For optics, photometry consists of measuring light according to the perception of the eye
Photometry is the area of optics that is dedicated to the study of the laws linked to light intensity . The term also refers to the process that allows measuring the intensity of light .
Light measurement
What photometry does is analyze how electromagnetic radiation stimulates vision . For this, it takes into account the visible range of the spectrum , which is the part of the radiation that stimulates the human eye.
It is important not to confuse photometry with the concept of radiometry , which consists of measuring light according to absolute power. In short, we can say that the first returns a value that is not proportionally linked to the amount of energy associated with the emitted light, but rather to the perception of the human eye.
Our eyesight is not equally sensitive to any wavelength of the visible spectrum . Photometry considers the various radiometric quantities taken for each wavelength, with their respective ocular sensitivity factors. In this framework we can also talk about the luminosity function , which defines the link between light understood physically and the sensation of light from our perspective.
The luminosity function can vary depending on the lighting conditions to which the human eye must adapt, whether we are dealing with photopic vision (good lighting) or scotopic vision (poor lighting). In the first case, the curve reaches a maximum of 555 nm; in the second, however, at 507nm.
For astronomy
The idea of photometry also appears in the field of astronomy . In this framework, the concept refers to the observation technique that is oriented towards measuring the color and apparent brightness of the stars .
When we talk about stars , we are referring to a broad group of concepts, including the following: comets, planets, asteroids, satellites and stars . Another name by which the stars are known is astronomical objects , and these are physical entities whose existence in the observable universe has been proven by science.
In the case of stars , the apparent brightness is measured using a system of magnitudes. These magnitudes can be recorded in infrared, ultraviolet or visible light, for example, using filters that only allow the passage of certain radiation frequencies. In total, Nicaea recognized five magnitudes and their system was used for many centuries.
The scale used to measure this property was invented by Hipcarchus of Nicaea , an astronomer who was born in Greece in 190 BC. C. and that also left us the first catalog of stars, the time division of the day into 24 equal parts, and more. Much later, at the beginning of the 17th century, the Italian astronomer Galileo Galilei created the telescope and thanks to it, humans were able to observe many new stars, so an expansion of the Nicene scale was necessary.
The brightness of the stars has been studied for more than two millennia
Types of photometry
It is possible to use photoelectric photometry , photographic photometry , CCD photometry or visual photometry . These methods work with different bands according to the optical filter used.
CCD photometry is the most modern. It uses a CCD (charge coupled device) chip as a receiver, which also quantifies the light it receives and presents the measurements in digital format.
Absolute photometry is known as that which measures the lumens (unit of luminous flux) emitted by a particular light source. Relative photometry , meanwhile, measures how the light emitted by various sources that have different luminous fluxes is distributed.