Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D4039-09 – Standard Test Method Technical Guide
🧪 Scope and Principle of Reflection Haze
ASTM D4039-09 (Reapproved 2023) defines a standard test method for evaluating the reflection haze of high-gloss, nonmetallic surfaces. This method is particularly useful for assessing the clarity of clear finishes applied to nonglossy substrates and the distinctness of images reflected by opaque, glossy pigmented finishes. The test relies on two simple specular gloss measurements to compute a single numerical haze index (H).
As defined in Section 3 of the standard, haze in reflection refers to the scattering of light at the glossy surface of a specimen, which is directly responsible for the apparent reduction in contrast of objects viewed by reflection. Section 5.2 notes that this scattering is caused by microscopic irregularities in the reflecting surface that affect the distribution of flux around the specular angle.
💡 Key Application: This method is a cornerstone quality control tool for industries requiring high-appearance surfaces, such as automotive clear coats, high-gloss plastics, and optical coatings. The simple index allows for quick pass/fail analysis during production.
⚙️ Test Procedure and Haze Index Calculation
The procedure is straightforward. First, the specular gloss of the specimen is measured using the 60° geometry (G60) specified in Test Method D523. Second, the same specimen is measured using the 20° geometry (G20) from the same standard. The Haze Index (H) is then calculated using the formula from Section 3.2.3:
H = G60 − G20
| 🟦 Feature | 📏 60° Gloss (G60) | 📐 20° Gloss (G20) |
|---|---|---|
| Angle of Incidence | 60° | 20° |
| Primary Range | Medium to High Gloss | High Gloss |
| Sensitivity to Haze | Lower | Higher |
| Refractive Index Sensitivity | Lower | Higher (see Section 4.3) |
| Role in Index (H) | Baseline gloss level | Haze-sensitive component |
This test method is applicable only to specimens having a 60° specular gloss value greater than 70 (per Section 4.2). If the gloss is lower than this threshold, the test loses sensitivity to haze. Proper specimen selection per Practice D3964 is also critical for valid results.
📊 Significance and Critical Limitations
The Haze Index provides a practical measure of the scattering of light around the specular angle. A perfectly smooth high-gloss surface will have nearly identical G60 and G20 values (H near zero). As surface irregularities increase, the 20° gloss (G20) drops more rapidly than the 60° gloss (G60), resulting in a higher calculated Haze Index.
⚠️ Important Limitation – Refractive Index (Section 4.3): The Haze Index (H) is not a universal material constant. Because G20 changes much more rapidly with refractive index than G60, the calculated value of H is inherently dependent on the material’s refractive index. The standard explicitly states that valid comparisons of reflection haze by this method are strictly limited to specimens of essentially the same refractive index.
For a deeper analysis of the angular distribution of scattered light, Test Method E430 (Abridged Goniophotometry) is the preferred reference standard. Uniform film thickness for test panels can be achieved using Practices D823.
❓ Frequently Asked Questions
🔍 What is the Haze Index (H)?
The Haze Index is a computed value representing the light scattering properties of a glossy surface, calculated as the difference between the 60° specular gloss (G60) and the 20° specular gloss (G20) as defined in Section 3.2.3. A high haze index indicates a “milky” or “hazy” appearance in reflected images, typically