b ≈ 2898 µm·K

Example:

70°C object

Convert first:

70 + 273 = 343 K

Then:

2898 / 343 ≈ 8.45 µm

Remember

Higher temperature → shorter peak wavelength.

Lower temperature → longer peak wavelength.

For the test, know how to calculate the wavelength, not only the general principle.

MRTD

MRTD = Minimum Resolvable Temperature Difference

MRTD describes the ability of the complete imaging system to distinguish a temperature difference between small spatial features.

It combines aspects of:

• thermal contrast

• spatial resolution

• image-system performance

Think of it as:

Can I actually distinguish two small features that have only a small temperature difference?

This is different from simply detecting that some temperature difference exists.

NETD vs MRTD

Although NETD is introduced at Level 1, Level 2 requires understanding its relationship with MRTD.

Simple memory aid

NETD = Detect

MRTD = Resolve

A camera can theoretically detect a small temperature difference but may not necessarily resolve it clearly on a small target.

Target Size, Distance & IFOV Application

At Level 2, IFOV becomes a measurement problem, not merely a definition.

The target must occupy enough detector pixels for reliable temperature measurement.

Approximate relationship:

Target size ≈ Distance × IFOV

Practical consequence

As distance increases:

• each pixel covers a larger physical area

• small targets occupy fewer pixels

• temperature measurements become less reliable

If the target is too small: move closer.

Focusing the camera better does not compensate for insufficient detector coverage.

Important principle

Being able to see an object does not necessarily mean you can accurately measure it.

Measurement IFOV vs Detection IFOV

For temperature measurement, the target normally needs to cover more than one detector pixel.

This is why:

• detection distance

• recognition distance

• accurate measurement distance

are not the same thing.

A hot object may be visible at a large distance while its reported temperature is still inaccurate because surrounding pixels influence the measurement.

Cavity Effect

A cavity can have an effective emissivity higher than the original surface emissivity.

Radiation entering the cavity undergoes repeated:

• reflections

• absorption

• re-emission

before leaving the opening.

This causes the cavity to behave more like a blackbody.

Practical implication

A hole or recess may appear thermally different from the surrounding flat surface even when both are:

• the same material

• at similar actual temperatures

Key idea

Geometry itself can affect apparent emissivity.

Effect of Viewing Angle on Emissivity

Level 2 goes beyond simply avoiding reflections.

As the viewing angle becomes increasingly oblique:

• apparent emissivity can decrease

• apparent reflectivity can increase

• measured temperature can become less reliable

For accurate quantitative measurements, viewing close to normal/perpendicular is preferred whenever practical.

Important

The material’s emissivity should not always be treated as one fixed number independent of:

• angle

• wavelength

• temperature

• surface condition

• camera spectral response

Emissivity vs Target Temperature Practical

Changing the emissivity setting changes the camera’s calculated target temperature.

The effect becomes particularly significant when:

• target emissivity is low

• target temperature differs substantially from reflected surroundings

General relationship

For a given amount of radiation reaching the camera:

Lower entered emissivity
→ camera attributes less radiation to the target itself
→ calculated target temperature generally increases.

Level 2 lesson

Do not treat emissivity merely as a number to enter into the camera.

Understand how sensitive the final result is to the assumption being made.

Reflected Apparent Temperature — Measurement Importance

For quantitative measurements, the camera receives radiation consisting partly of:

target emission + reflected environmental radiation

The lower the emissivity:

→ the greater the reflected component.

Therefore Reflected Apparent Temperature becomes increasingly important for low-emissivity targets.

For high-emissivity surfaces in moderate environments, its effect may be comparatively small.

Practical lesson

Do not adjust RAT blindly for every inspection.

Assess whether reflected radiation is actually significant to the measurement uncertainty.

Slit Response Function

Slit Response Function (SRF) demonstrates how target size influences the temperature reported by an infrared camera.

A controlled thermal target is viewed through a slit.

As slit width increases:

• more detector pixels are filled by the target

• influence from the surrounding background decreases

• indicated temperature approaches the true target temperature

What it demonstrates

A camera may:

1. Detect an object

2. Display the object

3. Yet still be unable to accurately measure its temperature

until the target occupies sufficient detector area.

Connection to field inspections

Particularly important for:

• wires

• small terminals

• distant components

• narrow pipes

• small electrical contacts

Lens Filters

An infrared filter allows selected infrared wavelengths to reach the detector while reducing or blocking others.

Filters can be used for specialised applications such as:

• high-temperature measurement

• furnaces

• process monitoring

• viewing through particular materials

• suppressing unwanted radiation bands

Important principle

The filter must be matched with:

• the target

• required wavelength

• camera detector

• camera spectral range

Adding a filter changes the amount and wavelength distribution of radiation reaching the detector.

Infrared Windows — Measurement Considerations

An IR window allows infrared radiation to pass through an enclosure while keeping the enclosure physically closed.

For quantitative temperature measurement, the window introduces another factor: transmittance

Radiation reaching the camera may now be affected by:

• target emissivity

• reflected radiation

• window transmission

• window temperature

• spectral response

Important

Seeing the target through an IR window does not automatically mean its temperature is correct.

The transmission characteristics of the window need to be considered.

Qualitative vs Quantitative Thermography

Qualitative Thermography

Concerned mainly with:

thermal patterns and relative differences

Questions include:

• Where is the anomaly?

• What thermal pattern is abnormal?

• How does it compare with surrounding areas?

Precise absolute temperature may not be necessary.

Quantitative Thermography

Concerned with:

temperature measurement

Greater attention is therefore required for:

• emissivity

• reflected apparent temperature

• target size

• distance

• transmittance

• atmospheric effects

• camera accuracy

Easy distinction

Qualitative = Pattern

Quantitative = Measurement

Camera Image Clarity — System Approach

A clear thermal image is not determined by detector resolution alone.

Image performance depends on the complete imaging system, including:

• detector

• optics/lens

• electronics

• display

• thermal sensitivity

• spatial resolution

• focus

Level 2 principle

Do not judge camera performance using a single specification.

A camera with excellent NETD but poor spatial resolution may still be unsuitable for a small target.

Likewise, very high detector resolution does not automatically guarantee good thermal sensitivity.

Measurement Before Interpretation

The thermographer must first establish whether the apparent thermal condition is real and measurable.

Before interpreting a thermal anomaly, consider:

Target

• size

• material

• emissivity

• geometry

Camera

• focus

• detector resolution

• measurement spot size

• spectral range

Environment

• reflected radiation

• atmospheric effects

• wind

• external heat sources

Equipment

• operating state

• load

• expected thermal behaviour

Only after these factors are considered should the thermographer determine the likely significance of the anomaly.

HIGH-PRIORITY FORMULAS & NUMBERS

LAST-MINUTE LEVEL-2 MEMORY MAP

THERMAL MATERIAL PROPERTIES

Specific Heat → Energy required to change material temperature

Thermal Capacitance → Energy storage capability of the whole object

Thermal Resistance → Resistance to heat flowing through material

CAMERA PERFORMANCE

NETD → Can the detector sense the ΔT?

MRTD → Can the imaging system resolve the ΔT as a feature?

IFOV → How much target does one pixel see?

SRF → How target size affects the measured temperature

QUANTITATIVE MEASUREMENT

Accurate temperature depends on:

Target + Camera + Environment

Pay particular attention to:

• emissivity

• reflected apparent temperature

• target size

• viewing angle

• transmission

• distance