Photometry is the science of the measurement of light in terms of perceived brightness to the human eye. Chapter 22 Photometry deals with the luminous part. By principle, photometers help examine the interaction of light with reflective materials. Thus, photometry has a broad range of applications in different fields of study. A certain kind of photometers is used to observe and understand how light reflects wavelengths and/or absorbs them. Other instruments can be used to convert light into electric current and measure such current the light produces.
Luminous efficiency is the ratio formed between the total luminous flux radiated by a given source and the total radiant flux from the respective source. It is measured in lumens per watt. Moreover, luminous flux is also known as energy per unit time (dQ/dt) which is radiated from a source over visible wavelengths.
Luminous intensity is used to characterise a light source. Simply put, it is defined as the luminous flux per unit solid angle. Various formulas exist to calculate luminous efficiency, intensity, and flux. Moreover, there is a significant difference between luminance and illuminance. Luminance is nothing but the amount of light that passes through an object. On the other hand, illuminance refers to the amount of light falling onto a given surface area. Illuminance is calculated by the formula E= Icosθ/r2
where "I" denotes luminous intensity. This chapter also explains Inverse Square law and Lambert's cosine law. The illuminance of a small area is inversely proportional to the square of the distance of the area from the source is known as Inverse Square law. The intensity is maximum along the normal to the surface and decreases as we consider directions away from this normal. For many surfaces, if the luminous intensity along the normal is Io , it is I =Io cosθ. A photometer is used to compare the intensities of two-point sources.