Understanding the tools used to measure different physical quantities is fundamental in science and engineering. This question asks about an instrument specifically designed to measure the intensity of light by comparing it to a known standard.

Identifying the Right Instrument for Light Intensity Measurement

The question requires identifying the instrument that measures light intensity relative to a standard source. Let's examine the given options:

• Calipers: Calipers are instruments used to measure the physical dimensions of an object, such as its length, diameter, or thickness. They work by using jaws that can be adjusted to fit the object. Calipers have nothing to do with measuring light intensity.
• Photometer: A photometer is an instrument that measures luminous intensity, luminous flux, illuminance, or luminance. Specifically, many types of photometers work by comparing the light from an unknown source to the light from a standard source to determine the unknown source's intensity. This aligns directly with the description in the question.
• Ammeter: An ammeter is an instrument used to measure the electric current flowing through a circuit. It measures the rate of flow of electric charge and is not related to light intensity measurement.
• Dynamometer: A dynamometer is a device used for measuring force, torque, or power. It is commonly used to test the performance of engines or motors. It has no function in measuring light intensity.

Based on the functions of these instruments, the one used to measure the intensity of light produced by an unknown source in terms of a standard source is the photometer.

How a Photometer Measures Light Intensity

A common type of photometer, historically like the Bunsen photometer, works on the principle of comparing the illuminance produced by two light sources. The instrument allows the user to adjust the distance of either the standard source or the unknown source until the illuminance they produce on a screen or surface appears equal. Since illuminance is proportional to intensity and inversely proportional to the square of the distance from the source ($\text{E} \propto \text{I}/\text{d}^2$), comparing distances allows for the determination of the unknown intensity based on the known intensity of the standard source.

Other modern photometers use photoelectric sensors to directly measure the light falling on them, often calibrated against known standards.

Comparing Measurement Instruments

Here is a brief comparison of the instruments mentioned:

Therefore, the correct instrument for measuring the intensity of light from an unknown source relative to a standard source is the photometer.

Revision Table: Key Measurement Instruments

Reviewing the functions of these common instruments helps solidify understanding.

Additional Information on Photometry and Light Measurement

Photometry is the science of the measurement of light in terms of its perceived brightness to the human eye. It differs from radiometry, which is the measurement of radiant energy (including light) in terms of absolute power. Key photometric quantities include:

• Luminous Flux: The total perceived power of light (measured in lumens).
• Luminous Intensity: The luminous flux per unit solid angle emitted by a point source in a particular direction (measured in candelas). This is often what is meant by "intensity of light source".
• Illuminance: The total luminous flux incident on a surface, per unit area (measured in lux or lumens/m$^2$). This is how bright a surface appears.
• Luminance: The luminous intensity per unit area of light traveling in a given direction (measured in candelas/m$^2$). This is how bright a surface appears when viewed from a specific angle.

Photometers are essential tools in various fields, including lighting design, photography, cinematography, environmental monitoring, and manufacturing, ensuring consistent and appropriate light levels.