Photometry is the science that deals with the measurement of the intensity of light, such as the brightness perceived by the human eye. That is, it studies the ability of electromagnetic radiation to stimulate the visual system. It should not be confused with Radiometry, which deals with the measurement of light in terms of absolute power. When intensity is measured at various angles of a luminaire, the process is called goniophotometry.
Photometry is supported by a tool known as Photometric Report, essential in the design process, since it contains all the data required for the correct selection and application of various light sources in the space to be illuminated.
The report provides information that allows the lighting professional to predict the performance of a lighting system, in addition to calculating the number of luminaires required and providing specific information on illuminance.
The human eye does not have the same sensitivity for all wavelengths that make up the visible spectrum. Photometry introduces this fact by weighting the different radiometric magnitudes measured for each wavelength by a factor that represents the sensitivity of the eye for that wavelength.
The function that introduces these weights is called the spectral luminosity function or relative luminous efficiency of a model eye, which is usually denoted as V? (This model or standard observer is very similar to those of Colorimetry). This function is different, depending on whether the eye is adapted to conditions of good lighting in daylight (photopic vision) or poor vision in night light (scotopic).
Thus, under photopic conditions, the curve reaches its peak at 555 nm (nanometers), while under scotopic conditions it does so at 507 nm.
Why use photometry?
Photometry is applied for the correct selection of equipment and/or devices to be used or proposed for its correct application in the lighting design process. Those involved in the design process or special applications must go to the photometric extensions to have the necessary tools that will help them make a correct selection of luminaires.
Knowing the photometric report in depth will provide all the necessary elements to extract the necessary information from each type of luminaire. Practitioners in the various fields where a visual stimulus is applied must support their intentions and transmit their reasoning and decisions in the design process. If photometry is not used, there will be a design model without foundations that will not have solid arguments for its application.
A poorly lit environment will result in unnecessary or poorly managed consumption, poor distribution of light or inadequate selection of luminaires based on the spaces or surfaces to be illuminated.
