**Explanation:**

The instrument used to measure the heat energy absorbed by a known area in a fixed time is called a **pyrheliometer**.

**Pyrheliometer:**

A pyrheliometer is a scientific instrument used to measure the direct solar radiation received from the Sun at a given location on the Earth's surface. It is specifically designed to measure the solar energy or heat energy absorbed by a known area in a fixed time.

**Working Principle:**

The working principle of a pyrheliometer is based on the measurement of the amount of solar radiation incident on a thermopile, which is a device that generates an electric current when exposed to heat. The thermopile is located at the focal point of the instrument and absorbs the solar radiation.

**Components:**

A pyrheliometer consists of the following components:

1. **Thermopile:** It is the main component of the pyrheliometer that absorbs the solar radiation and generates an electric current.

2. **Blackened Receiver:** The thermopile is housed in a blackened receiver to enhance its absorption of solar radiation.

3. **Temperature Sensors:** Pyrheliometers are often equipped with additional temperature sensors to measure the ambient temperature and the temperature of the instrument itself. These measurements are used to correct the readings obtained from the thermopile.

4. **Data Logger:** A data logger is used to record and store the measurements obtained from the pyrheliometer. It can be connected to a computer or other storage device for further analysis.

**Measurement of Heat Energy:**

By measuring the electric current generated by the thermopile, the pyrheliometer can quantify the amount of solar radiation incident on its surface. This current measurement is directly proportional to the heat energy absorbed by the known area in a fixed time.

Therefore, the instrument used to measure the heat energy absorbed by a known area in a fixed time is a pyrheliometer.