Refractive index
Current AP PSL pages describe approximately 1.59 at 589 nm and 25°C; the exact response at the instrument wavelength requires the appropriate model.
Optical response
A particle counter does not directly see geometric diameter
A light-scattering instrument detects optical interaction and maps that response to an equivalent size using its calibration model. Material refractive index, wavelength, medium and collection geometry influence that response.
Medium
The contrast between particle and surrounding medium changes scattering.
Shape
Spherical PSL reduces orientation effects, but real process particles may be irregular.
Size parameter
Scattering behavior changes with diameter relative to wavelength.
Equivalent size
Calibration material defines the reporting convention
ISO 21501-2 explains that the reported size from a light-scattering liquid counter depends on particle and liquid refractive indices and is equivalent to the calibration particles in pure water. This is why a PSL-equivalent diameter may not equal the geometric diameter of a process particle with different optical properties.
Interpretation boundary: do not convert a PSL response into a universal geometric truth without an optical model and method-specific evidence.
PSL vs process particles
Use a consistent reference, then understand the difference
| Attribute | PSL reference sphere | Typical process particle |
|---|---|---|
| Shape | Spherical | Often irregular |
| Composition | Polystyrene | Metal, oxide, organic, biological or mixed |
| Refractive index | Known/reference value | Material and wavelength dependent |
| Distribution | Controlled and documented | Often broad or unknown |
| Purpose | Instrument reference or test material | Actual contamination or sample population |
Selection consequences
Ask the optical questions before ordering
- What wavelength and detector geometry does the instrument use?
- What medium surrounds the particles?
- Is the reported result PSL-equivalent size or another convention?
- What size nodes are needed to characterize the response curve?
- Does the procedure require PSL specifically?
- How will real-particle morphology and refractive index affect interpretation?
Applied Physics technical pathway
Match the standard to the measurement—not the other way around.
Send the instrument, medium, target size, concentration, certificate and packaging requirements to Applied Physics for a technical recommendation.