Question

Pyrheliometer is an instrument used for measuring

• A. Diffuse radiation
• B. Direct radiation
• C. Both direct and diffuse radiation
• D. Global radiation

Hint:

Pyrheliometer = Direct (Beam) radiation.
Pyranometer = Global (Direct + Diffuse) radiation.
Sunshine Recorder = Duration of bright sunshine hours.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Solar radiation reaching the earth's surface comprises direct beam radiation, diffuse scattered radiation, and reflected radiation. Specialized radiometers are tailored for specific components.
Key Formula or Approach:
\[ I_{\text{global}} = I_{\text{direct, beam}} \cdot \cos \theta_z + I_{\text{diffuse}} \]

Step 2: Detailed Explanation:
1. Pyrheliometer: Uses a collimating tube with a narrow field of view (typically \(5.7^\circ\)) mounted on a two-axis solar tracker to measure direct beam solar radiation at normal incidence.
2. Pyranometer: Has a hemispherical glass dome with a \(180^\circ\) field of view to measure global solar radiation (direct plus diffuse radiation).
3. Shaded Pyranometer: Uses a shadow ring/disk to block beam radiation and measure diffuse radiation alone.

Step 3: Final Answer:
Therefore, a pyrheliometer is used for measuring Direct radiation, corresponding to option (B).