Measurement of Physical Quantities

A measurand is the physical quantity intended to be determined (commonly referred to in metrology as the “measurand”). Only when it is clearly defined what exactly is being measured – for example, the length of an edge at a specified location, the temperature of a surface at a defined emissivity, or the profile along a laser line – does the measurement become reproducible.

A measurement result consists of a numerical value and a unit; formally, it can be written as:

x = {x} · [x]

Transduction describes the conversion of a physical quantity into an evaluable signal (usually electrical).

Optical systems, for example, convert geometric information via light projection and camera imaging into pixel coordinates. Thermal systems detect infrared radiation and derive temperature values from it.

The measurement chain includes the sensor, signal conditioning (amplification/filtering), digitization (ADC), evaluation, and, if necessary, compensation. Each element of the measurement chain contributes to measurement uncertainty.

Physical quantities are defined within the International System of Units (SI). The base quantities include:

• Length in meters (m)
• Time in seconds (s)
• Temperature in kelvin (K)

Derived quantities include:

• Velocity v = s / t
• Acceleration a = Δv / Δt
• Frequency f = 1 / T
• Force F = m · a
• Pressure p = F / A

In geometric inspection, additional characteristic quantities include:

• Angle
• Form deviation
• Roughness

In thermography, relevant quantities include:

• Temperature fields
• Gradients

In practice, this means that every measurand becomes accessible only through an interaction between the measurement object and the measurement system. This interaction is implemented by a measurement principle that converts the quantity into an evaluable signal – the process of transduction.

Optical sensing relies on light–matter interactions (reflection, scattering, imaging), while thermal sensing utilizes electromagnetic radiation in the infrared range.