Quick answer: Haze is the percentage of transmitted light that deviates from the incident beam by more than 2.5 degrees because of wide-angle scattering. It is calculated as diffuse transmittance divided by total transmittance, times 100. A high haze number means the material looks cloudy or milky; it does not necessarily mean the material transmits less light. Always state which standard the haze value follows, because the angle threshold is part of the definition.
Haze, clarity and transmittance are three different numbers
| Quantity | What it counts | Typical question it answers |
|---|---|---|
| Total luminous transmittance | All light passing through the sample | How much light gets through? |
| Haze | Light scattered wider than the standard’s threshold (2.5° in ASTM D1003) | Does the material look cloudy? |
| Clarity | Narrow-angle scattering just around the incident beam | Do details stay sharp when viewed through the material? |
A film can pass most of the light and still look hazy; it can also show low haze and poor clarity. Reporting only one of the three leaves the optical picture incomplete. For the wider context of optical quantities, see our optical measurement overview.
Why the standard is part of the result
ASTM D1003 defines haze through wide-angle scattering beyond 2.5° from the incident beam and is written for essentially transparent plastics. ISO 14782:2021, the international counterpart for transparent plastics, uses a different scattering threshold, so the same specimen does not automatically produce the same number on both scales. ASTM D1003 itself notes that it is not equivalent to ISO 13468-1 and ISO 14782. Practical consequences:
- Quote the standard and edition with every haze value, for example “haze 2.4 % per ASTM D1003-21”;
- Do not compare two haze figures taken under different standards without recalculating;
- For contractual work, follow the standard the specification names — not the one the instrument happens to use.
How a haze meter reaches the number
Both the haze meter (Procedure A in ASTM D1003) and the spectrophotometer route (Procedure B) use an integrating sphere. The sphere collects the transmitted light uniformly; with the specular beam trapped, the detector reads only the scattered component, and the ratio of that component to total transmittance gives haze. A few points from the standard worth knowing:
- Procedure A haze values are normally slightly higher and less variable than Procedure B values;
- The specimen is placed at the sphere entrance port so that total hemispherical transmittance is measured — not the regular transmittance of a clear window;
- Material with haze above 30 % counts as diffusing and should be tested by other means; ISO 14782 covers haze values below 40 %;
- Abraded or matted surfaces can read lower than the true haze because some scattering stays near the incident beam (ISO 14782 note).
Sample handling and what to record
- Use flat, planar specimens representative of the material; a single scratch or moulding defect can dominate the reading.
- Condition and measure the sample the way the specification says; surface contamination is read as haze.
- Record specimen identity, conditioning, instrument, standard and edition, and individual readings.
- State the illumination basis when it matters — ASTM D1003 and the ISO methods do not use identical illuminants.
A disciplined record template keeps optical results auditable in the same way thickness records do.
Common misconceptions
- “Haze means light loss” — a hazy sample can still show high total transmittance; haze describes how the light spreads, not how much survives.
- “Haze and clarity are the same” — they split at the scattering angle; a material can pass one and fail the other.
- “Haze meter and turbidity meter are interchangeable” — haze is defined for solid transparent materials in percent; turbidity describes liquids in NTU-like scales.
- “One haze number fits every report” — without the standard and edition, the number is not comparable.
Technical sources
Related instruments: DONGRU DR-W300 Haze Meter · Request a quote