Understand the question

We need to identify the instrument that detects infrared radiation.

Recall the functions of the given instruments

• Spectrometer: Used to measure properties of light such as wavelength, frequency, and spectrum.
• Pyrometer: Used to measure high temperatures, especially by detecting the thermal radiation emitted by hot bodies, including infrared radiation.
• Nanometer: This is a unit of length ($1\ \text{nm} = 10^{-9}\ \text{m}$), not an instrument.
• Photometer: Used to measure the intensity of visible light.

Evaluate each option

A. Spectrometer
A spectrometer analyzes spectra, but it is not specifically the standard instrument referred to for detecting infrared radiation in this context. So, this is not the best answer.

B. Pyrometer
A pyrometer detects thermal radiation emitted by bodies and is commonly associated with infrared detection for temperature measurement. Hence, this is correct.

C. Nanometer
This is a unit, not a detecting instrument. So, incorrect.

D. Photometer
A photometer measures visible light intensity, not specifically infrared radiation. So, incorrect.

Final answer

The correct option is: $\boxed{\text{B: pyrometer}}$

Comparison with stored correct answer

Stored correct answer = B
Derived answer = B

Therefore, the answers agree.