Photometry

Photometry

Introduction of Photometry in Physics

The word "Photometry" comes from two Greek words, namely "photos" and "-merry," which denote something to measure with light. The meaning is that photometry deals with measuring light. It is the science of measuring the intensity of light. Quantitative measurement of light has great importance in various fields of physics, astronomy, and technology. The early photometry was done by direct inspection of objects with eyes visibly responding to different intensities. It helped the persons to distinguish between different luminous sources, but it did not provide information about how much light they emitted. In 1838, the French physicist A Camille Chevreul developed a law that defines that brightness or sensation of vision is proportional to 1/6th power of intensity of stimuli. This stimulated the scientists towards further development in this field, and J W von Bezold (1874) placed a blackened silver lining the room of a dark box. He introduced a small hole in the center of this box with which he obtained standard candlelight. This device was named "Bezold's Lumimeter."

Thus, photometry deals with the quantitative measurement of light using some suitable instruments capable of measuring various parameters of light. These are:-

Luminous intensity: It is the power emitted by a light source in particular. The SI unit of luminous intensity is the candela (cd).
Luminous flux: It is the total power emitted by a light source in all directions. The SI unit of luminous flux is lumen (lm).
Luminous emittance: It is the power emitted by a light source in a particular direction. The SI unit of luminous emittance is lux (lx).
Radiance: It is the total power emitted per unit solid angle by a light source in all directions. The SI unit of radiance is watt per steradian (W sr).
Illuminance: It is the power of flux incident on a surface per unit area. The SI unit of illuminance is lux(lx).

Luminous intensity and Illumination: Luminous intensity and illumination are closely related to each other. They depend upon the nature and position of diffusing surface, intensity, and wavelength of light. For example, a candle flame about 1 cm in diameter emits more illumination on a surface perpendicular to its axis as compared to another placed at 45° to the axis. This is because the candle flame spreads over a larger area as it becomes inclined with respect to the horizontal plan