---
title: "Solar radiation measuring instruments"
subject: "Energy Resources"
chapter: 2
key: "semester_6/energy_resources:pyranometer-pyrheliometer-sunshine-recorder"
source: https://www.bctengineer.com/q/semester_6/energy_resources/pyranometer-pyrheliometer-sunshine-recorder
site: BCT Engineering
---

# Solar radiation measuring instruments

Energy Resources · Chapter 2: Solar energy

- Asked in: year not given, 5 marks
- Video: [Solar Radiation Measurements | Renewable Energy Sources](https://www.youtube.com/watch?v=tDbjRyIRe6Y) — Engineering Xplained
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/energy_resources/pyranometer-pyrheliometer-sunshine-recorder
- Whole chapter: https://www.bctengineer.com/md/semester_6/energy_resources/chapter-2

## Question

Describe the working of a pyranometer, a pyrheliometer and a sunshine recorder. State what each one measures.

## Short answer

A pyranometer uses a thermopile under glass domes to measure global (beam+diffuse) radiation. A pyrheliometer has a collimated tube and thermopile to measure direct beam radiation. A Campbell-Stokes sunshine recorder uses a glass sphere to burn a card, recording sunshine duration in hours.

## Answer

Solar radiation instruments convert incident radiation into a measurable signal, usually heat or voltage.

#### Pyranometer
Measures **global radiation** (beam + diffuse) on a horizontal surface; with a shade ring or disc it measures diffuse radiation only.

The Eppley type has a black and white sensing surface (thermopile) under two glass domes. Black parts absorb radiation and get hotter than the white parts, so the thermopile produces a small voltage (about 10 µV per W/m²) proportional to the radiation. The domes protect from wind and rain, and a desiccant keeps them dry. A photodiode type is cheaper but spectrally selective.

```text
   glass domes
     ____
    /    \
   |B|W|B|W|  <- thermopile hot (black)
   |______|      and cold (white) junctions
   heat sink --> voltage out
```

#### Pyrheliometer
Measures **beam (direct) radiation** at normal incidence. A long collimating tube with a narrow field of view (about 5–6°) is kept pointed at the sun by a tracker. The radiation falls on a thermopile or a blackened receiver. In the Angstrom compensation type, one of two strips is shaded and heated electrically until the temperatures equal; the electrical power then equals the absorbed radiation. It is also the reference instrument for calibration.

#### Sunshine recorder
Measures the **duration of bright sunshine** (hours), not intensity. In the Campbell-Stokes recorder a solid glass sphere focuses the sun's rays onto a card strip; the burn trace length gives the sunshine hours. The data are used with Angstrom regression $\bar H/\bar H_0 = a + b\,(\bar n/\bar N)$ to estimate radiation where no pyranometer is available.

| Instrument | Measures | Principle |
|---|---|---|
| Pyranometer | Global/diffuse, W/m² | Thermopile |
| Pyrheliometer | Beam, W/m² | Collimated thermopile |
| Sunshine recorder | Hours of sunshine | Burn trace |

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does a pyranometer measure, and how can it measure diffuse only?
   A. Beam radiation; a lens focuses the sun
   B. Global radiation; a shade ring or disc blocks the beam
   C. Sunshine hours; a glass sphere burns a card
   D. Global radiation; the dome is removed
   Answer: B. The thermopile gives global radiation, and shading the direct sun leaves diffuse. (Key point: Pyranometer)

2. How does a pyrheliometer measure beam radiation?
   A. A collimating tube with a narrow field of view is kept pointed at the sun by a tracker
   B. A horizontal thermopile under two domes
   C. A glass sphere focusing on a card
   D. A shade ring blocking the sky
   Answer: A. Narrow field of view (about 5-6°) admits only direct radiation at normal incidence. (Key point: Pyrheliometer)

3. What does the Campbell-Stokes recorder give?
   A. Intensity of radiation in W/m²
   B. Diffuse radiation in W/m²
   C. Sun's declination
   D. Duration of bright sunshine in hours from the burn-trace length
   Answer: D. It measures hours, not intensity, and the data are used with Angstrom regression to estimate radiation. (Key point: Sunshine recorder)

## Related in Energy Resources

- [Energy audit instruments and energy balance](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Aenergy-audit-instruments-measurements) — Energy Resources, chapter 9
- [Earth's motion, solar constant and seasons](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Aearth-sun-geometry-solar-constant) — Energy Resources, chapter 2
- [Flat plate collector and solar water heater](https://www.bctengineer.com/question.md?key=semester_6%2Fenergy_resources%3Aflat-plate-collector-construction-water-heater) — Energy Resources, chapter 2

## Related in other subjects

- [Solar radiation, solar cell and plant](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-radiation-cell-plant) — Energy Environment and Society, chapter 3
- [Importance of solar radiation, humidity, evapotranspiration](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Aimportance-solar-rh-et) — Engineering Hydrology, chapter 3
- [Working of a solar cell](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Asolar-cell-working) — Energy Environment and Society, chapter 3
- [Types of frequency meter and one working](https://www.bctengineer.com/question.md?key=semester_4%2Finstrumentation_i%3Afrequency-meter-types-working) — Instrumentation I, chapter 6